Class 7 Chemistry - Acids Bases And Salts

Acids contain replaceable __________.

Monobasic acid contains one replaceable hydrogen atom in a molecule. For organic acids, basicity is the number of $$-COOH$$ groups present in the molecule. A tribasic acid contains three replaceable hydrogen atoms in a molecule. So, acids contain replaceable hydrogen.

Why would it be useful for a gardener or a vegetable farmer to use litmus paper to test soil samples?

A gardener or a farmer might use litmus paper to test the soil samples for acidic or basic character. Knowing the nature of soil is important to ensure successful growth of crops and plants.

A restaurant owner has a few bottles of soft drinks in his restaurant. But, unfortunately, these are not labeled. He has to serve the drinks on the demand of customers. One customer wants an acidic drink, another wants a basic and the third wants a neutral drink. How will the restaurant owner decide which drink is to be served to whom?

The restaurant owner can decide by using an indicator like litmus paper. If the sample of drink turns red litmus blue, it is basic. If it turns blue litmus red, it is acidic. If the color of litmus does not change then it is neutral.

A yellow curry stain on a white shirt turns red when it is washed with soap. Explain why?

This happens because the soap solution is basic in nature which changes the colour of turmeric in the curry stain to red. Turmeric is a natural indicator.

How do you prepare your own indicator using beetroot? Explain.

Acid and Bases can be identified using natural indicators like beetroot, cabbage, plums etc.

To use beetroot as an indicator, boil the beetroot in water for 30 to 60 minutes. Purple water is obtained, which acts as a natural pH indicator.

Name a synthetic indicator.

Answer: Phenolphthalein

An indicator prepared from artificial substances is known as a synthetic indicator. Phenolphthalein is an example of a synthetic indicator.

Give two important uses of washing soda and baking soda.

Washing soda $$[Na_2CO_3]$$:

$$1.$$ Cleaning agent for domestic purposes.

$$2.$$ Used to remove permanent hardness in water.

Baking soda $$[NaHCO_3]$$:

$$1.$$ Relieve skin irritation/ heartburn.

$$2.$$ Used widely in cooking purposes.

Explain why:
(a) An antacid tablet is taken when you suffer from acidity.
(b) Calamine solution is applied on the skin when an ant bites.
(c) Factory waste is neutralized before disposing it into the water bodies.

(a) We take an antacid such as milk of magnesia to neutralises the excessive acid released in the stomach.

(b) Ant injects an acidic liquid(Formic acid) into the skin on biting which causes inflammation, to the skin. The effect of the acid can be neutralised by rubbing. Calamine solution contains zinc carbonate which is a very weak base and causes no harm to the skin.

(c) The waste of factories contain acids. If acids are disposed of in the water body, the acids will harm the organisms. So factory wastes are neutralised by adding basic substances.

Dry ammonia has no action on litmus paper but a solution of ammonia in water turns red litmus paper blue. Why is it so?

$$NH_3$$ is basic in nature but only in its aqueous form. When $$NH_3$$ reacts with water, $$NH_4OH$$ is formed which is a base and contains $$NH^+_4$$ and $$OH^-$$ ions. Hence, the red litmus turns blue.

Litmus paper is a__________ indicator.

Litmus paper is a $$\text { natural}$$ indicator. It is used to check the acidic or basic nature of substance. It is a purple dye which is extracted from a type of plant called lichen.

Cutting onions taken from fridge is more comfortable than cutting onions at room temperature. Explain why?

At low temperature, vapour pressure is low. Also the vapours of the tear producing chemical are less. Hence it is more comfortable to cut onions taken out from the fridge.

China rose indicator is turned _______by a basic solution.

China rose indicator is turned green by a basic solution. On the other hand, acidic solutions turn it to dark pink.

Write the scientific reason.
Lemon sherbat has sweet, sour and salty taste and it can be poured in a glass.

Lemon sherbet has a sweet and salty taste and it can be poured in glass because lemon is citrus extract and corrosive in nature.

Blue litmus solution is added to two test tubes A and B containing dilute hydrochloric acid and sodium hydroxide solutions, respectively. In which test tube a colour change will be observed? State the colour change and give its reason.

Test tube $$A$$ contains dilute hydrochloric acid and test tube $$B$$ contains dilute sodium hydroxide. On adding blue litmus solution to both the test tubes, the colour of test tube $$A$$ will change from blue to red while the colour of test tube $$B$$ will remain the same. This is because hydrochloric acid is acidic and acid turns blue litmus to red. Sodium hydroxide being basic will not change the colour of blue litmus.

What is an acid? Write some of the properties of acids.

Curd, lemon juice, orange juice and vinegar taste sour. These substances taste sour because they contain acids. The chemical nature of such substances is acidic. The word acid comes from the Latin word $$acere$$ which means sour.

Some of the properties of acids are:

Acids change the colour of litmus from blue to red.

Some acids are highly corrosive in nature

Acids tend to evolve hydrogen gas whilst reacting with metals.

Why does dry HCl gas not change the colour of the dry litmus paper?

Hydrogen ions change the colour of litmus.

In dry $$ HCl $$, there are no $$ H^+ $$ ions.

Ions are formed in aqueous solution. Both $$ HCl $$ and litmus are dry.

Hence there is no change in dry $$ HCl $$ and dry litmus paper.

Define the following terms:
(a) Standard solution (b) Normal solution (c) Indicator

A. In analytical chemistry, a standard solution is a solution containing a precisely known concentration of an element or a substance.

B. Normal solution is a solution made by dissolving 1 gram equivalent weight of a substance in distilled water to make 1 L of the solution.

C. A substance that indicates whether a solution is acidic, basic, or neutral by usually a color change is termed as an indicator. Methyl orange is an indicator that is reddish-orange in an acidic medium, and in the basic medium, it turns yellow.

If someone is suffering from the problem of acidity after overeating, which of the following would you suggest as a remedy?
Lemon juice, vinegar, baking soda solution.
Give reason for your choice.

$$\bullet$$ If someone is suffering from the problem of acidity after overeating, a $$\text{baking soda}$$ solution would be suggested as a remedy as it is basic in nature. It neutralizes excess acid in the stomach.

$$\bullet$$ Lemon juice, vinegar are acidic so they would not be helpful.

(a) Define an acid and a base. Give two examples of each.
(b) Give the names and formulae of two strong bases and two weak bases.
(c) What type of ions are formed:
(i) when an acid is dissolved in water?
(ii) when a base (or alkali) is dissolved in water?
(d) Write the neutralisation reaction between acids and bases in terms of the ions involved.
(e) Write any two important uses of bases.

(a) Acids are those chemical substances that have a sour taste. Example: Acetic acid and citric acid.

Base is a chemical substance that has a bitter taste. Example: Caustic soda and washing soda.

(b) Strong bases – Sodium hydroxide (NaOH), potassium hydroxide (KOH).

Weak bases – Calcium hydroxide $$(Ca(OH_2))$$, ammonium hydroxide $$(NH_4OH)$$.

(c)

(i) When an acid is dissolved in water Hydrogen ions are found.

(ii) When a base (or alkali) is dissolved in wate Hydroxide ions are found.

(d) Neutralisation reaction between acids and bases:

$$\underset{Hydrogen\ ions \ (From \ acid)}{H^+ (aq)}+ \underset{Hydroxide\ ions \ (from \ bases)}{OH^- (aq)} \xrightarrow[Reaction]{Neutralisation} \underset{Water}{H_2O(l)}$$

(e) Two important uses of bases are:

(i) Sodium hydroxide is used in the manufacture of soap, paper and rayon.

(ii) Calcium hydroxide is used in the manufacture of bleaching powder.

What would be the color of litmus in a solution of sodium carbonate?

We know that sodium carbonate is a basic salt. Hence, its aqueous solution will also be basic in nature. So, when red litmus paper is dipped in sodium carbonate solution it will change to blue colour.

Give any two practical applications of neutralisation reaction.

$$\textbf{Concept: }$$Neutralisation reaction are those reaction in which an acid reacts with base or can say neutralises the base to form salt and water.
Following are two practical applications of neutralisation reaction:

Use of antacid to neutralise the excess release of acid in our stomach.
Basic fertilizers are used to neutralise the acidic soil which then become fit for cultivation.

A knife that is used to cut fruit was immediately dipped into water containing drops of blue litmus solution. The color of the solution changes to red. What is the nature of the fruit?

The fruit is acidic as it turns blue litmus solution to red.

What type of reaction is represented by the digestion of food in our body?

$$\bullet$$ Digestion of food in our body is an example of $$\textbf{decomposition reactions. }$$

$$\bullet$$ The starch decomposes into sugar in the body and proteins get decomposed into smaller substances called amino acids.

(i) Name one antacid. How does it help to relieve indigestion in the stomach?
(ii) A farmer treats the soil with quicklime or calcium carbonate. What is the nature of soil? Why does the farmer treat the soil with quicklime?

(i) Name of an antacid is Magnesium hydroxide (Milk of magnesia).

During indigestion, our stomach produces too much acid and this irritates us. Hence, when we take a base, like milk of magnesia, it neutralises the effect of the excess acid and relieves us from irritation and indigestion.

(ii) Quicklime or calcium carbonate is basic in nature. Hence, the nature of the soil must be acidic. Plants grow well in the pH range of 6-8. So, when the soil becomes acidic in nature the farmers add bases like quicklime or calcium carbonate to neutralise the excess acid and maintain the pH of the soil.

Give reason why antacids are required when there is pain or irritation in the stomach ?

$$\text{Pain or irritation is caused in our stomach when there is the production of an excess amount of acid in our body.}$$ $$\text{Antacids are basic in nature . Hence antacids are required when there is pain or irritation in the stomach to}$$ $$\text{neutralize the effect of the excess acid. }$$

Hritik tested rainwater and found a pH of 5-6 due to presence of $$H^+$$ ions formed by reaction of rainwater with carbon dioxide. When pH of the rainwater drops below 5-6, then it is called acid rain. Oxides of nitrogen and sulphur are acidic in nature. Burning of fossil fuels such as coal and oil in power stations and furnace produce oxides of nitrogen and sulphur. He told all of his classmates living nearby to use bicycle instead of school bus so as to reduce air pollution and prevent acid rain.

(a) Which acid is formed by sulphur dioxide in the presence of air?
(b) Which acid is formed by nitrogen dioxide in the presence of air?
(c) Why is acid rain harmful to agriculture, trees and plants?
(d) How can we prevent from acid rain?
(e) What values are possessed by Hrithik?

$$\textbf{Concept Applied:}$$ Non- metals react with oxygen to form acidic oxides or neutral oxides. The non-metal oxides are covalent in natures which are formed by the sharing of electrons. The acidic oxides of non-metals dissolve in water to form acids. The acidic oxides of non-metals turn blue litmus solution to red.

$$\bullet$$ Sulphurous acid is formed by sulphur dioxide, in the presence of water, in the air.

$$\bullet$$ Nitric acid is formed by nitrogen dioxide in presence of air.

$$\bullet$$ Plants, agricultural crops and trees require an optimum pH for their survival. Due to acid rain, the pH balance of the soil is disturbed and the soil becomes more acidic. This acidic nature of the soil is very much harmful to the healthy growth of plants, agricultural crops and trees. In this way, acid rain is harmful to agriculture, trees and plants.

$$\bullet$$ We can prevent acid rain by controlling pollution, by controlling the rate of consumption and burning of fossil fuels such as coal and petroleum oil, by controlling automobile combustion, reducing the burning of wastes, filtering out particulate matters emitted from the industries, using cleaner fuels like CNG in the automobiles and start using renewable sources of energy like solar energy, wind energy, tidal energy, etc. for electricity generation.

$$\bullet$$ Hritik possesses the value of social responsibility. He is very much concerned for his environment and the health of society as a whole.

Match the substances in Column I with those in Column II
Column I Column II
a) Tartaric acid i) Soap
b) Calcium hydroxide ii) curd
c) Formic acid iii) Unripe mangoes
d) Sodium hydroxide iv) Ant's sting
e) Lactic acid v) Lime water

The correct table is given below:

Column I	Column II
a) Tartaric acid	i) Unripe mangoes
b) Calcium hydroxide	ii) Lime water
c) Formic acid	iii) Ant's sting
d) Sodium hydroxide	iv) Soap
d) Lactic acid	v) curd

Look at Figure which shows solutions taken in test tubes A,B,C and D. What colour is expected when a piece of red litmus paper is dropped in each test tube?
Nature of the solutions is given in the table for your help.
Test Nature of solution Change in colour of red litmus
A Neutral
B Basic
C Acidic
D Neutral

According to the given condition:

Test	Nature of solution	Change in colour of red litmus
A	Neutral	No Change
B	Basic	Colour will change to blue
C	Acidic	No change
D	Neutral	No change

Look at the given reaction.
Hydrochloric acid + Sodium hydroxide (base) → Sodium Chloride(salt) + Water
Solium chloride formed in this reaction remains in solution form. Can we get solid sodium chloride from this solution? Suggest a method (if any).

In water 'sodium chloride is soluble and it gives a solution of sodium chloride. By the $$\text{'evaporation method,'}$$ all the water gets evaporated out and we will be left with the solid sodium chloride.

Form a sentence using the following words - baking soda, ant bite, moist, effect, neutralised, and rubbing.

The arranged form of given words:
"The effect of ant bite can be neutralized by rubbing moist baking soda on the affected area".

While playing in a park, a child was stung by a wasp. Some elders suggested applying paste of baking soda and others lemon juice as remedy. Which remedy do you think is appropriate and why?

The nature of baking soda is basic whereas lemon juice is acidic in nature. So applying the paste of baking soda is good remedy as compared to the paste of lemon juice because wasp sting releases methanoic acid into the sting region which causes itchness.

Fill in the cross word given as Figure with the help of the clues provided.
Across
(2) The solution which does not change the colour of either red or blue litmus.
(4) Phenolphthalein gives pink colour in this type of solution.
(7) Colour of blue litmus in lemon juice.
Down
(1) It is used to test whether a substance is acidic or basic.
(3) It is a natural indicator and gives pink colour in basic solution.
(5) Nature of ant's sting.
(6) It is responsible for increase in temperature during a neutralisation reaction.

1636739_1788970_ans_491471515757476aa1d74a0c3aa74866.png

Explain two neutralisation reactions related to daily life situation.

$$\textbf{Concept:}$$ neutralization reaction. a chemical reaction in which an acid and a base interact with the formation of a salt
$$\bullet$$ The neutralisation reactions related to daily life situations are:

a) Digestive disorders: When too much 'hydrochloric acid' is produced by stomach, which is called 'hyperacidity', it causes digestive disorders. The acid can be treated by intake of basic substances such as 'milk of magnesia' or 'baking soda'.

b) Ant-bite: Sting of an ant releases 'methanoic acid' or 'formic acid' in the body. These acids cause burning sensation and irritation. This issue is fixed by rubbing moist baking soda because moist baking soda is basic in nature.

Fill the blanks in the following sentences
Phenolphthalein gives ________ colour with lime water.

Phenolphthalein gives $$\text{pink}$$ colour with lime water.

Lime water is a base.

You are provided with four test tubes containing sugar solution, baking soda solution, tamarind solution, salt solution. Write down an activity to find the nature (acidic/basic/neutral) of each solution.

By tasting the solution present in test tube, we can identify the nature (acidic/basic/neutral) of each solution.

Sugar solution: The taste of sugar solution will be sweet. Its taste is not sour or bitter. So, it comes in the category of neutral solution.

Baking soda solution: The taste of baking soda solution will be bitter. So, it comes in the category of basic solution.

Tamarind solution: The taste of tamarind solution will be sour. So, it comes in the category of acidic solution.

Salt solution: The taste of salt solution will be salty. Its taste is not sour or bitter. So, it comes in the category of neutral solution.

A farmer was unhappy because of his low crop yield. He discussed the problem with an agricultural scientist and realised that the soil of his field was either too acidic or too basic. What remedy would you suggest the farmer to neutralise the soil?

$$\textbf{Concept:}$$ a chemical reaction in which an acid and a base interact with the formation of a salt; with strong acids and bases the essential reaction is the combination of hydrogen ions with hydroxyl ions to form water.
$$\bullet$$ When the soil of his field is too acidic, it is to be treated with bases like slaked lime(calcium hydroxide) or quicklime(calcium oxide).

The organic matters are added to the soil if the soil of his field is too basic. Organic matter releases acids that neutralize the basic nature of the soil.

Fill in the blanks with correct options given with question:
Turmeric and litmus are ________ acid-base indicators. (natural/synthetic)

Turmeric and litmus are $$\text{natural}$$ acid-base indicators.

You are provided with three test tubes A, B and C as shown in figure with different liquids. What will you observe when you put:
a) A piece of blue litmus paper in each test tube.
b) A piece of red litmus paper in each test tube.
c) A few drops of phenolphthalein solution to each test tube.

Test tube	Effect on blue litmus	Effect on red litmus	Effect on phenolphthalein
A	Changes to red	Remains red	Colourless
B	Remains blue	Changes to blue	Pink colour
C	Remains blue	Remains red	Colourless

Test tube A has an acidic solution. Test tube B has a basic solution. Test tube C has a neutral solution.

Fill the blanks in the following sentences.
Lemon juice and vinegar taste _ because they contain __.

Lemon juice and vinegar taste $$\text{sour}$$ because they contain $$\text{acids}$$.

Paheli observed that most of the fish in the pond of her village were gradually dying. She also observed that the waste of a factory in their village is flowing into the pond which probably caused the fish to die.
i) Explain why the fish were dying.
ii) If the factory waste is acidic in nature, how can it be neutralised?

The wastes of factory may contain high ratio of basic or acidic chemicals. These chemicals make aquatic life difficult.
Base will neutralize the acids. So, by adding the base (limestone, lime etc) into water we can make it less harmful and neutral too.

What will be the action of the following substances on litmus paper?
Dry HCl gas, Moistened $$NH_{3}$$ gas, Lemon juice, Carbonated soft drink, Curd, Soap solution.

$$\begin{array}{|l|l|l|}\hline {\text { Substance }} & \text { Action on Litmus paper } & \text { Red Litmus paper } \\\hline \text { Dry HCl gas } & \text { None } & \text { None } \\\hline \text { Moistened } \mathrm{NH}_{3} & \text { None } & \text { Turns blue } \\\hline \text { Lemon juice } & \text { Turns red } & \text { None } \\\hline \text { Carbonated soft drink } & \text { None } & \text { Turns blue } \\\hline \text { Curd } & \text { Turns red } & \text { None } \\\hline \text { Soap solution } & \text { None } & \text { Turns blue } \\\hline\end{array}$$

Name the acid present in ant sting and give its chemical formula. Also, give the common method to get relief from the discomfort caused by the ant sting.

Methanoic acid is present in ant stings. Its chemical formula is $$HCOOH$$. When baking soda is rubbed on the affected part, it gives relief from pain.

What do you observe when the following substance is burned and then tested with moist blue and red litmus paper?
Phosphorous

Phosphorus on burning produces dense white fumes of phosphorus pentoxide. When test with blue and red litmus paper, it changes the colour of blue litmus to red as it is acidic in nature. The red litmus remains unaffected.

Fill the blanks in the following sentences
When an acidic solution is mixed with a basic solution, they each other forming and water.

When an acidic solution is mixed with a basic solution, they $$\text{neutralize}$$ each other forming $$\text{salt}$$ and water.

Paheli is suffering from indigestion due to acidity. Is it advisable to give her orange juice in this situation and why?

Orange juice is acidic. So, it is not advisable to take orange juice in her situation because orange juice increase acidity.

What do you observe when the following substance is heated and then tested with moist blue and red litmus paper?
Magnesium

Magnesium on heating produces magnesium oxide. Magnesium oxide turns moist red litmus paper to blue as it is basic in nature. The blue litmus remains unaffected.

Give reason:
A wasp sting is treated with vinegar.

$$\bullet$$ Wasp stinger contains venom and that can be transmitted to humans during a sting. This venom can cause significant irritation and pain.

$$\bullet$$ These wasp stings are alkaline and can be neutralized by vinegar which is a weak acid.

What do you observe when blue litmus paper is dipped in dilute hydrochloric acid?

When a blue litmus paper is dipped in dilute $$HCl$$ acid, its color changes to red.

Blue litmus turns red when it is dipped in acid.

Write down the acids present in vinegar, grapes and lemon.

Acids present in following are:
Vinegar : Acetic acid
Grapes : Tartaric acid and Malic acid
Lemon : Citric acid

Give reason:
A person suffering from acidity is advised to take an antacid.

When a person suffers from acidity, due to an increased concentration of acid in the stomach, it is suggested that a base or antacids be administered to the person to neutralize the acid in the stomach. That is why substances such as milk of magnesia (antacid) which contains magnesium hydroxide are advised to take.

What do you understand by the terms acid and base.

$$\text{Acid}$$ are molecule or ions capable of donating a proton i.e $$H^+$$.

$$\text{Example:}$$ $$HCl, CH_3COOH\ etc.$$

$$\text{Base}$$ are molecule or ions capable of accepting a proton.
$$\text{Example:}$$ $$NaOH, NH_3\ etc.$$

You have been provided with three tubes. One of them contains distilled water and the other two have an acidic solution and a basic solution respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

The test tube containing distilled water does not affect the red litmus paper.

The test tube containing acidic solution does not change the red litmus paper.

But the test tube containing the basic solution turns red litmus paper blue.

Write the ion that turns red litmus blue.

Hydroxide ions $$(OH^-)$$ can change the colour of red litmus into blue.

What do you observe when blue litmus solution is added to water?

Pure water is neutral to litmus which means that no change in the colour of blue or red litmus solution is observed when treated with water.

Give reason:
Acidic soil is treated with quick lime.

Quick lime is basic in nature. Most of the soil contains acidic substances, which lowers the pH of the soil. In such cases, the growth of plants is badly affected.

Therefore, quick lime is added to the soil to neutralise the acid present in it and bring the pH to 7 which is favourable for growth.

Write the names of any two natural indicators.

Indicators obtained from natural sources are called natural indicators. Litmus, turmeric, red cabbage, china rose, etc. are some common natural indicators used widely to show the acidic or basic character of substances.

Write down the changes that will be seen in the instance and explain the reason behind it.
A litmus paper was dropped into 2ml dilute HCI. Then 2ml concentrated NaOH was added to it and stirred.

When litmus paper is dipped into 2 mL of dilute HCI solution , then blue litmus paper is turned into red colour and there is no effect on red litmus paper. Again, if the same litmus paper is dipped into 2 mL of concentrated NaOH solution, then red litmus paper turns into blue colour but there is no effect on blue litmus paper.This is due to the respective properties of blue and red litmus paper with acid and base.

Dry HCl gas does not change the colour of dry litmus paper. Why ?

Dry HCl gas does not change the color of dry litmus paper $$\text{because HCl ionizes only in aqueous solution.}$$ In the case of dry HCl gas, there is no presence of water and so, the dye on litmus paper cannot react with the gas. So, any dry gas will not give any change in the color of litmus paper.

Fill in the blanks using appropriant words.
Main constituent of acid is _________

$$\textbf{Concept:}$$ Acid: A solution that has an excess of H+ ions. It comes from the Latin word acidus, which means "sharp" or "sour". Base: A solution that has an excess of OH- ions. Another word for the base is alkali.
$$\bullet$$ Acid: A solution that has an excess of H+ ionsMain means the main constituent of acid is the H+ ion

Bases turn red litmus to ....................

Properties of bases

Bases are slippery to touch when in aqueous form.
Bases turn the color of red litmus paper to blue.
Acids and Bases react to form salt and water.

So the correct answer is blue.

Match the columns

$$\textbf{Correct Answer:}$$ Certain chemicals have common names. Here are the correct names for given chemical formula:

$$List \;1$$ $$List \;2$$

A. Sodium chloride   3. $$NaCl$$
B. Acid of sulphur                  1. $$H_{2}SO_{4}$$
C. Baking soda                      2. $$NaHCO_{3}$$
D. Vinegar                              5. $$CH_{3}COOH$$
E. Quick lime      4. $$CaO$$

Fill in the blank.
If M is a non-metal, then the blue litmus will turn _______.

Non-metal, M burns in oxygen to form an oxide. Non-metallic oxides form an acidic solution in water.

So, when tested with a blue litmus, it will turn red.

If a solution changes the colour of litmus from red to blue, what can you say about its pH?

Litmus paper is used to test whether a substance is acidic or basic. If the solution changes the colour of litmus from red to blue, it indicates that the solution is basic. For the solution that is basic in nature, the pH is above 7 and lies in the range from 7-14.

Write down only the word which will correctly complete each of the following sentences.
If M is a metal, then the litmus will turn__________.

When Metal $$M$$ burns with oxygen it form its oxide which is soluble in water. That means it is a basic oxide and a base turns red Litmus $$blue$$ color.

Write three differences between acids and bases.

Difference between acids and bases:

$$Acids$$ $$Bases$$
1. Sour in taste 1. Bitter in taste
2. Change blue litmus red 2. Change red litmus blue
3. Not soapy to touch 3. Soapy to touch

Write the name of two artificial indicators.

Two artificial indicators are: (i) Phenolphthalein and (ii) Methyl orange.

What are indicators? Name any two indicators. Explain the effect of these indicators on acid and bases.

Indicators are substances that show a change in at least one of their properties(like colour or smell) when brought in contact with acids and bases. These indicators indicate the nature of a substance by this change.

Name of two indicators: Litmus, turmeric.

Effects of indicators on acids and bases:

Acids turn blue litmus red and bases turn red litmus blue.

Bases mixed in a solution of turmeric powder give red colour. Turmeric solution does not change its colour with acid.

What happens if waste materials of industries is drained off without treatment?

Most industrial waste contains toxic and harmful chemicals (mostly acidic). If we directly drained this waste into a river or any other water body then it will be hazardous for aquatic animals like fishes, aquatic plants, and other organisms.

What are acids? Write the name of two food items which contains acids.

$$\bf{Concept-}$$

An acid is any substance that in water solution tastes sour, changes blue litmus paper to red, reacts with some metals to liberate hydrogen, reacts with bases to form salts.

$$\bf{Explanation-}$$

$$\bullet$$ The word acid is derived from the Latin word’ Acids’ which means sour. Hence, we can say that the substances which are sour in taste are called acids.

$$\bullet$$ Two food items which contain acid: Orange and curd.

Orange contains citric acid.

Curd contains lactic acid.

What are antacids?

Antacids are a class of medicines that neutralize the acid in the stomach. They contain ingredients such as aluminium, calcium, magnesium, or sodium bicarbonate which act as bases to counteract stomach acid and make its pH neutral.

When a blue litmus is dipped in a solution it becomes red. Write the nature of the solution. Explain it

When blue litmus paper turns red in a solution, then the nature of the solution is acidic. In acidic solution, blue litmus paper turns red while in basic solution red litmus turns blue.

The substances which have no effect on litmus paper are called ...................... substances.

The main use of litmus is to test whether a solution is acidic or basic.

Blue litmus paper turns red under acidic conditions.
Red litmus paper turns blue under basic or alkaline conditions.

The substances which have no effect on litmus paper are called $$\text {neutral}$$ substances.

Acid turns blue indicator to ________.

The main use of litmus is to test whether a solution is acidic or basic.

Blue litmus paper turns red under acidic conditions.

Red litmus paper turns blue under basic or alkaline conditions, with the color change occurring over the $$pH$$ range $$4.5–8.3$$ at $$25 ^oC$$.

The neutral litmus paper is purple.

Acid is derived from word .....................

The word acid is derived from the Latin $$\textbf {acidus/acēre}$$, meaning 'sour'.

Give examples of natural indicators.

Litmus, turmeric and China rose are natural indicators.

Write the conjugate acid of the following:
$$S^{2-}, NH_3, H_2 PO^-_4, CH_3NH_2$$

According to Bronsted-lowry, conjugate acid-base pair consists of two substances that differ only by the presence of a proton. A conjugate acid is formed when a proton is added to a base and, a conjugate base is formed when a proton is removed from an acid.

Species Conjugate acid
$$S^{2-}$$ $$HS^-$$
$$NH_3$$ $$NH_4^{+}$$
$$H_2PO^-_4$$ $$H_3PO_4$$
$$CH_3NH_2$$ $$CH_3NH^+_3$$

What is the role of bases in disposal of wastes from factories?

$$\textbf{Concept:}$$ A neutralization reaction is when an acid and a base react to form water and salt and involves the combination of H+ ions and OH- ions to generate water.
$$\bullet$$ Acidic substances are present in industrial wastes. If these are directly disposed into the water, then they may be harmful to fishes, aquatic plants, and other aquatic animals. Therefore, basic substances are mixed in this waste to neutralize them.

What will you take to get relief from hyperacidity?

$$\bf{Concept-}$$

Hyperacidity refers to a set of symptoms caused by an imbalance between the acid-secreting mechanism of the stomach and proximal intestine and the protective mechanisms that ensure their safety.

$$\bf{Explanation-}$$

$$\bullet$$ Milk of magnesia is an alkaline suspension, meaning that it will cause neutralization when it encounters anything acidic.

$$\bullet$$ This is why it makes an excellent antacid, as it neutralizes excess stomach acid (hydrochloric acid, $$HCl$$) when swallowed.

Write some properties of acids.

Acids taste sour.
They turn blue litmus red.
They react with base and neutralise each other to form a salt.

What is an acid?

A substance that is sour in taste and which turns blue litmus to red is called acid.

An acid is a substance that produces $$H^+$$ ions in an aqueous solution.

Find the characteristics of the substances given in the table below using litmus papers.
Substance Change in the color of litmus Characteristics
Vinegar
Lime water
Soap solution
Hydrochloric acid

Substance	Change in the color of litmus	Characteristics
Vinegar	Blue turns red	Acidic
Lime water	Red litmus turns blue	Alkaline
Soap solution	Red litmus turns blue	Alkaline
Hydrochloric acid	Blue litmus turns red	Acidic

What can be inferred about the nature of the following substance from the litmus test?
Distilled water

If we test distilled water with red and blue litmus paper, they will not show any change. This indicates distilled water is neutral in nature.

Organic acids are present in a number substances we use in our daily life.
(eg. Tomato, orange, apple, grapes, curd, etc.) Identify the organic acids in each of them and tabulate.

Tomato - oxalic acid
Orange - Citric acid
Apple - Malic acid
Grapes - Tartaric acid
Curd - Lactic acid, Butiric acid.

From the characteristics given below, find out those that are suitable for acids and put $$(\checkmark)$$ Mark.
Have a bitter taste.
Turn blue litmus red.
React with carbonates to form carbon dioxide gas
Soapy to touch
Liberate hydrogen on reaction with highly reactive metals like $$Mg$$ and $$Zn$$
Have a sour taste
Turns red litmus blue.

1. Have a bitter taste, (characteristic of a Base)
2. Turn blue litmus red $$(\checkmark)$$,
3. React with carbonates to form carbon dioxide gas $$(\checkmark)$$
4. Soapy to touch, (characteristic of a Base)
5. Liberate hydrogen on reaction with highly reactive metals like $$Mg$$ and $$Zn$$ $$(\checkmark)$$
6. Have a sour taste $$(\checkmark)$$
7. Turn red litmus blue, (characteristic of a Base)

Take equal quantities of dilute hydrochloric acid, sodium hydroxide solution and distilled water in three test tubes. Use red litmus paper and blue litmus paper to find out the nature of the solutions. Also, add two or three drops of phenolphthalein solution to the three test tubes. Record the observation and find out the nature of the substances.
Do you notice any colour change in distilled water? What property of water is revealed here?

Answer: (i) No (ii) Neutral property of water

There is no change in colour of the indicators in case of distilled water. That's because distilled water is neutral, and so will not change the colour of any of these indicators.

Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain

The solution can be either a base or a neutral solution. Acidic solutions change blue litmus red, hence it is not acidic. Bases change red litmus to blue hence there is no change in colour of blue litmus paper. Neutral solutions have no effect on litmus paper.

Dip a red litmus paper in dilute hydrochloric acid. What change do you observe?

Answer: No change

Dilute hydrochloric acid has no effect on red litmus paper. However, it will change blue litmus paper red as it is an acid. Acids turn blue litmus paper red.

Solution of sodium carbonate, potassium chloride, and ammonium sulfate are taken in separate beakers. Dip a litmus paper (red, blue) in each beaker.
Salt Colour of litmus paper Nature of the substance
A
B
C

Salt

Colour of litmus paper

Nature of the substance

Red litmus turns to blue

Basic

No color change

Neutral

Blue litmus turns to red

Acidic

i) Salts formed by the reaction between a strong base and weak acid will be slightly basic. Example: $$Na_{2}CO_{3}$$
ii) Salts formed by the reaction between a strong acid and a strong base will be neutral in nature. Example: $$KCl$$
iii) Salts formed by the reaction between a weak base and strong acid will be slightly acidic. Example: $$(NH_{4})_{2}SO_{4}$$.

Wet blue and red litmus papers are placed over the watch glass containing soap solution. Which litmus paper shows a color change?

Blue litmus paper will not show any change in colour but red litmus will turn blue because soap solution is basic nature. In basic solution, blue litmus remains blue but red litmus changes to blue.

Give a method that is used to identify acids and bases.

Using indicators like litmus, phenolphthalein and methyl orange, we can distinguish between acids and bases. For example, acids turn blue litmus paper red, and bases turn red litmus paper blue.

Fill in the blanks:
China rose indicator turns acidic solutions to dark __________ and basic solutions to green.

China rose indicator turns acidic solutions to dark $$pink$$ (magenta) and basic solutions to green.

When an ant bites, it injects _________ into the skin.

$$\textbf{Correct Answer:}$$ Formic acid

$$\textbf{Explanation:}$$ When an ant bites, it injects formic acid into the skin. This is why it hurts. It can be treated using a base like baking soda to neutralise the acid.

Formula of Formic Acid: HCOOH

Litmus is extracted from ______.

Litmus is extracted from $$lichens$$.

Lichens are a source of which natural indicator?

Litmus indicator is obtained from lichens.

The color of a blue litmus paper when dipped in a solution remains blue. The solution is .......... or ..........

Litmus paper is used to test the acidic and basic nature of a substance.

In basic medium-red litmus changes to blue and blue litmus remains blue.

In the acidic medium- Blue litmus changes to red and red litmus remains red.

If the solution is neutral no change in colour is observed.

In the given question the blue litmus remains blue when dipped in a solution. Thus the solution is basic or neutral.

You are provided with three test tubes A, B and C which contain distilled water, acidic solution and basic solution respectively. If you are given blue litmus paper only, how will you identify the contents of each test tube.

The given blue litmus paper is dipped in the given solutions. The solution which turns blue litmus red is acidic solution. Now tear this blue litmus turned red into 2 halves. Now, the solution which turns it blue again is the acidic solution and the one which does not affect it is distilled water.

(a) What is litmus? How is litmus paper obtained?
(b) Write the method of preparation of turmeric paper.

(a) $$\text {Litmus}$$: To determine acidity or basicity of any substance, litmus is most commonly used as natural indicator. Litmus is obtained from lichen. It is used both as a solution and as strips of paper. These strips of paper are called litmus paper.

There are two types of litmus papers: red litmus and blue litmus.

(b) $$\text{Method of preparation of turmeric paper:}$$
Take some turmeric powder in a spoon. Mix small amount of water in it and make a paste. Spread this paste on a piece of white paper. Cut into small strips after drying. Now, turmeric paper is ready to use.

Match the List I(Indicator) with List II(Types of titration)

A .Phenolphthalein is used to titrate strong acid and strong base like $$HCl+NaOH,H_2SO_4+KOH$$

B .Phenol red is used to titrate weak acid and weak base like $$CH_3COOH+NH_4OH$$

C .Bruno cresol green is used to titrate strong acid and weak base like $$H_2SO_4+NH_4OH$$

D .Methyl orange is used to titrate strong acid and strong base and also stong acid - weak baselike $$HCl+NaOH,H_2SO_4+KOH,H_2SO_4+NH_4OH$$

The pH values of three solutions A, B and C are given in the table. Answer the following questions:
Solution pH value
A
B
C 12
2
7
(i) Which solution will not affect the litmus solution?
(ii) Which solution will liberate $$CO_2$$ when reacted with sodium carbonate?
(iii) Which solution will turn red litmus solution blue?

(i) C (Because pH=7 indicates neutral solution).
(ii) B (Because acids liberate $$CO_2$$ gas when treated with carbonates and acids have pH less than 7)
(iii) A (Bases turn red litmus blue and they have pH more than 7)

Solution	pH value
A B C	12 2 7

Acids Bases And Salts - Class 7 Chemistry - Extra Questions