Questions are taken verbatim from the NCERT textbook; answers were grounded against the chapter's content during generation. Items needing review are marked.
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1Exercises12 questions
Q.1Classify 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 foodv
SolutionA chemical change forms new substances, while a physical change changes only physical properties such as shape, size or state.
Answer:(a) Chemical change
(b) Physical change
(c) Chemical change
(d) Physical change
(e) Physical change
(f) Chemical change
Q.2State 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.
(b) Formation of manure from leaves is a physical change.
(c) Iron pipes coated with zinc do not get rusted easily.
(d) Iron and rust are the same substances.
(e) Condensation of steam is not a chemical change.v
SolutionCutting and condensation do not form new substances. Manure formation and rusting form new substances. Zinc coating helps prevent rusting.
Answer:(a) False; cutting wood is a physical change.
(b) False; formation of manure is a chemical change.
(c) True.
(d) False; iron and rust are different substances.
(e) True.
Q.3Fill 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.v
SolutionCarbon dioxide reacts with lime water to form insoluble calcium carbonate. Baking soda is sodium hydrogen carbonate. Painting and galvanisation keep air and moisture away from iron.
Answer:(a) calcium carbonate
(b) sodium hydrogen carbonate
(c) painting; galvanisation
(d) physical
(e) chemical
Q.4When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.v
Answer:It is a chemical change because a new gas, carbon dioxide, is formed. The bubbles are due to the release of this gas when baking soda reacts with the acid in lemon juice.
Q.5When 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.v
Answer:In burning a candle, melting of wax is a physical change because only the state changes. Burning of wax vapour is a chemical change because new substances such as carbon dioxide and water vapour are formed. Another example is burning of LPG: the gas changes flow and mixes physically, and then chemically burns to form new substances.
Q.6How would you show that setting of curd is a chemical change?v
Answer:Setting of curd is a chemical change because milk changes into a new substance, curd, with a different taste and properties. The curd cannot be changed back into milk by a simple physical process.
Q.7Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.v
Answer:Burning of wood is a chemical change because it produces new substances such as ash, carbon dioxide and water vapour. Cutting wood into small pieces is a physical change because only the size and shape change; no new substance is formed.
Q.8Describe how crystals of copper sulphate are prepared.v
Answer:Take water in a beaker, add a few drops of dilute sulphuric acid and heat it. Dissolve copper sulphate powder in the hot water until no more dissolves. Filter the solution and allow it to cool undisturbed. Blue crystals of copper sulphate form on cooling.
Q.9Explain how painting of an iron gate prevents it from rusting.v
Answer:Rusting needs oxygen and water. Paint forms a protective layer on the iron surface and prevents air and moisture from coming in contact with iron, so rusting is slowed or prevented.
Q.10Explain why rusting of iron objects is faster in coastal areas than in deserts.v
Answer:Coastal air contains more moisture and salts, and rusting requires moisture and oxygen. Desert air is much drier, so iron objects rust more slowly there than in coastal areas.
Q.11The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exists 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.v
SolutionChange A is a physical change because LPG changes state from liquid to gas. Change B is burning, which forms new substances, so it is a chemical change.
Answer:(ii) Process B is a chemical change.
Q.12Anaerobic 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.v
SolutionDigestion of animal waste by bacteria produces new substances including biogas, so Change A is chemical. Burning biogas forms new substances and releases heat, so Change B is also chemical.
Answer:(iii) Both processes A and B are chemical changes.