Class 7 Chemistry - Physical And Chemical Changes

Write some changes happening in our body.

Changes that happen in our body are:

(a) Growth of nails
(b) Growth of hairs
(c) Increase in height
(d) Increase in weight

To walk through a waterlogged area, you usually shorten the length of your dress by folding it. Can this change be reversed?

Yes, it can be reversed by unfolding the dress. The change is temporary and can be easily reversed.

Fill in the blanks:
A new substance is formed in a _____________ change.

Fill up is chemical.

Give two examples of a chemical change.

Two examples of a chemical change are:

1. Burning of wood.

2. Breathing.

Classify the following as physical or chemical change:
Boiling

Boiling is a physical change.

Churning of milk cream to get butter is a _______ change.

When milk cream is churned, we get butter. Here, no new substance is formed. Instead, the fat droplets in milk cream join together to form clumps of fat (butter). There is no chemical change happening here, only a physical change.

_____, water, and water vapour look different and display different physical properties but they are chemically the same.

Ice, water and water vapour are $$3$$ different states or physical forms with same molecular formula $${ H }_{ 2 }O$$.

Water at temperature less than $${ 0 }^{ 0 } C$$ exists as ice and water above $${ 100 }^{ 0 }C$$ exists as water vapor.

So, the correct answer is Ice.

Change in the color of black tea on adding lemon juice is an example of _________ changes.

Adding lemon juice changes the acidity of tea.

Tea has polyphenol pigment $$7-20\%$$ and on adding acidic lemon juice, hydrogen ions suppress the ionization of polyphenols and that changes the color of tea making it lighter.

Therefore, the change in the color of black tea on adding lemon juice is an example of chemical change.

Rising of hot air over a radiator is a chemical or physical change? Give reason.

It is a physical change when water is evaporating to form hot air vapors over the radiator as no new chemical or substance is formed.

Drying of a shirt in the sun is an example of __________ change.

Answer: Drying of a shirt in the sun is an example of $$\underline {physical}$$ change.

Drying of a shirt in the sun is a physical change because water gets evaporated in the form of vapours into the atmosphere and no new substance is formed.

Burning of kerosene in a lantern is an example of _______ changes.

In the burning of the lantern, the kerosene is the fuel which used to burns rapidly in the presence of oxygen to give light along with carbon dioxide and water vapour.

Hence, the Burning of kerosene is an example of chemical change.

Dissolution of salt in water is a ______ (physical/chemical) change.

The dissolution of salt in water is the process of dissolving salt in water. It is a physical change as no new substance is formed and, the salt can be recovered back on evaporation of water.

Freezing of water and evaporation of water are both physical changes.

A physical change is a temporary and reversible change in which the physical properties of the substance change without altering the composition of the substance. Here in both processes, only the state of matter changes so they are physical changes only.

List six physical changes that matter can go through.

Some physical changes that matter can go through are breaking, melting, freezing, cutting, crushing and dissolving.

Iron is a solid, grey metal. Oxygen is a colourless gas. When they chemically combine, rust is made. Rust has a reddish brown colour. Why is rust different from the iron and oxygen that it is made of?

When elements combine to form a compound, the compound's properties are different from the properties of the individual elements.

An object was originally a copper colour. After being exposed to the air, it turned a greenish colour. What kind of change happened? Explain your answer.

A chemical change occurred. The copper in the object combined with oxygen in the air and formed new copper compounds with different properties.

A .............. change is a change that does not change the identity of the substance.

A Physical change is a change that does not change the identity of the substance. for example, water changes from ice to liquid or from liquid to vapor its identity is still the same.

Dissolution of salt in water is a _________ change

Dissolution of salt in water is a $$\text{Chemical}$$ change

A chemical change involves a chemical reaction, with new substances produced as a result of the change. When you dissolve salt in water the sodium chloride dissociates in $$Na^+$$ ions and $$Cl^-$$ ions, which may be written as a chemical equation:

$$NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)$$

Therefore, dissolving salt in water is an example of a chemical change.

Which of the following are chemical changes?
$$(a)$$ Growth of a plant
$$(b)$$ Rusting of Iron
$$(c)$$ Mixing of iron fillings and sand
$$(d)$$ Cooking of food
$$(e)$$ Digestion of food
$$(f)$$ Freezing of water
$$(g)$$ Burning of a candle.

A chemical change is a change that brings a change in the chemical properties of matter and a new substance is formed.

Examples of chemical changes from the given list of changes are:
Rusting of iron, Cooking of food, Digestion of food, and Burning of a candle.

Can you change the shape of an eraser after erasing it?

Yes, the shape of the eraser changes after erasing because by erasing a part of the eraser is used.

Why does a blacksmith heat the metal rim to fix it on a cart wheel?

A blacksmith heats the metal rim to fix it onto a cart wheel because a metal rim is made slightly smaller. On heating, the rim expands and fits onto the wheel. Then on cooling, the rim contracts and fits tightly onto the wheel.

Can the rusting of iron nails occur in distilled water? Justify your answer.

The essential thing required for rusting is atmospheric oxygen.

In distilled water, it will readily absorb oxygen from the atmosphere, and hence rusting of iron will take place.

$$4Fe+3{ O }_{ 2 }+2x{ H }_{ 2 }O\longrightarrow 2{ Fe }_{ 2 }{ O }_{ 3 }.x{ H }_{ 2 }O$$

(rust)

What is a chemical change? Explain with example.

The changes in which new substance with new chemical properties are formed are called chemical changes.

Example: Reaction between washing soda and lemon juice in which $$CO_2$$ and other substances are formed.

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Give two examples of fast changes.

Examples of fast changes:

1. Bursting of crackers

2. Thunder

Why does a lump of cotton wool shrink in water? This is a physical or a chemical change?

Lump of cotton wool has air among gaps of cotton fibres. When water replaces the air from these gaps, the cotton lump becomes heavy and also shrinks due to removal of air gaps. This is a physical change.

When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

When the candle burns,

wax $$\longrightarrow { CO }_{ 2 }+$$ heat $$+$$ light

(vapours)

This is a chemical change, it can't be reversed. Whereas, melting of wax is a physical change.

$${ wax }_{ \left( e \right) }\rightleftharpoons wax\left( solid \right) $$

When molten wax cools it solidifies again.

Another example of a process in which both physical and chemical changes take place is the digestion of food.

Physical change $$\longrightarrow $$Breaking down of larger food particles into smaller particles

Chemical change $$\longrightarrow $$Digestion of food

How would you show that setting of a curd is a chemical change?

$$\textbf{Concept: }$$When any substance or object undergo any change which changes its chemical composition or form new substance. This type of change is known as chemical change.

Setting of curd from milk is a chemical change because chemical property of that substance get entirely changed and also you can not reverse that change.

When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

$$\textbf{Concept: }$$ When any substance or object undergo any change which changes its chemical composition or form new substance. This type of change is known as chemical change.

When baking soda is mixed with lemon juice, Carbon dioxide$$(CO_2)$$ gas is released in form of bubbles. As chemical composition of substance is getting changed and also new product is formed after mixing. Thus it is a chemical change.

Give the two methods by which rusting of iron can be prevented.

$$\textbf{Concept: }$$Rusting of iron refers to the formation of iron oxides on the surface of Iron which degrades Iron gradually.

$$\text{Following are two methods by which rusting of iron can be prevented:}$$

Galvanization: A thin coating of Zinc is done on Iron. Zinc coating prevents Iron to come in contact with oxygen and moisture.
Greasing on Iron surface: Grease is a thickened oil. Water and oil are immiscible liquids. So after greasing, water can not come in contact with the Iron and hence prevents rusting.

Explain why rusting of iron objects are faster in coastal area than in deserts?

$$\bf{Hint-}$$

Study about the rusting process of iron.

$$\bf{Concept-}$$

Rusting is a chemical phenomenon in which iron comes in contact with air $$(O_2)$$ or moisture $$(H_2O)$$ and form iron oxide $$(Fe_2O_3)$$ which is called rust.

$$2Fe+ \dfrac 32{ O }_{ 2 }\longrightarrow { Fe }_{ 2 }{ O }_{ 3 }$$ [Water acts as catalyst]

(iron) (rust)

$$\bf{Explanation-}$$

In coastal areas, the quantity of moisture present in the air is higher than in the deserts. Hence, rusting of iron objects is faster in coastal areas than in deserts.

Explain how painting of an iron gate prevents it from rusting?

$$\textbf{Concept:}$$ Rust forms when iron or its alloys are exposed to moist air. The oxygen and water in the air react with the metal to form the hydrated oxide($$Fe_2O_3.XH_2O$$). The oxygen needs moisture as a catalyst and reactant to accelerate the reaction, so in the absence of moisture, iron won't rust.
$$\bullet$$ The essential component for rusting is the contact of metal with oxygen and moisture. When the iron gate is painted, it protects the iron from getting in contact with oxygen and moisture. Hence, prevents it from rusting.

State two differences between rusting and burning.

(a) Rusting is a slow process while burning is a fast process.

(b) Rusting takes place in the presence of oxygen and moisture while burning requires only oxygen.

Explain energy changes in a chemical change.

When a chemical reaction occurs, some bonds will break, while new bonds may form. Energy changes in chemical reactions result from the breaking and forming of bonds. For bonds to break, energy must be absorbed. When new bonds form, energy will be released.

Because the new product has lower energy than the `in-between' stage of the reaction when the bonds in the reactants have just been broken.

Which of the following is a physical change?
i. Burning of magnesium
ii. Dissolving of sugar in water
iii. Formation of compound from its element
iv. Rusting of iron

$$(i)$$ Burning of magnesium can be shown by the reaction,

$$Mg+{ O }_{ 2 }\longrightarrow MgO$$ $$-$$ It is a $$\text{chemical change}$$

$$(ii)$$ It is $$\text{physical change}$$, as there is no formation of any new substances. This change can be reversed.

$$(iii)$$ $$\text{Chemical change}$$ as new compound is formed from its element.

$$(iv)$$ Rusting is a $$\text{chemical change}$$ since rust is formed (a new substance).

The burning of a piece of magnesium ribbon in air is a chemical change. Explain.

When the magnesium ribbon is burnt, a new substance with different chemical property is formed.

$$2Mg+{ O }_{ 2 }\longrightarrow 2MgO$$

This property shows it is a chemical change.

Why do we apply paints on iron goods?

Iron articles are painted to prevent them from rusting. After painting iron surface does not come in contact with moisture and air which causes rusting.

What is rust?

The brown, flaky coating observed on the surface of iron is called rust. It appears when iron is exposed to oxygen and moisture. Chemically, rust is hydrated iron oxide.

What kind of change is shown by tearing of paper?

Tearing of paper is a physical change.

Classify the following process into physical or chemical changes:
Digestion of food.

Digestion of food is a chemical change because in digestion of food larger macromolecules are broken down into simpler molecules.

Match the items of Column I with the items of Column II.
Column I
Column II
a) Large crystals
i) Turns lime water milky.
b) Depositing a layer of zinc on iron
ii) Physical change
c) Souring of milk
iii) Rust
d) Carbon dioxide
iv) Sugar candy (mishri)
e) Iron oxide
v) Chemical change
f) Dissolving common salt in water.
vi) Galvanisation

Column I	Column II
a) Large crystals	i) Sugar candy (Mishri)
b) Depositing a layer of zinc on iron	ii) Galvanisation
c) Souring of milk	iii) Chemical change
d) Carbon dioxide	iv) Turns lime water milky
e) Iron oxide	v) Rust
f) Dissolving common salt in water	vi) Physical change

Fill in the blanks:
Grinding of the wheat grains is done to change their size. It is a ___________ change.

It is a physical change as only the size of the wheat grain changes, but its chemical composition remains the same. Although it's an irreversible change as we cannot restore the original wheat grain after grinding it.

Fill in the blanks:
Making sugar solution is a __________ change.

Making sugar solution is a physical change.

Burning of crackers is a physical or chemical change?

Burning of crackers is a chemical change because it produces new products with different properties.This chemical change also produces heat, light and smoke.

Melting of wax is a change where a solid changes to a liquid state. Give one more such change which you observe in your surroundings.

When a candle is burning two things occur simultaneously. First, melting of wax and second, burning of wax. In these, the melting of wax is a physical change but the burning of wax is a chemical change. Because in burning oxidation of wax actually takes place which is a chemical change.

According to the given condition, another example is 'Melting of ice'.

Give two examples for the following case
Physical changes which are reversible.

A change in which a substance undergoes a change in its physical properties such as shape, size, state of a substance, colour, etc is called a physical change. A physical change is generally reversible. In such a change no new substance is formed.

Physical changes which are reversible:

Melting of ice
Folding of clothes

Classify the following process into physical or chemical changes:
Cutting of a log of wood into pieces.

Cutting of a log of wood into pieces is a physical change.

Classify the following process into physical or chemical changes:
Beating of aluminium metal to make aluminium foil.

The beating of aluminium metal to make aluminium foil is a physical change.

Give two examples for the following case:
Chemical changes.

Chemical changes:

i) Burning of wood

ii) Digesting food.

There are some more examples of chemical changes like rotting, cooking, rusting of iron, etc.

Define rusting? What are two necessary condition for rusting of iron. Give the chemical name of rust.

$$Rusting$$: Slow conversion of iron into its hydrated oxide in the presence of moisture and air is called rusting.

$$Conditions$$ $$for$$ $$rusting$$:

1. Presence of moisture (water).

2. Presence of oxygen (air).

Chemical name of rust is $$hydrated$$ $$iron$$ $$oxide$$.

Which among the following are physical or chemical changes?

(a) Evaporation of petrol
(b) Burning of Liquefied Petroleum Gas (LPG)
(c) Heating of an iron rod to red hot
(d) Curdling of milk

Evaporation of petrol and heating of iron rod are physical changes since new substances are not being formed. Burning of LPG and curdling of milk are chemical changes as the original substances are changing to new substances.

What do you observe when an iron nail is kept in tap water for few days?

We observe reddish-brown coating on the nail which is called rust (an entirely new substance). It is a chemical change.

Give two examples for the following case:
Physical changes which are not reversible.

Physical changes which are not reversible:

i) Breaking of glass

ii) Cutting of tree

Classify burning of kerosene in a lantern as a physical or a chemical change. Give reasons.

When kerosene burns, it combines with atmospheric oxygen to form new products. Thus in a chemical change, new products are formed. Hence burning of kerosene is a chemical change.

Classify drying of a shirt in the sun as a physical or a chemical change. Give reasons.

Drying a shirt in the sun is a physical change because, during drying, water present in liquid form in the shirt is converted into vapor. No chemical reaction happens during the drying of the shirt under the sun.

Classify rising of hot air over a radiator as a physical or a chemical change. Give reasons.

Rising of hot air over a radiator is a physical change. Water in a radiator converts to vapours. Hot air becomes lighter and rises.

Classify change in the colour of black tea on adding lemon juice to it as a physical or a chemical change. Give reasons.

The colour of black tea changes when lemon juice is added. This is a chemical change. Lemon is a source of citric acid. ascorbic acid and malic acid. Black tea contains theaflavin antioxidants.

State whether the following is physical or chemical change:
Burning of wood.

Burning of wood is a chemical change because wood transforms into a different substance.

State whether the following is physical or chemical change:
Water cycle in nature.

The water cycle is a physical change, which means that the water that is cycled never chemically changes into another substance. It just cycles through its physical states(solid, liquid and gaseous).

Fill in the blanks:
An earthquake is a _____________ change.

Fill up is non-periodic.

A physical change is temporary. Explain.

A physical change is defined as a change in which no new substance is formed. A physical change is temporary because this change can be easily reversed by reversing the conditions.

For example, when ice is melted, it gets converted into water by absorbing heat. Water so formed can be easily converted to the ice by cooling, thus heat is released. This example shows that physical change is temporary.

State whether the following is physical or chemical change:
Burning of sugar

A change in which new substance is formed is known as chemical change.

Burning of sugar is a chemical change because on burning sugar it yields carbon dioxide and water.

Sublimation is classified as a physical change. Why?

Physical change are those in which no new substance is formed. In sublimation, there is direct conversion of solid into gas and vice versa. Thus the substance remains the same.

Give reasons:
Melting of solid (wax) is a physical change.

Melting of solid (wax) is a physical change because no new substance is formed during this change.

Classify the following as a physical or a chemical change:
Drying of wet clothes

Drying of wet clothes is a Physical change. because in this process there is no chemical change

Give reason :
Freezing of water to ice and evaporation of water are physical changes .

Freezing of water to ice and evaporation of water are physical changes because water can be brought back to its original (liquid) form by
1. We can heat the ice to bring it back to water.
2. We can cool down the vapours to bring it back to water.

What is rusting?

Iron objects often acquire a thin film of brownish substance on their outer coating. This substance is rust and the process of its formation is rusting. Chemically, when iron comes in contact with oxygen and water, a reaction takes place to form iron oxide (rust).

This can be represented by the equation:
$$Fe + O_{2} + H_{2}O \rightarrow Fe_{2}O_{3}$$

Fill in the blanks :
There is no change in the __________ of the substance during a physical change.

There is no change in the chemical composition of a substance during a physical change.

Give reason: Cutting of a cloth piece is a physical change, though it cannot be reversed.

Cutting of a cloth piece is a physical change because it does not change the chemical composition of the cloth. However, it is an irreversible change because the cloth cannot be put back in the original state.

Classify the following as a physical or a chemical change:
Manufacture of salt from sea water

Manufacture of salt from sea water is a Physical change.

How can you say that the process of digestion is a chemical change?

During digestion, new substances are formed which cannot be converted back to the same food which we have eaten. Besides energy is also released. These factors prove that digestion is a chemical change.

Name the phenomenon which causes conversion of Ice into water.

Conversion of ice into water is called melting.

Three experiments to study the process of rusting are given below. Observe the three test tubes and answer the following questions.
A. Why the nail in the test tube $$2$$ is not rusted?
B. Why is the nail in the test tube $$1$$ is rusted highly?
C. Would the nail in the test tube $$3$$ get rusted?

The essential requirement for corrosion are:

Supply of oxygen
Presence of water
Material itself

(A) The nail in the test tube $$2$$ is not rusted because in test tube $$2$$, we take boiling water. So, all the dissolved oxygen is removed from the water and if the iron nail does not get a supply of oxygen then corrosion is not carried out.

(B) The nail in the test tube $$1$$ is rusted highly because in the test tube $$1$$, the iron nail meets all the requirements which are essential for the process of corrosion. That is the supply of oxygen in dissolved form, the presence of water and material itself.

(C) The nail in the test tube $$3$$ is not rusted because calcium chloride is one of the best absorbents. So, it absorbs all the dissolved oxygen present in water. Hence corrosion process does not take place.

What will you take into account while identifying the following?
A physical change in a substance.

Physical changes are those changes in which only the forms of substances get modified; the chemical natures and compositions of the substances involved are not altered.
Characteristics:
$$\cdot$$ No new substance is formed during a physical change.
$$\cdot$$ Most physical changes can be reversed easily.
$$\cdot$$ The chemical composition of a substance undergoing physical change remains the same.

Fill in the blank:
A .......... change is a temporary change.

Temporary means anything which remains only for a certain period of time and it's same with physical change because it can be reversed again in its original form.

Hence $$\text{physical change }$$ is a temporary change.

When hydrogen burns in oxygen, water is formed; when electricity is passed through water, hydrogen and oxygen are given out. Name the type of chemical changes involved in the two cases.

Hydrogen gas burns in oxygen, to form water involved a combination of hydrogen and oxygen gas and this type of reaction is called a combination reaction.

$$H_{{2}{(g)}} + O_{{2}{(g)}}\longrightarrow H_2O_{{(liq)}}$$

When electricity is passed through water, hydrogen, and oxygen gases are given out, the chemical changes involved here are decomposition and the type of reaction is a decomposition reaction.

$$H_2O_{{(liq)}}\longrightarrow H_{{2}{(g)}} + O_{{2}{(g)}}$$

Complete the statement by filling the gap using the appropriate term.
Rusting of iron is a ..........change.

When a piece of iron is left in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting.

During rusting of iron, iron metal combines with air in the presence of moisture to form iron oxide(rust). Rust formed is a new substance.

Therefore, rusting of iron is a chemical change.

State four ways to prevent rusting.

Rusting can be prevented by:

1. Galvanizing - iron metal is coated with zinc.

2. Enameling - the iron surface is baked with a mixture of silicates at a high temperature.

3. Coating with red lead oxide paint or tar on the iron surface.

4. Oils and grease coating on iron surface cuts off moist air and prevents rusting.

Give a scientific explanation for following statement:

The aluminum article is used as an anode in the anodizing process.

The aluminum article is used as an anode in the anodizing process to deposit a thick layer of aluminum oxide, on the article to protect the article from further rusting.

What is rusting? Give the equation for the formation of rust.

Rusting is the slow oxidation of iron by atmospheric oxygen in the presence of water.

Equation:

$$Iron\ (Fe) + Oxygen\ (O_2) + Water\ (H_2O) \rightarrow Rust\ (iron\ oxide; Fe_2O_3)$$

How does the painting of an iron object prevent rusting?

By painting an iron object, iron does not come in contact with atmospheric oxygen and moisture directly. This prevents rusting.

What kind of change tearing of paper is?

Tearing of paper is a $$physical$$ change, since no new products are formed.

Conversion of wheat grains into wheat flour is a .................. change.

The conversion of wheat grains into wheat flour is a physical change because it doesn't change the chemical composition of wheat but only changes the size of the particles of wheat.

When carbon dioxide is passed in lime water, it turns milky. This is a .................change

This is a $$\textbf {chemical}$$ change, since it cannot be reversed.

Explain the chemical change and physical change with proper example.

$$\textbf {Physical Change}$$: Properties like size, shape, state (solid, liquid or gas), temperature, pressure etc., are physical properties. Those changes in which only physical properties of a substance changes are called physical changes. For example, lightening of bulb is a physical change.

$$\textbf {Chemical Change}$$: The change in which new substance is formed and chemical composition of a substance is changed is called a chemical change. A substance loses its identity in a chemical change and cannot be get back into its original form. For example, burning of candle, formation of curd etc.

What is crystallisation?

Crystallization is defined as a process by which a chemical is converted from a liquid solution into a solid crystalline state.. The crystallisation method is used to purify solids. Crystallisation is an example of physical change.

Write the chemical equation for reaction of oxygen with magnesium.

Magnesium burns in oxygen to give magnesium oxide.

$$2Mg+O_{2}\longrightarrow 2MgO$$

To make solution of sugar is a....................change.

To make solution of sugar is a $$\textbf {Physical}$$ change.

Physical changes are .................

Physical changes are temporary whereas chemical changes are permanent. So the correct option is temporary.

Which factors are responsible for rusting?

This process usually occurs when environmental changes affect the physical properties of metal. The four main factors of iron corrosion are the presence of impurities on the metal surface, variation in temperature around metal, inappropriate surface conditions, and o much exposure to metal in the atmosphere.

The iron pillar of Mehrauli, Delhi is known as magic pillar. What is the reason of its not being rusted?

In Delhi, near Qutub Minar in Mehrauh, an iron pillar is situated which is constructed by ironsmith around 400 BC. The pillar is protected even after centuries and it is not rusted. This shows that our ancestors had developed a technique to prevent rusting.
Researches shows that a layer of iron oxide is present on its surface. They had mixed phosphorous at the time of its formation. It had been painted by mixture of different substances and was heated strongly.

Explain crystallisation with the help of labelled diagram.

$$\textbf {Crystallization}$$: A process that separates a pure solid in the form of its crystals from a solution is called crystallization. No new substance is formed, only crystals of the substance are formed. Therefore, crystallization is an example of physical change. To obtain a big crystal, a small piece of alum is hanged in the solution of alum with the help of a thread. After some time, we observe that piece of alum becomes bigger in size. This is due to the fact that small pieces of alum get deposited on alum.

1723679_1820559_ans_7cc6b7b75f274dbda915e137190bcd72.png

Can we see the crystals of alum?

Yes, you can see small alum crystals at the bottom of the jar. If you see crystals starting to grow on the sides or bottom of your jar, carefully remove your crystal, pour the liquid into the clean jar, and put the crystal in the new jar.

Write the name of new substance formed when magnesium is burnt in air.

When magnesium is burnt in air, it reacts with oxygen to form magnesium oxide.

Write down four examples of physical and chemical changes in our daily life.

$$Examples \; of\; physical \;changes$$:
1. Making squash by mixing sugar in water.
2. Making of salt powder by grinding solid pieces of salt.
3. Wetting of clothes.

4. Freezing of water in the freezer of the refrigerator.

$$Examples \;of \;chemical \;change$$:
1. Burning of the candle.
2. Making of curd from milk.
3. Dissolution of lime in water for whitewashing.
4. Formation of ash by burning wood.

Changes in which physical properties of substances change are called ..................changes.

Those changes in which only physical properties of substances change are called $$\textbf {physical }$$changes. Also, such changes do not involve the formation of any new substances.

Explain the process of rusting and how could it be prevented?

$$\textbf {Rusting}$$ : If an iron piece is kept open for sometime, a brown layer is obtained on the surface of iron. This brown layer is rust and the process is called rusting. Rust is not iron but a new substance iron oxide is formed as a chemical reaction.

The process of rusting can be represented as:

$$\underset{Iron}{Fe} + \underset{oxygen}{O_{2}(in\; air)} + \underset{water}{H_{2}O} \longrightarrow \underset{Iron\,oxide}{Fe_{3}O_{3}}$$

For rusting of iron, air and moisture are two important factors. Rust gradually destroys iron. As iron is used in structure of ships, cars, trucks, bicycle etc and in the formation of bridges and big buildings. Therefore, to prevent rusting, it is necessary to protect it from air and moisture both.

A simple solution to this is painting, oiling, greasing etc. Iron can be prevented from rusting by applying a layer of Chromium or zinc. To protect iron and steel from rusting, a layer of zinc is applied on it. This process is known as $$\textbf {Galvanisation}$$. Stainless steel is obtained by mixing carbon, manganese, nickel and chromium with iron. It is hard and resistant to rust.

Potato and Apple when kept in open for sometime after cutting becomes brown. Which type of change is it?

When potatoes and apples are kept open after cutting, they become brown. It is because the starch in potatoes and the iron in apples undergo a chemical reaction in presence of oxygen. So it is a chemical change.

Explain the reason: Colour of apple changes when left open after cutting.

Colour of apple changes when left open after cutting: Iron is present in apple. Iron present in apple changes to iron oxide after reacting with oxygen present in air. As a result, the colour of surface which is cut is changed to brown.

How does rusting of iron occurs?

When an iron object is left in damp air for a considerable time, it gets covered with a reddish-brown flaky substance called rust. This is called rusting of iron.

It occurs in the presence of oxygen and water.

$$Iron(Fe) + Oxygen(O_2 ) + Water(H_2O) \rightarrow Rust(iron\ oxide \ Fe_2O_3)$$

Classify the following as chemical change or physical change:
Melting of butter in a pan.

Melting of butter in a pan is a physical change. In this change, new substance is not formed and the melted butter can change back to solid when it is cooled.

What is a physical change ? explain with an example.

The changes which occur without a change in composition and in the chemical nature of the substance are called physical changes.

For example, The change of water in ice is a physical change because chemically ice and liquid water both are the same.

These changes are mostly reversible in nature i.e. the original substance can be further obtained by reversing the reaction conditions.

Why crystallisation is called a physical change?

In the process of

crystallisationcrystallisation, no new substance is formed, only a salt is obtained in its crystal form.

For example, to obtain a big crystal, a small piece of alum is hanged in the solution of alum with the help of a thread. After some time, we observe that the piece of alum becomes bigger in size. This is due to the fact that small crystals of alum get deposited on the larger crystal of alum.

In this process, chemical composition of alum is not changed. Therefore, it is a physical change.

Similarly, when seawater is heated and cooled, salt crystallizes out of the mixture. Salt does not change into another chemical substance. It only separates in pure form. So, it is not a chemical change. It is a physical change.

Write examples of two physical and chemical changes seen by you.

Physical Change:

1. formation of ice

2. rain from clouds

Chemical change:

1. yellow stain of vegetable becomes red in contact with soap.
2. Mehndi becomes red after drying.

Three test tubes are taken and labelled as $$A, B$$ and $$C$$. In test tube $$A$$ iron nail is dipped in water. In test tube $$B$$ iron nail is dipped in mixture of water and oil and in test tube $$C$$ iron is added with dry calcium chloride.
Name the test tube in which the iron nail will rust and why?

The iron nail will rust in test tube $$A$$ because it provides the condition required for rusting, both moisture as well as air is present in it.
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Name the conditions necessary for rusting.

The presence of water and oxygen is essential for the rusting of iron.

Rusting of iron takes place in the presence of moisture and air. So the presence of oxygen and water vapour in air are two necessary conditions for rusting of iron.

Classify the following as chemical change or physical change:
(I) Boiling of water to form steam.

The boiling of water to form steam is a physical change because the vapors formed can be converted into the water on cooling.

What is physical change of matter? Give three examples.

When a substance undergoes a change in its physical form or physical property without undergoing any change in its chemical composition then it is termed as the physical change of matter.

Examples: chopping of wood, melting of ice, paper shredding, etc.

What kind of change is rusting of iron?

Rusting of iron is a chemical change since a new substance (rust) is formed from iron in the presence of air and moisture.

Classify the following as chemical change or physical change:
Making a fruit salad with raw fruits.

Making a fruit salad with raw fruits is a physical change because in this process chemical composition of fruits is not changed.

Classify the following as chemical change or physical change:
Rusting of the almirah.

Rusting of the almirah is a chemical change. During, rusting a chemical reaction takes place and, a new substance called iron oxide is formed.

Classify the following as chemical change or physical change:
Dissolving common salt in water.

Dissolving common salt in water is a physical change because the process can be reversed. If we boil the saltwater solution, the water will be evaporated, and the salt can be obtained. The evaporated water can be condensed to liquid form again. Thus dissolving common salt in water is a physical change in which just the state of compound changes and no chemical reactions occur.

Classify the following as chemical change or physical change:
Passing of electric current through water and the water breaking down into hydrogen and oxygen gases.

Passing of electric current through water and the water breaking down into hydrogen and oxygen gases is a chemical change because redox reaction takes place during the electrolysis of water.

Reaction:

$$\mathrm{2H_2O(l) \rightarrow 2H_2(g) + O_2(g)}$$

Why is the rusting of iron faster in saline water than in pure water?

Strong electrolytes such as sodium chloride are present in saline water. The ions produced from $$NaCl$$ help in the reduction of oxygen to form water. Hence, the rusting of iron is faster in saline water than in pure water.

Is photosynthesis a chemical change?

A change in which one or more new substances are formed is called a

chemical change.

Photosynthesis is a chemical change because, during photosynthesis, water and carbon dioxide combine to form glucose(carbohydrate) and oxygen in presence of chlorophyll and light. New substances like glucose and oxygen are being formed here hence it's a chemical change.

Classify the following as chemical change or physical change:
Burning of paper and wood.

Burning of paper and wood is a chemical change because on heating paper or wood, ash is obtained which has a different chemical composition than that of paper or wood and we can't obtain paper again from ash.

How we prepare magnesium hydroxide?

When magnesium ribbon burns.

When it is completely burnt it leaves behind powdery ash of magnesium oxide.

Magnesium ($$Mg$$) + Oxygen ($$O_2$$) $$\rightarrow$$ Magnesium oxide ($$MgO$$)

When we dissolve the ash, from the above reaction, in the water we get magnesium hydroxide.

Magnesium oxide + water $$\rightarrow$$ Magnesium hydroxide

$$MgO + H_{2}O \rightarrow Mg(OH)_{2}$$

Define physical change, give example.

A change in which a substance undergoes a change in its physical properties such as shape, size, color, and state is called a physical change.

A physical change is generally reversible, (some exceptions). In this change, no new substance is formed.

Example: An ice cube melting into water

What are the other changes occur during chemical reaction?

In addition to new products, the following may accompany a chemical change:

1. Heat, light or any other radiation may be given off or absorbed.

2. Sound may be produced.

3. A change in smell may take place or a new smell may be given off.

4. A colour change may take place.

5. A gas may be formed.

Some examples for the above statements

1. Burning of any substance is always accompanied by the production of heat and light.

2. Explosion of fireworks produces heat, light and sound. During this, the unpleasant gases will produce and pollute the atmosphere. (It is not advisable, so it is avoided).

3. The stale food gives a bad odour (smell)

4. The cut fruits and vegetables change their colour after some time.

Fill in the blanks in the following statements:
Changes in which only _________ properties of a substance change are called physical change.

Changes in which only physical properties of a substance change are called physical change.

Define Crystallisation.

The process of obtaining large crystals of pure substances from their solutions is called Crystallisation.

Give four reasons why burning of a magnesium ribbon in air is considered a chemical change.

Burning of a magnesium ribbon in air is considered a chemical change because:
i. It is an irreversible change.
ii. New product is formed with different chemical properties i.e MgO.
iii. Composition of magnesium changes.
iv. Heat energy is required for this reaction to occur.

Physical And Chemical Changes - Class 7 Chemistry - Extra Questions