Physical and Chemical Changes
Science Class Seven
NCERT Exercise Solution
Question (1) Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
Answer : Chemical change.
Explanation
In the process of photosynthesis, glucose and oxygen are formed. Since in photosynthesis new substances are formed, hence it is a chemical change.
(b) Dissolving sugar in water
Answer : Physical change.
Explanation
Since in the dissolving of sugar in water, no new substance is formed, and we can get the sugar back from its solution in water. This means it is a reversible change. Hence it is a physical change.
(c) Burning of coal
Answer : Chemical change.
Explanation
In the burning of coal, heat, gas and smoke are formed. Thus it is a chemical change, as other burnings are.
(d) Melting of wax
Answer : Physical change.
Explanation
Since, in the melting of wax, only the physical state of wax changes from solid to liquid. Thus, the melting of wax is a physical change.
(e) Beating aluminium to make aluminium foil
Answer : Physical change.
Explanation
By beating aluminium to make the aluminium foil, only the shape of the aluminium changes and no new substance is formed. Hence it is a physical change.
Question (2) State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
(a) Cutting a log of wood into pieces is a chemical change. (True/False)
Answer False
The Correct Statement Cutting a log of wood into pieces is a physical change.
Explanation
Because by cutting a log of wood, no new substance is formed, hence it is a physical change.
(b) Formation of manure from leaves is a physical change. (True/False)
Answer False
The Correct Statement Formation of manure from leaves is a chemical change.
Explanation
In the formation of manure from leaves, new substances are formed. This happens because of the decaying of leaves by the action of bacteria over them. A chemical reaction takes place in the formation of manure from leaves. Thus, it is a chemical change.
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
Answer True
Explanation
For rusting, water and/or oxygen should be in contact with iron objects. The zinc coating over the iron pipe prevents water and oxygen to come in contact with pipes. So, such pipes are not get rusted easily.
The process of zinc coating over iron pipes is called Galvanisation.
(d) Iron and rust are the same substance. (True/False)
Answer False
The Correct Statement iron and rust are different substances.
Explanation
Rust is iron oxide and not iron. Hence both are different substances.
(e) Condensation of steam is not a chemical change. (True/False)
Answer True
Explanation
By condensation, steam gets turned into water. This is only the change of state of steam from a gas into a liquid. Thus, it is not a chemical change, rather it is a physical change.
Question (3) Fill in the blanks in the following statements:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of _____.
Answer When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.
(b) The chemical name of baking soda is ______.
Answer The chemical name of baking soda is sodium hydrogen carbonate or sodium bicarbonate.
(c) Two methods by which rusting of iron can be prevented are ____ and _______
Answer Two methods by which rusting of iron can be prevented are painting and galvanisation.
(d) Changes in which only ______ properties of a substance changed are called physical changes.
Answer Changes in which only physical properties of a substance changed are called physical changes.
(e) Changes in which new substances are formed are called _______ changes.
Answer Changes in which new substances are formed are called chemical changes.
Question (4) When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Answer This is a chemical change.
When baking soda is mixed with lemon juice, carbon dioxide gas is formed. In this change, a new substance is formed. It is a chemical reaction, hence it is a chemical change.
Question (5) 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.
Answer
Two changes take place in the burning of candles, one is melting of wax and another is burning of wax.
The melting of wax is a physical change. And the burning of wax is a chemical change.
The burning of wood is another similar example, in which chemical and physical change both take place.
The burning of coal is another similar example, in which chemical and physical change both take place.
In the burning of coal, the coal starts burning after the vaporisation of water present in it.
In this vaporisation of water is a physical change and burning of coal is a chemical change.
Question (6) How would you show that setting of curd is a chemical change?
Answer
After adding a small amount of curd, the lukewarm milk is left for some hours, the whole milk turns into curd. In this process, we get a new product curd which is completely different from milk.
The setting of curd is an irreversible process, as we can not get the milk back from curd.
Since, in the setting of curd, a new product is formed and it is an irreversible process, so, this process is a chemical change.
Question (7) Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer
In the burning of wood, we get heat, vapour, light, ash, and carbon dioxide. As we get new products by burning wood, hence it is a chemical change.
In the cutting of wood in small pieces, only the shape of the wood is changed and no new product is formed. As in cutting of wood in small pieces only shape, i.e. only physical property changes, so it is a physical change.
Question (8) Describe how crystals of copper sulphate are prepared.
Answer
The crystal of copper sulphate is prepared from its concentrated solution after cooling it.
Steps of preparing the crystal of copper sulphate
(a) Take a beaker or a glass.
(b) Fill about half of the beaker with water.
(c) Add a few drops of dilute sulphuric acid to it.
(d) Start heating the water.
(e) When water starts boiling, add some copper sulphate slowly to it with stirring.
(f) Keep adding copper sulphate with stirring until no more copper sulphate powder can be dissolved.
(g) Now, filter the solution and leave it to cool.
(h) Do not disturb the solution while cooling.
(i) After cooling, the crystals of copper sulphate is obtained.
Question (9) Explain how painting of an iron gate prevents it from rusting.
Answer
For rusting, oxygen and moisture must be in contact with iron objects for a long time. In painting, a coat of layers is applied over an object. A layer of paint over iron objects prevents them to come in contact with moisture and oxygen present in the air. And this prevents iron objects from rusting.
Thus, the painting of an iron gate prevents it from rusting.
Question (10) Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer
Water and oxygen are necessary for rusting. Without coming in contact for a long time, iron does not get rusted.
Because more humidity is present in the coastal area compared to a desert, increases the rate of rusting. This is the cause that rusting of iron is faster in coastal areas than in deserts.
Question (11) the gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder it exist as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer (ii) Process – B is a chemical change.
Explanation
The change of liquid petroleum gas into gas is a physical change, and the burning of gas is a chemical change. Thus, option (ii) "Process – B is a chemical change" is the correct answer.
Question (12) Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burn as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer (iii) Both processes A and B are chemical changes.
Explanation
The production of biogas after digestion of leaves by bacteria and burning of biogas, both are chemical changes. Thus, option (iii) "Both processes A and B are chemical changes" is the correct answer.
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