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CHAPTER 6 PHYSICAL AND CHEMICAL CHANGES BOOK ..., Exams of Painting

Correct statement: Iron and rust are two different substances. Rust is the oxide of iron. (e) Condensation of steam is not a chemical change - True.

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Class Notes
Class: VII
Topic: CHAPTER 6
PHYSICAL AND CHEMICAL CHANGES
BOOK EXERCISES AND EXTRA
QUES/ANSWERS
Subject: SCIENCE
TO BE WRITTEN IN SCIENCE NOTES COPY
Q1.Classify the changes involved in the following processes as physical or chemical
changes:
(a) Photosynthesis - Chemical change
(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
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 - False
Correct statement: Cutting a log of wood into pieces is an irreversible physical
change.
(b) Formation of manure from leaves is a physical change False
Correct statement: Formation of manure from leaves is a chemical change.
(c) Iron pipes coated with zinc do not get rusted easily True
(d) Iron and rust are the same substances - False
Correct statement: Iron and rust are two different substances. Rust is the oxide of
iron.
(e) Condensation of steam is not a chemical change - 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 calcium carbonate.
(b) The chemical name of baking soda is sodium hydrogen carbonate.
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Class Notes Class: VII Topic: CHAPTER 6 PHYSICAL AND CHEMICAL CHANGES BOOK EXERCISES AND EXTRA QUES/ANSWERS

Subject: SCIENCE

TO BE WRITTEN IN SCIENCE NOTES COPY

Q1.Classify the changes involved in the following processes as physical or chemical changes: (a) Photosynthesis - Chemical change (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

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 - False Correct statement: Cutting a log of wood into pieces is an irreversible physical change.

(b) Formation of manure from leaves is a physical change – False Correct statement: Formation of manure from leaves is a chemical change.

(c) Iron pipes coated with zinc do not get rusted easily – True

(d) Iron and rust are the same substances - False Correct statement: Iron and rust are two different substances. Rust is the oxide of iron.

(e) Condensation of steam is not a chemical change - 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 calcium carbonate. (b) The chemical name of baking soda is sodium hydrogen carbonate.

(c) Two methods by which rusting of iron can be prevented are painting and galvanization. (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.

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. Ans- (i) When baking soda (sodium bicarbonate) is mixed with lemon juice (citric acid), a chemical change takes place. (ii) In this reaction new substances like carbon dioxide is formed and heat is evolved. This change is chemical and irreversible. Lemon juice (citric acid) + Baking soda (sodium bicarbonate) Sodium citrate + carbon dioxide + water + heat

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. Ans- Physical change – The melting of wax is a physical change because on cooling the melted wax becomes solid again. Chemical change – The burning of wax is a chemical change because new substances like carbon dioxide, soot and water vapour are formed.

Cooking of food is another example in which both chemical and physical change takes place. Raw vegetables getting cooked is a chemical change and changing of water of the vegetables into steam is a physical change.

Q6. How would you show that setting of curd is a chemical change? Ans- Setting of curd is a chemical change because we cannot get the original substance (milk) back. The new substance formed is curd which is different from milk in taste, smell and chemical properties.

Reason : Bacteria acts on waste and convert it to biogas (change A). Hence, it is a chemical change. During biogas production it works as a fuel (change B) and produces CO 2 and heat. Hence, both processes A and B are chemical changes.

CHAPTER 6-PHYSICAL AND CHEMICAL CHANGES-EXTRA QUESTION ANSWERS

Q1. Why a chemical change cannot be normally reversed? Ans: In a chemical change the products formed are different with different chemical properties from the reactants. So a chemical change cannot be normally reversed.

Q2. What happens when an iron blade of knife is dipped in copper sulphate solution? What kind of change takes place? Ans: When an iron blade of a knife is dipped in copper sulphate solution, iron blade is coated with reddish brown deposits of copper and blue colour of copper sulphate solution changes to light green due to the formation of iron sulphate. So, it is a chemical change. (Draw figure 6.4 from the book)

Q3. Is cloud formation a physical change or a chemical change? Explain. Ans: Formation of clouds is a physical change. Reason: Clouds are formed by the condensation of water vapour(change in state) present in the atmosphere. This water goes back on the earth in the form of rain. No new product is formed. Therefore, it is a physical change.

Q4. Explosion of a cracker is a chemical change. Explain. Ans: When we burn a cracker it explodes. Explosion produces heat, light, sound and unpleasant gases that pollute the atmosphere. As new substances are formed in explosion of the cracker, so it is considered as a chemical change.

Q5. What is galvanisation? Ans: The process of depositing a layer of zinc on iron objects is called as galvanisation.

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