Q1. Write the change involved in:(i) Mixing of beans and peanuts(ii) Heating chicken in a pan
Solution
(i) Physical change(ii) Chemical change
Q2. Explosion of a cracker is a chemical change. Explain.
Solution
Explosion of a cracker is a chemical change because the explosive reactants are transformed into gaseous products along with heat, light and sound which cannot be reversed. Hence, it is a chemical change.
Q3. A fraction of ship's iron has to be replaced every year. Explain.
Solution
We know that ships are made of wood with some fraction of iron and a part of them remains under water. On the part above water, drops keep clinging to the ship's outer surface. Moreover, the water of sea contains many salts. The salt water makes the process of rust formation faster. Therefore, ships suffer a lot of damage from rusting in spite of being painted. For this reason a fraction of ship's iron has to be replaced every year.
Q4. Why are cooking utensils usually made of stainless steel?
Solution
Cooking utensils are usually made of stainless steel as it does not rust at all.
Q5. Rusting of iron is faster in coastal areas than in deserts. Give reason.
Solution
Since content of moisture in the air in coastal areas is higher than in the air in deserts so, the process of rusting becomes faster in the coastal areas.
Q6. What is charring of sugar? Is it a chemical change?
Solution
When sugar is continuously heated in a dish, then it starts evaporating and becomes foggy due to water vapour. A black powdery substance is left behind, which is charcoal. This process is called charring of sugar. It is a chemical change as it cannot be reversed.
Q7. The same substance can undergo a physical change or a chemical change depending upon the conditions. Explain with an example.
Solution
The melting of wax is a physical change but burning of wax is a chemical change. So, the same substance wax can undergo both physical and chemical change. On melting, only change in the state of wax occurs but on burning wax, it produces carbon dioxide gas, water vapour, soot, heat and light. Hence, it is a chemical change.
Q8. Which of the given properties change during a physical change? Shape, chemical composition, size, colour, state
Solution
Properties such as shape, size, colour and state of a substance change during a physical change.
Q9. Write an experiment to show that air and moisture are necessary for rusting.
Solution
Experiment to show that rusting of iron requires both air and water. We take three test tubes and put clean iron nail in each of the them. 1.In the first test tube containing iron nail, we put some anhydrous calcium chloride and closed its mouth with a tight cork. The anhydrous calcium chloride is added to absorb water or moisture from the damp air present in the test tube and make it dry. In this way, the iron nail in the first test tube is kept in dry air. This test tube is kept aside for about half an hour. After one week, we observe that there is no rust on the surface of iron nail. This shows that rusting of iron does not take place in air alone. 2.In the second test tube containing iron nail, we put boiled distilled water. Boiled distilled water does contain any dissolved air or oxygen in it. A layer of oil is put over boiled water in the test tube to prevent the outside air from mixing with boiled water. In this way, the iron nail in the second test tube is kept in air free, boiled water. The mouth of this test tube is closed with a cork and it is kept aside for about a week. After one week, we observe that no rust is there on the surface of iron nail. This shows that rusting of iron does not take place in water alone. 3.In the third test tube containing an iron nail, we put unboiled water so that about two-thirds of the nail is immersed in water and the rest is above the water, exposed to damp air. In this way, the iron nail has been placed in air and water together. The mouth of this test tube is closed with a cork and it is kept aside for about a week. After one week, we observe that red brown rust is seen on the surface of iron nail kept in the presence of both air and water.
Q10. Explain how galvanization of an iron object prevents it from rusting.
Solution
In the method of galvanisation, surface of iron is covered with layer of more active metal like zinc. In this way, iron is prevented to come in contact with air and moisture which are required for rusting. Zinc metal does not undergo corrosion and hence prevents the rusting of iron.
Q11. When a chemical change takes place, what changes accompany the formation of new substance?
Solution
In addition to new product, the following may accompany a chemical change: (i) Heat, light or any other radiation may be given off or absorbed. (ii) Sound may be produced. (iii) A change in smell may take place or a new smell may be given off. (iv) A colour change may take place. (v) A gas may be formed.
Q12. Mention two ways of protecting iron from rust.
Solution
Rusting of iron can be prevented by (i) Painting iron articles. (ii) Galvanisation.
Q13. What is rusting? Is it a physical change or chemical change?
Solution
The chemical reaction of iron with oxygen in presence of water vapour to produce iron oxide (rust) is known as rusting. Rusting of iron is a chemical change
Q14. Formation of clouds is a physical change. Explain.
Solution
Formation of clouds is a physical change as it is phase transformation of water from liquid to gas during water cycle and then, gas to liquid. Hence, the physical property of water undergoes change in the formation of clouds.
Q15. Most physical changes are reversible. Give two examples to support this statement.
Solution
(i) Melting of ice: During this change, the water changes from its solid form to liquid form. It can be solidified again. The water remains water in both cases hence this is a reversible change. (ii) Lightening of an electric bulb: During this change, electricity is passed through the filament which becomes white hot and glows, but when switch is off, the filament returns to its original shape and condition, hence it is totally reversible.
Q16. (i) What kind of a change is rusting? (ii) Can damp air cause rusting?
Solution
(i) Rusting is a chemical change.(ii) Damp air contains both oxygen and water required for the rusting of iron to occur. So, damp air can cause rusting.
Q17. Identify the type of change in each of the following: (i) Burning of candle (ii) Lightening of bulb (iii) Preparation of food by green plants (iv) Volcanic eruption (v) Lightening (vi) Spinning of wheels of cycle
Solution
(i) Burning of candle: Chemical change (ii) Lightening of bulb: Physical change (iii)Preparation of food by green plants: Chemical change (iv)Volcanic eruption: Chemical change (v) Lightening: Physical change (vi)Spinning of wheels of cycle: Physical change
Q18. Write the change involved in: (a) A piece of an apple rots on the ground. (b) A tyre is inflated with air.
Solution
(a) Rotting of apple is a chemical change.(b) Inflation of tyre is a physical change and can be reversed.
Q19. Differentiate between a physical and chemical change on basis of reversibility of reaction?
Solution
A physical change is generally reversible while a chemical change is irreversible in nature.
Q20. Differentiate between physical and chemical changes (four points).
Solution
Physical changes
Chemical changes
1. In a physical change, only physical properties such as physical state, colour, volume etc. of the reacting substance undergo change. Chemical properties remain unchanged.
1. In a chemical change, the chemical composition and chemical properties of the reacting substances undergo change.
2. No new substance is formed.
2. One or more new substances are formed.
3. These are temporary changes that can be easily reversed.
3. These are permanent changes and cannot be reversed back.
4. The original form of the substance can be obtained easily by simple physical methods.
4. The original substances cannot be obtained by simple physical methods.
Q21. Name the process of depositing a layer of zinc on iron. Give two examples of objects on which zinc coating is done.
Solution
Galvanisation is the process of depositing a layer of zinc on iron. Rusting of an iron object can be prevented by the process of galvanization. Ex: Iron sheets used for making buckets and sheds are coated with zinc to prevent rusting.
Q22. What is photosynthesis? Why is it considered a chemical change?
Solution
The process in which carbon dioxide reacts with water in presence of chlorophyll and sunlight to produce carbohydrates (glucose) and oxygen is known as photosynthesis. New products (glucose and oxygen) are obtained during photosynthesis, thus it is a chemical change.
Q23. Identify the following changes as physical or chemical change. (a) Log of wood burns to form ash. (b) Steam condenses to form water. (c) A bicycle chain rusts. (d) Water is absorbed by a paper towel. (e) A piece of a mango rots on the ground. (f) Dissolving sugar in water.
(g) Eggs turn into omelet.
(h) Stretching metals to form wires.
Solution
Physical Change
Chemical Change
(i) Steam condenses to form water. (ii) Water is absorbed by a paper towel.
(iii) Dissolving sugar in water.
(iv) Stretching metals to form wires.
(i) Log of wood burns to form ash. (ii) A bicycle chain rusts. (iii) A piece of a mango rots on the ground. (iv) Eggs turn into an omelet.
Q24. (i)What is the chemical formula of rust? (ii) Explain how applying oil or grease to the surface of an iron object prevents it from rusting?
Solution
(i)Formula of rust is Fe2O3.xH2O (iron oxide). (ii)When some oil or grease is applied to the surface of an object, then air and moisture cannot come in contact with it and hence rusting is prevented. For example, the tools and machine parts made of iron and steel are smeared with grease to prevent their rusting.
Q25. Explain how applying paint on an iron object prevents it from rusting?
Solution
We know that the process of rusting requires both air (oxygen) and water (moisture). Painting prevents the surface of iron object to come in contact with oxygen and moisture thus prevents it from rusting.
Q26. A boy wants to experiment with sugar cubes. Select the method/s that cause sugar cube to change chemically only.(i) Chewing the sugar cube and digesting it(ii) Crushing the sugar cube(iii) Adding sugar cube in water(iv) Burning the sugar cube with a matchstick
Solution
The methods that cause sugar cube to only change chemically are:(i) Chewing the sugar cube and digesting it(iv) Burning the sugar cube with a matchstick
Q27. Give reasons - "Melting of ice is a physical change".
Solution
On heating, ice melts to form water while on cooling, water solidifies back to ice. It is a reversible process so it is considered a physical change.
Q28. In a physical change, no new substances are formed. Explain your answer.
Solution
A physical change involves a change in physical properties of the substances such as shape, size, colour and state. It does not involve a change in the chemical composition of the substances hence there are no new products formed.
Q29. Why is digestion classified as a chemical change?
Solution
Digestion is a process where food nutrients such as carbohydrates, proteins, fats and other materials are broken down into simpler substances with release of energy.New substances are formed thus digestion is a chemical change.
Q30. What is crystallisation? Give its main use.
Solution
Crystallisation is the process of obtaining large crystals of pure substances from their solution. Crystallisation is a technique used to purify solid compounds.
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