Last updated on August 30th, 2023 at 01:00 am
NCERT Solutions Class 7 Science Chapter 6 Physical and Chemical Changes
Class 7 Science Chapter 6 Physical and Chemical Changes for cbse/icse students.
Q1 :Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f) Digestion of food
Answer :
- Chemical change
- Physical change
- Chemical change
- Physical change
- Physical change
- Chemical change
Q2 :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)
(b) Formation of manure from leaves is a physical change. (True/ False)
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
(d) Iron and rust are the same substances. (True/False)
(e) Condensation of steam is not a chemical change. (True/False)
Answer :
(a) False [ it’s a physical change]
(b) False [ chemical change]
(c) True
(d) False
(e) True
Q3 :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 _________.
(b) The chemical name of baking soda is _________.
(c) Two methods by which rusting of iron can be prevented are _________and ____ ____.
(d) Changes in which only _____ __ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called _________changes.
Answer :
- calcium carbonate (CaCO3)
- sodium hydrogen carbonate
- galvanization and painting
- physical
- chemical changes.
Q4 :When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Baking soda (Sodium bicarbonate) forms bubbles when mixed with lemon juice (citric acid). Bubbles are formed by the evolution of carbon dioxide gas. It is a chemical change.
As a result of the chemical reaction, the citric acid contained in lemon juice reacts with baking soda or sodium bicarbonate) resulting in the evolution of carbon dioxide gas.
Q5 :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.
Both physical and chemical changes occur during the burning of a candle.
Changes in physical properties → Melting of wax
Changes in chemical composition → Burning of wax
Q6 :How would you show that setting of curd is a chemical change?
It is impossible to obtain milk from curd after it is formed. The properties of milk and curd also differ. This indicates that setting of curd is a chemical change.
Q7 :Explain why burning of wood and cutting it into small piece are considered as two different types of changes.
Coal is formed when wood is burned. As a result, it is a chemical change. When we cut wood, we only change its shape and size. There are no new substances created. Thus, it is a physical change.
Q8 :Describe how crystals of copper sulphate are prepared.
Crystals of copper sulphate are prepared by the method of crystallization.
Copper sulphate crystals are prepared as follows:
- In a beaker, water is taken.
- Add a few drops of diluted sulphuric acid.
- As soon as it starts boiling, add copper sulphate powder slowly while stirring continuously.
- Copper sulfate powder is added to saturation level.
- Filter and cool the solution.
- Let it remain undisturbed for a few hours, then we’ll observe copper sulphate crystals.
Q9 :Explain how painting of an iron gate prevents it from rusting.
During rusting, both moisture (water) and air (oxygen) contribute to the process. Painting an iron gate prevents it from coming into contact with the air and moisture present in the atmosphere. Thus, rusting is prevented.
Q10 :Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt 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.
