Class 7 NCERT Solutions Science Chapter 6 - Physical and Chemical Changes
Physical and Chemical Changes Exercise 65
(b) Dissolving sugar in water Physical change
(c) Burning of coal Chemical change
(d) Melting of wax Physical change
(e) Beating aluminium to make aluminium foil Physical change
(f) Digestion of food Chemical change
Concept insight: In physical change, no new substance is formed and in a chemical change, new substances are formed.
Concept insight: Cutting a log of wood into pieces is a physical change.
Concept insight: Formation of manure from leaves is a chemical change.
Concept insight: because zinc forms zinc oxide and prevents iron from coming in contact with air.
Concept insight: Iron and rust are different substances. Rust is iron oxide (Fe2O3)
Concept insight: In condensation, water vapors changes into water.
Physical and Chemical Changes Exercise 66
(b) Sodium hydrogen carbonate.
(c) Galvanization and painting
When baking soda (Sodium hydrogen carbonate) is mixed with lemon juice (citric acid), bubbles are formed. The bubbles are formed due to the evolution of carbon dioxide gas. This is a chemical change involving the formation of new substances.
Lemon juice + Baking soda Carbon dioxide + other substances
Concept insight: It is an example of chemical change.
Physical change Melting of wax
Chemical change Burning of wax
Eating of food is another example where both physical and chemical changes occur simultaneously.
Physical change Breaking down of larger food particles into smaller particles
Chemical change Digestion of food
Once the curd is formed, milk cannot be re-obtained from it. Also, both milk and curd have different properties. Since these are the properties of a chemical change, setting of curd is a chemical change.
Concept insight: When milk is converted into curd, lactose sugar is converted into lactic acid which is a new substance.
When we burn wood, a new substance, coal is formed. Therefore, it is a chemical change. However, when we cut wood, only the shape and size of the wood are changed. No new substance is formed. Therefore, it is a physical change.
Concept insight: In cutting of wood, only physical appearance of the wood changes while on burning, new substance is produced.
i.Take a cupful of water in a beaker.
ii.Add few drops of dilute sulphuric acid to this.
iii.The water is then heated and when it starts boiling, copper sulphate powder is added with stirring.
iv.Copper sulphate powder should be added on till the solution becomes saturated. It is then filtered into a china dish and allowed to cool.
v..The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.
Concept insight:Crystallisation is a technique to obtain pure substance.
Concept insight: Important question.
Concept insight: For rusting, both moisture and oxygen are required.