1. What among the following changes physically?
(a) Rusting of iron
(b) Combustion of magnesium ribbon
(c) Burning of candle
(d) Melting of wax
1. What among the following changes physically?
(a) Rusting of iron
(b) Combustion of magnesium ribbon
(c) Burning of candle
(d) Melting of wax
Explanation:
The physical change is a change in which the identity of the substance remains the same. Out of the given options, the melting of wax is a physical change because the identity of the wax remains the same even after melting. The other options involve a chemical change in which the identity of the substance changes. For example, rusting of iron involves the chemical reaction of iron with oxygen, which forms a new compound, i.e., rust. Similarly, combustion of magnesium ribbon and burning of a candle involve chemical reactions.
2. What among the following changes chemically? (a) Twinkling of stars
(b) Cooking of vegetables
(c) Cutting of fruits
(d) Boiling of water
2. What among the following changes chemically? (a) Twinkling of stars
(b) Cooking of vegetables
(c) Cutting of fruits
(d) Boiling of water
Explanation:
Cooking of vegetables is correct answer.
A chemical change is a process in which one or more substances are converted into new substances that have different chemical properties. During a chemical change, the chemical composition and chemical properties of the substances involved in the reaction are altered, resulting in the formation of a new substance or substances. In other words, chemical change involves the breaking of chemical bonds in the reactants and the formation of new chemical bonds in the products. Examples of chemical changes include combustion, rusting, cooking, and digestion.
3. A chemical change may occur when:
(a) change in colour only
(b) change in temperature only
(c) evolution of gas only
(d) any or all of the above
3. A chemical change may occur when:
(a) change in colour only
(b) change in temperature only
(c) evolution of gas only
(d) any or all of the above
Explanation:
A chemical change may involve any or all of the changes mentioned in the options - change in colour, change in temperature, and evolution of gas. In addition, it may also involve the formation of a precipitate, emission of light, or production of sound. These changes occur due to the breaking and formation of new chemical bonds between the reactant molecules, leading to the formation of new products with different chemical properties than the reactants.
4. When milk turns into curd, which of the following statement(s) is/are true?
i. It transitions from a liquid to a semi-solid condition.
ii. Its colour changes.
iii. It alters the flavour.
iv. The modification is irreversible. Choose the right response from the list below:
A) Both I and (ii) are true.
(b) Is true for (ii) and (iii)
(C) I, III, and IV are accurate
All of (d) I to (iv) are true.
4. When milk turns into curd, which of the following statement(s) is/are true?
i. It transitions from a liquid to a semi-solid condition.
ii. Its colour changes.
iii. It alters the flavour.
iv. The modification is irreversible. Choose the right response from the list below:
A) Both I and (ii) are true.
(b) Is true for (ii) and (iii)
(C) I, III, and IV are accurate
All of (d) I to (iv) are true.
Explanation:
The correct option is (d) (i) to (iv) are correct.
When milk changes into curd, it undergoes a chemical change known as curdling. The milk proteins coagulate and form a semi-solid mass, which changes the state of milk from liquid to semi-solid. The change in pH during the curdling process also causes a change in colour and taste of the milk. This change is irreversible, as it involves the formation of new chemical compounds (curd) that cannot be converted back into milk by simply reversing the process.
5. To 1. stop it from rusting, a man painted his iron main gate.
2. shield it from the sunlight.
3. Make it appear lovely.
4. Remove all dust.
Which of the aforementioned statements is true?
only (ii); (B) only (ii); (C) only (iii); (D) only I and (iii)
5. To 1. stop it from rusting, a man painted his iron main gate.
2. shield it from the sunlight.
3. Make it appear lovely.
4. Remove all dust.
Which of the aforementioned statements is true?
only (ii); (B) only (ii); (C) only (iii); (D) only I and (iii)
Explanation:
The correct option is (a) (i) and (ii). The man painted his main gate made up of iron to prevent it from rusting, which is a chemical change caused by the reaction of iron with oxygen in the presence of moisture. He also painted it to protect it from the sun, which can cause the gate to fade and become discolored due to prolonged exposure to sunlight.
6. The following are some notable facts about an iron pillar in Delhi near the Qutub Minar. Which of these things best explains its extended stability?
(a) It is taller than 7 metres.
(b) It is around 6000 kg in weight.
(c) It has been standing for more than 1600 years.
(d) Despite the length of time, it has not corroded.
6. The following are some notable facts about an iron pillar in Delhi near the Qutub Minar. Which of these things best explains its extended stability?
(a) It is taller than 7 metres.
(b) It is around 6000 kg in weight.
(c) It has been standing for more than 1600 years.
(d) Despite the length of time, it has not corroded.
Explanation:
The fact responsible for the long stability of the Iron pillar near the Qutub Minar in Delhi is that it has not rusted after such a long period of over 1600 years.
7. Which of the following is subject to the process of galvanization to stop it from rusting?
(a) Iron
(b) Zinc
(c) Aluminium
(d) Copper
7. Which of the following is subject to the process of galvanization to stop it from rusting?
(a) Iron
(b) Zinc
(c) Aluminium
(d) Copper
Explanation:
Galvanisation is a process used to prevent the rusting of iron. In this process, a layer of zinc is coated on the surface of iron. This layer of zinc acts as a barrier and prevents the contact of iron with air and moisture, thereby preventing rusting.
8. Sugar was dissolved in hot water to create a concentrated sugar syrup by Paheli's mother. Sugar crystals split as it cooled. This suggests a -
(A) a reversible physical alteration.
(B) the chemical transformation that is reversible.
(c) a permanent physical modification.
(d) the irreversible chemical alteration.
8. Sugar was dissolved in hot water to create a concentrated sugar syrup by Paheli's mother. Sugar crystals split as it cooled. This suggests a -
(A) a reversible physical alteration.
(B) the chemical transformation that is reversible.
(c) a permanent physical modification.
(d) the irreversible chemical alteration.
Explanation:
The separation of sugar crystals after cooling indicates a physical change that cannot be reversed. Therefore, option (c) is the correct answer.
9. Which of the following statements about chemical reactions is false?
(A) Heat can be released, but it can never be absorbed.
(b) The sound might be made.
(c) There might be a colour shift.
(d) Gases can develop.
9. Which of the following statements about chemical reactions is false?
(A) Heat can be released, but it can never be absorbed.
(b) The sound might be made.
(c) There might be a colour shift.
(d) Gases can develop.
Explanation:
The incorrect statement for a chemical reaction is:
(a) Heat can be released, but it can never be absorbed.
Explanation: In a chemical reaction, heat energy can be either given out or absorbed, depending on whether the reaction is exothermic or endothermic, respectively.
10. One gramme of copper sulphate powder was mixed with two drops of weak sulfuric acid, which was then partially dissolved with hot water (step I). Beautiful blue crystals split off during cooling (step II). Steps I and II consist of:
(a) physical and chemical changes, respectively.
(b) chemical and physical changes, respectively.
(c) both physical change.
(d) both chemical change.
10. One gramme of copper sulphate powder was mixed with two drops of weak sulfuric acid, which was then partially dissolved with hot water (step I). Beautiful blue crystals split off during cooling (step II). Steps I and II consist of:
(a) physical and chemical changes, respectively.
(b) chemical and physical changes, respectively.
(c) both physical change.
(d) both chemical change.
Explanation:
The given steps I and II involve the following changes:
Step I: Addition of dilute sulphuric acid to copper sulphate powder and addition of hot water to dissolve it. This is a chemical change as a new substance is formed due to the reaction between copper sulphate and sulphuric acid to form copper sulphate pentahydrate and water.
Step II: Cooling of the solution and separation of blue crystals of copper sulphate pentahydrate. This is a physical change as no new substance is formed, and the change is only in the state or appearance of the substance. Hence, the correct option is (a) physical and chemical changes, respectively.
11. State whether the following statements are true or false:
(a) When a candle burns, both physical and chemical changes take place.
(b) Anaerobic bacteria digest animal waste and produce biogas.
(c) Ships suffer a lot of damage though they are painted.
(d) Stretching of the rubber band is not a physical change.
11. State whether the following statements are true or false:
(a) When a candle burns, both physical and chemical changes take place.
(b) Anaerobic bacteria digest animal waste and produce biogas.
(c) Ships suffer a lot of damage though they are painted.
(d) Stretching of the rubber band is not a physical change.
Explanation:
(a) True. When a candle burns, the wax melts and vaporizes, which is a physical change. The vaporized wax then undergoes combustion with oxygen in the air, which is a chemical change.
(b) True. Anaerobic bacteria digest animal waste and produce methane gas, which is the main component of biogas.
(c) True. Ships are constantly exposed to the corrosive effects of saltwater, which causes the metal to rust and deteriorate over time, despite being painted.
(d) False. Stretching of the rubber band involves the rearrangement of polymer chains, which is a chemical change.
12. Wax melting is a transformation from a solid to a liquid state. Provide one more example of a change you've seen in your environment.
12. Wax melting is a transformation from a solid to a liquid state. Provide one more example of a change you've seen in your environment.
Explanation:
One more example of a change where a solid changes to a liquid state is the melting of ice into water.
13. What kind of change is demonstrated by paper tearing?
13. What kind of change is demonstrated by paper tearing?
Explanation:
Tearing of paper is a physical change, as the chemical composition of paper remains the same. The paper is simply being physically broken down into smaller pieces without changing its chemical structure.
14. Match the items of Column I with the items of Column II.
14. Match the items of Column I with the items of Column II.
Explanation:
15. Fill in the blanks in the following statements using the words given in the box.
(a) Making sugar solution is a ____________ change.
(b) A physical change is generally____________.
(c) Grinding of wheat grain changes its size. It is a ____________ change.
(d) Iron benches kept in lawns and gardens get____________. It is a _________ change because a new _________ is formed.
15. Fill in the blanks in the following statements using the words given in the box.
(a) Making sugar solution is a ____________ change.
(b) A physical change is generally____________.
(c) Grinding of wheat grain changes its size. It is a ____________ change.
(d) Iron benches kept in lawns and gardens get____________. It is a _________ change because a new _________ is formed.
Explanation::
(a) Making sugar solution is a physical change. (Reason: The sugar molecules do not undergo any chemical reaction and the resulting solution can be reversed back to sugar and water by evaporation.)
(b) A physical change is generally reversible. (Reason: The properties of the substance undergoing a physical change remain the same and the change can be reversed.)
(c) Grinding of wheat grain changes its size. It is a physical change. (Reason: The chemical composition of the wheat grain does not change during grinding.)
(d) Iron benches kept in lawns and gardens get rusted. It is a chemical change because a new substance (iron oxide) is formed. (Reason: The iron reacts with oxygen and water to form a new substance that cannot be reversed back to its original form without undergoing a chemical reaction.)
16. Indicate whether the processes listed below result in physical or chemical changes:
(i) The beating of aluminium metal to make aluminium foil.
(ii) Digestion of food.
(iii) Cutting of a log of wood into pieces.
(iv) Burning of crackers.
16. Indicate whether the processes listed below result in physical or chemical changes:
(i) The beating of aluminium metal to make aluminium foil.
(ii) Digestion of food.
(iii) Cutting of a log of wood into pieces.
(iv) Burning of crackers.
Explanation:
(i) Physical change - The beating of aluminium metal to make aluminium foil is a physical change because it does not involve any chemical reaction. The shape and size of the metal are changed, but the chemical composition of the metal remains the same.
(ii) Chemical change - Digestion of food is a chemical change because it involves the breakdown of complex food molecules into simpler ones with the help of enzymes and other chemicals in the body.
(iii) Physical change - Cutting of a log of wood into pieces is a physical change because it does not involve any chemical reaction. The size and shape of the wood are changed, but the chemical composition of the wood remains the same.
(iv) Chemical change - Burning of crackers is a chemical change because it involves the reaction of chemicals present in the crackers with oxygen in the air, resulting in the formation of new substances with different properties.
17. Write word equations for two chemical reactions with the help of materials given in the box.
17. Write word equations for two chemical reactions with the help of materials given in the box.
Explanation:
(1) Iron + Air + Water _ Iron oxide
(2) Copper sulphate + Iron _ Iron sulphate + Copper
18 . Explain the following: (a) When carbon dioxide gas is added to lime water, it turns milky.
(b) Acetic acid causes bubbles to form when it is applied to a sodium hydrogen carbonate solution.
18 . Explain the following: (a) When carbon dioxide gas is added to lime water, it turns milky.
(b) Acetic acid causes bubbles to form when it is applied to a sodium hydrogen carbonate solution.
Explanation:
(a) Lime water turns milky on passing carbon dioxide gas into it because the carbon dioxide gas reacts with calcium hydroxide present in lime water to form calcium carbonate, which is insoluble and appears as a white precipitate. The chemical reaction can be represented as follows:
Ca(OH)2 + CO2 → CaCO3 + H2O
(b) Bubbles are produced when acetic acid is added to a solution of sodium hydrogen carbonate because the two react with each other to form carbon dioxide gas, which gets evolved as bubbles. The chemical reaction can be represented as follows:
CH3COOH + NaHCO3 → CO2 + H2O + NaCH3COO
19. Describe each of the following situations using two examples:
(a) Alterations to the body can be reversed.
(a) Physical modifications cannot be undone.
(c) Chemical alterations.
19. Describe each of the following situations using two examples:
(a) Alterations to the body can be reversed.
(a) Physical modifications cannot be undone.
(c) Chemical alterations.
Explanation:
(a) Examples of physical changes that are reversible:
1.Melting of ice: When ice melts, it changes from a solid to a liquid state. This change is 2.reversible, as the liquid water can be frozen again to form solid ice.
Dissolving salt in water: When salt is dissolved in water, it forms a solution. This change is reversible, as the salt can be recovered by evaporating the water.
(b) Examples of physical changes that are not reversible:
1.Breaking a glass: When a glass is broken, it cannot be repaired to its original state. The broken pieces can be reused to make something else, but the glass cannot be restored to its original form.
2.Cutting a paper: When a paper is cut into pieces, the pieces cannot be joined back to form the original paper. The paper is permanently changed.
(c) Examples of chemical changes:
1.Burning of a matchstick: When a matchstick is burnt, it undergoes a chemical change. The chemicals in the matchstick react with oxygen in the air to produce heat, light, and smoke.
2.Rusting of iron: When iron is exposed to air and moisture, it undergoes a chemical change called rusting. Iron reacts with oxygen and water to form a new substance, iron oxide, which is the reddish-brown rust that we see on iron objects.
20. For every one of the following scenarios, provide an example of a chemical reaction:
(A) A modification in colour is noticed.
Gas is evolved (b).
(c) A sound is made.
20. For every one of the following scenarios, provide an example of a chemical reaction:
(A) A modification in colour is noticed.
Gas is evolved (b).
(c) A sound is made.
Explanation:
(a) A change in colour is observed: When iron nail is kept in copper sulphate solution, the colour of the solution changes from blue to light green as iron displaces copper from copper sulphate forming iron sulphate.
(b) Gas is evolved: When baking soda (sodium hydrogen carbonate) is mixed with vinegar (acetic acid), carbon dioxide gas is evolved.
(c) Sound is produced: When calcium carbide is added to water, acetylene gas is produced which reacts with the water to produce bubbles of hydrogen gas along with a popping sound.
