Changes Around Us - Class 6 Chemistry - Extra Questions

Folding paper to make an aeroplane is a reversible change. Why?

The aeroplane made by folding paper can be unfolded to get back the paper in its original form. The change made to the paper, by folding, can be undone. Hence it's a reversible change.


A bag of cement lying in the open gets wet due to rain during the night. The next day the sun shines brightly. Do you think the changes, which have occurred in the cement, could be reversed?

Water combines with cement to form a paste that binds it together. This paste is altogether a new substance and we can not get back cement and water from the mixture. Therefore, it is an irreversible chemical change.


Give two examples of irreversible changes.

$$a)$$ Milk to cheese
$$b)$$ Cooking of food 
These changes are irreversible.


A drawing sheet changes when you draw a picture on it. Can you reverse this change?

If the drawing was done using a soft pencil and can be erased thoroughly, we can reverse the change. But, if the drawing was done using a pen, the change cannot be reversed.


Explain how a metal rim slightly smaller than a wooden wheel can be fixed on it.

The metal rim is always made slightly smaller than the wooden wheel. The metal rim is heated. On heating, the rim expands and fits onto the wheel. Coldwater is then poured over the rim. Due to cooling the metal rim contracts and fits tightly onto the wheel.
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In places of hot climate, it is advised that the outer walls of houses be painted white. Explain.

In places of hot climate, it is advised that the outer walls of houses be painted white because white is a light color and because of that it reflects most of the heat that falls on it and a very less amount of heat is absorbed by the walls.
Therefore, a light color tends to keep the house cool.


Can you say deforestation is an irreversible or reversible change?

Deforestation is not a reversible change.it is an irreversible change. It can not take place in the reverse direction. because once we cut down a plant the plant's life and some animal's habitat is lost permanently, which cannot be retained


Give examples to explain the difference between changes that can or cannot be reversed.

The changes which can be reversed are called reversible changes.
The changes which cannot be reversed are called irreversible changes.

Examples of reversible and irreversible changes.

Reversible changes
Irreversible changes
1. Glowing of an electric bulb. (It glows when switched on and becomes dark when switched off.)
1. Burning of paper or wood. (It gives smoke and ash, which cannot form paper or wood again.)
2. Distillation of liquid.
$$Liquid \overset{evaporation}{\underset{condensation}{\rightleftharpoons}}Vapour$$
2. Rusting of iron. (Rust cannot be changed into iron again.)
3. Sublimation
$$solid \overset{heat}{\underset{cool}{\rightleftharpoons}}vapor$$
3. Making of curd from milk.
4. Collapsing of mimosa (touch me not) leaves on touching and opening up on removing the finger.
4. Growth of plants and animals.

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Some common changes are given in the table. Which of these changes do you think can be reversed?
Change
Can be reversed
(i) Raw egg to boiled egg
Yes/No
(ii) Batter to idli

(iii) Wet clothes to dry clothes

(iv) Woolen yarn to a knitted sweater

(v) Grain to its flour

(vi) Cold milk to hot milk

(vii) Straight string to a coiled string

(viii) Bud to flower

(ix) Milk to paneer

(x) Cow dung to biogas

(xi) Stretched rubber band to its normal size

(xii) Melting of ice-cream

$$\underline{Change}$$
$$\underline{Can\ be\ reversed}$$
(i) Raw egg to boiled egg
No
(ii) Batter to idli
No
(iii) Wet clothes to dry clothes
Yes
(iv) Woolen yarn to knitted sweater
Yes
(v) Grain to its flour
No
(vi) Cold milk to hot milk
Yes
(vii) Straight string to a coiled string
Yes
(viii) Bud to flower
No
(ix) Milk to paneer
No
(x) Cow dung to biogas
No
(xi) Stretched rubber band to its normal size
Yes
(xii) Melting of ice-cream
Yes


Give two examples of reversible changes.

1. Melting of ice
2. Folding a paper


How does curd made? Is this change reversible?

A small quantity of curd is added to warm milk. The milk is stirred and is set aside undisturbed for a few hours at a warm place. In a few hours, the milk changes into curd.
Curd formed from milk can not be changed into milk again. So, this is an irreversible (cannot be reversed) change.


Give one example of a change which occurs on heating but cannot be reversed.

Baking of bread is an irreversible change. It occurs as a result of heating and we cannot change the bread back to the dough.


A change in which the substance can be brought back to its original state is called a _____________ change.

A change in which the substance can be brought back to its original state is called a reversible change.


Are all the changes taking place in substances permanent, temporary or reversible? 

No. There are other kinds of changes as well. For example, there are irreversible changes, periodic changes, physical and chemical changes.


A piece of iron is heated until it becomes red-hot. It then becomes soft and is beaten to a desired shape. What kinds of changes are observed in this process reversible or irreversible?

The change occur in the above process is a reversible Change
$$\text{Explanation- }$$
$$\bullet$$ This process is reversible as if a piece of iron is heated till red-hot, desired shape can be obtained. Similarly, original piece of iron can be produced back from heating the product.

$$\text{Additional information:}$$ Reversible process are temporary and initial product can be obtained again by applying suitable condition.


Give reasons:
Dissolving salt in water is a reversible change.

Dissolving salt in water is a reversible change because salt and water can be again obtained by the distillation process.


Name any two changes under which the stretching of spring can be classified as?

The stretching of a spring can be classified as a physical change as there is no change in the chemical composition of the spring. Also, it's a reversible change because when the applied force is removed, the spring comes back to its original shape.


The substances cannot get back into their original form in a .................. change.

Chemical changes always result in forming new substances with different molecules. It is not easy to get back the original substances. Hence, the chemical changes are permanent and irreversible.
So, the substances cannot get back into their original form in a chemical change.


Conversion of liquid to vapors is a ...............change.

Conversion of liquid to vapour is a $$\textbf{physical change.}$$


Conversion of water into vapour, which type of change is this? Temporary or permanent?

Conversion of water into vapor is a $$temporary\ change$$. The vapour can change back into liquid water when it cools.


Define temporary change.

The conversion of a substance from one state to another is called $$\textbf {temporary change}$$.
Example: Formation of water vapor.


Explain reversible and irreversible changes giving examples.

  • Those changes which are possible in reversible direction also are called $$\textbf {reversible changes.}$$ Reversible changes are not permanent changes.
Examples: conversion of water into ice and vice-versa.
  • Those changes which are possible in forward direction only are called $$\textbf {irreversible changes.}$$ These changes are permanent.
For example: setting of curd, digestion of food etc.


Class 6 Chemistry Extra Questions