NCERT Solutions for Class 7 Science Chapter 6 Physical and Chemical Changes

NCERT Solutions for CBSE Class 7 Science Chapter 6 Physical and Chemical Changes are given below. These solutions help students to clear their doubts and to obtain good marks in final exam. Class 7 Science NCERT questions and answers provided in this article are strictly based on the CBSE syllabus and curriculum.

Class 7 Science Chapter 6 Physical and Chemical Changes NCERT Solutions

Class 7 NCERT Solutions for Science Chapter 6 includes all the intext and exercise questions. All these questions are solved by experts with a detailed explanation that will help students complete their assignments and homework.

EXERCISES

Question 1: 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: (a) Photosynthesis → Chemical change

(b) Dissolving sugar in water → Physical change

(d) Melting of wax → Physical change

(e) Beating aluminium to make aluminium foil → Physical change

(f) Digestion of food → Chemical 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)

(b) Formation of manure from leaves is a physical change. (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
Cutting a log of wood into pieces is a physical change.

(b) False
Formation of manure from leaves is a chemical change.

(d) False
Iron and rust are different substances. Rust is iron oxide.

(e) True

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 _________.

(b) The chemical name of baking soda is _________.

(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: (a) 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 Sodium hydrogen carbonate.

(d) Changes in which only physical properties of a substance change are called physical changes.

(e) 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: When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a carbon dioxide gas. This is a chemical change.

Lemon juice + Baking soda → Carbon dioxide + other substances

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: Physical Changes: On heating, candle wax gets melted is a physical change. Since it again turns into solid wax on cooling.

Chemical Changes: The wax near to flame burns and gives new substances like Carbon Dioxide, Carbon soot, water vapour, heat and light.

Cooking of food, is another example where both physical and chemical changes occurs.

Question 6: How would you show that setting of curd is a chemical change?

Answer: 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.

Question 7: Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Answer: Burning of wood is a chemical change because wood on burning converted to new substances like ash (carbon), carbon dioxide gas, water vapour, heat and light. The change is irreversible.

While cutting of wood in smaller pieces is a physical change because the original composition of wood does not change. No new substances are formed.

Question 8: Describe how crystals of copper sulphate are prepared.

Answer: Crystals of copper sulphate are prepared using the crystallization method, which is described as follows

Take a cupful of water in a beaker.
Add a few drops of dilute sulphuric acid to this.
Heat the water and when it starts boiling, add copper sulphate powder while still stirring.
Add the copper sulphate powder till the solution becomes saturated. Filter into a china dish and allow it to cool.
The solution should be kept undisturbed. Slowly, the crystals of copper sulphate separate out.

Question 9: Explain how painting of an iron gate prevents it from rusting.

Answer: For rusting, the presence of both oxygen and water (or water vapour) is required. The coat of the paint prevents direct contact of iron with air and oxygen and thus prevents it from rusting.

Question 10: Explain why rusting of iron objects is faster in coastal areas than in deserts.

Answer: For rusting, the presence of both oxygen and moisture (water vapour) is required. In coastal areas the quantity of moisture content in air is high (highly humid) as compared to desert regions (low humid), so the rusting becomes faster in coastal areas.

Question 11: The gas we use in the kitchen is called liquified 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.

Question 12: 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.