Introduction
Question 1
Change is the law of nature and occurs in our everyday life, at all times and in all places. Differentiate between the following changes with a suitable example.
(a) Desirable and undesirable change
(b) Periodic and non-periodic change
(c) Slow and fast change
(d) Natural and man-made change
(e) Reversible and irreversible change
Answer
(a) Difference between desirable and undesirable change :
| Desirable change | Undesirable change |
|---|---|
| Desirable changes produce good results. | Undesirable changes are unfavorable. |
| For example, rain bringing water to crops is a desirable change. | For example, rain causing destruction by floods is an undesirable change. |
(b) Difference between periodic and non-periodic change :
| Periodic change | Non - Periodic change |
|---|---|
| Changes which occur periodically at regular intervals of time. | Changes which do not occur periodically [do not repeat] at regular intervals of time. |
| For example, change of seasons is a periodic change | For example, change of weather is a non-periodic change. |
(c) Difference between slow and fast change
| Slow change | Fast change |
|---|---|
| Change which occur very slowly over days, months and years. | Change which occur very rapidly in seconds or minutes. |
| For example, the growth of a tree from a seed | For example, occurring of lightning. |
(d) Difference between natural and man-made change
| Natural change | Man-made change |
|---|---|
| Changes brought about by nature. | Changes brought by man |
| For example, growth in man and plants. | For example, conversion of iron to steel. |
(e) Difference between reversible and irreversible change
| Reversible change | Irreversible change |
|---|---|
| Changes which are temporary and can be reversed by change in conditions. | Changes which are permanent and cannot be reversed by change in conditions. |
| For example, melting of ice and freezing of water. | For example, burning of paper. |
Question 2
Differentiate between a physical change and a chemical change with reference to-
(a) Nature of change i.e., temporary and reversible or permanent and irreversible.
(b) Formation of products.
(c) Energy changes taking place during the respective change.
Answer
| Characteristics | Physical change | Chemical change |
|---|---|---|
| Nature of change | A physical change is a temporary and reversible change. E.g., Water on freezing converts to ice and ice on melting converts back to water | A chemical change is a permanent and irreversible change. E.g., lead dioxide gets permanently converted to lead monoxide on heating. |
| Formation of products | In a physical change, no new substances are formed. E.g., Water on freezing converts to ice and no new substance is formed. | In a chemical change, new substances are formed. E.g., on heating lead dioxide two new substances are formed, lead monoxide and oxygen. |
| Energy changes | Physical change is not accompanied by a marked evolution or absorption of heat. E.g., no energy change is seen during conversion of water to ice. | Chemical change is accompanied by a marked evolution or absorption of heat and sometimes emission of light. E.g., when a magnesium wire is heated, light energy is evolved. |
Question 3
Give three reasons why melting of wax, is considered a physical change while burning of a candle, a chemical change.
Answer
Melting of wax is considered a physical change, whereas burning of candle is a chemical change because of the following reasons:
(a) Melting of wax is a temporary and reversible change whereas, burning a candle is a permanent and irreversible change.
(b) During melting of wax, heat energy is absorbed and on cooling it is evolved forming the original substance i.e., wax. Hence, no marked evolution or absorption of heat takes place. So, it is a physical change.
However, during the burning of candle, the wax decomposes. The products formed cannot combine back to give wax. Hence, it is a chemical change.
(c) During melting of wax no new substance is formed. The original substance is wax and the final product is also wax. Hence, it is a physical change.
During burning of candle new substances are formed. The original substance is the burning candle. The final products are carbon dioxide and water vapour. Hence, it is a chemical change.
Question 4
State the observations seen, when milk in a dish is kept aside for a few hours or more. Is the change which occurs - a physical change or a chemical change. Give reasons.
Answer
When milk in a dish is kept aside for a few hours or more, it forms curd. On keeping aside longer, the curd does not change back to milk, hence it is an irreversible change.
New substances are formed. The original substance is milk, and the final product is curd. As the change is irreversible and new substance is formed, hence, it can be concluded that it is a chemical change.
Question 5
State what is meant by the term - interconversion of matter. Is interconversion of matter a physical change or a chemical change?
Answer
Interconversion of matter involves matter changing from one state to another and back to its original state, by change in temperature or pressure.
Interconversion of matter is a physical change because:
- it is a reversible change,
- no new products are formed,
- properties of the original substance are not altered.
Question 6
Ice kept in a beaker slowly melts and turns into water. The water in the beaker on solidification i.e., freezing turns back to ice. Give four reasons why the change from ice to water and water back to ice is considered a physical change.
Answer
Change from ice to water and water back to ice are considered as physical changes because:
- The change is temporary.
- It is a reversible change. Heating of ice turns it into water and cooling of water turns it back into ice.
- No new products are formed since ice and water are chemically H2O.
- Properties of the original substance are not altered.
Question 7
Explain the term- 'sublimation'. Is sublimation of naphthalene - a physical or a chemical change? Give reasons.
Answer
Sublimation is the conversion of a solid directly into gaseous [vapour] state and on cooling directly back to solid state without changing into liquid state.
Sublimation of naphthalene involves change of state and no new substance is formed. Moreover, the change is reversible as naphthalene vapour condense to form solid naphthalene. Hence, it is a physical change.
Question 8
Ammonium chloride is also a sublimable solid. Give a reason why sublimation of ammonium chloride involves a physical & a chemical change.
Answer
Ammonium chloride [NH4Cl] sublimes on heating and dissociates into ammonia [NH3] & hydrogen chloride [HCl], which recombine on cooling to give back NH4Cl again. As the change is reversible, hence, it is a physical change.
NH4Cl [s] ⇌ NH3 [g]+ HCl [g]
Dissociation of ammonium chloride involves formation of new products, hence it is a chemical change. Therefore, sublimation of ammonium chloride involves both – a physical and a chemical change.
Question 9
State why addition of sodium chloride to water is considered a physical change, while addition of dilute sulphuric acid to zinc is considered a chemical change.
Answer
Addition of sodium chloride (salt) to water is considered a physical change because:
(a) It is a temporary and reversible change. On adding, the salt dissolves in water and forms a salt solution. On evaporation of the salt solution the original salt remains behind. Hence, it is a reversible change.
(b) No new substances are formed. Original and the final product is the same [NaCl].
Addition of dilute sulphuric acid to zinc is a chemical change because :
Zinc + dil. Sulphuric Acid ⟶ Zinc Sulphate + Hydrogen gas
(a) It is a permanent and irreversible change.
(b) New substances zinc sulphate and hydrogen gas are formed.
Question 10
Photosynthesis is a natural process by which green plants manufacture food in the presence of sunlight.
(a) Are any new products formed during the above process?
(b) Can the change be reversed or is irreversible?
(c) State the conclusions which can be drawn, to represent photosynthesis as a physical or a chemical change.
Answer
(a) Glucose and oxygen are formed during photosynthesis. Hence, new products are formed.
(b) Carbon dioxide and water cannot be obtained back, from glucose, hence, the change is irreversible.
(c) Photosynthesis is a chemical change as it is irreversible and leads to the formation of new products.
Question 11
Give reasons why - separation of mixtures e.g., iron from a mixture of iron and sulphur is a physical change, but heating a mixture of iron and sulphur is considered a chemical change.
Answer
As separation of iron and sulphur from its mixture is a
- reversible process,
- no new products are formed and
- mixture has the same properties as iron and sulphur individually have. Hence, it is a physical change.
However, on heating of iron and sulphur,
- a new product iron sulphide is formed
- this new product [iron sulphide] has completely different properties from iron and sulphur.
- it is an irreversible process. Hence, it is a chemical change.
Question 12
Explain in brief the involvement of energy in - physical & chemical changes.
Answer
Physical changes use energy to change the state of matter.
Chemical changes release or absorb energy when changing a substance into a new substance.
- Energy is released during decomposition of a substance, while
- Energy is absorbed during formation of a new substance.
Objective Type Questions
Question 1
Fill in the blanks with the appropriate words for each of the statements given below:
Germination of seeds is considered a ............... [physical/chemical] change.
If a change is permanent and irreversible, it is considered a ............... [chemical/physical] change.
Heating a piece of iron is considered a ............... [physical/chemical] change, while heating a piece of coal [carbon] is considered a ............... [physical/chemical] change.
Chemical changes may involve exchange of energy in the form of ............... & ............... [light/sound/magnetic /heat].
Change of milk to curd is a ............... [desirable/undesirable] change.
Answer
Germination of seeds is considered a chemical change.
If a change is permanent and irreversible, it is considered a chemical change.
Heating a piece of iron is considered a physical change, while heating a piece of coal [carbon] is considered a chemical change.
Chemical changes may involve exchange of energy in the form of heat & light.
Change of milk to curd is a desirable change.
Question 2
State whether the following statements are true or false. If false, write- the correct statement.
Desirable changes are meaningful, while undesirable changes are unfavorable.
During physical changes, the composition of the original substance is not altered, but the properties of the original substance are altered.
During a chemical change the composition of the original substance is not altered, and the change is irreversible.
Melting of butter and wax are examples of chemical changes.
The melting of ice is an exothermic, irreversible chemical change.
Answer
True
False.
Corrected Statement — During physical changes, the composition and properties of the original substance are not altered.False.
Corrected Statement — During a chemical change the composition of original substance is altered and the change is irreversible.False.
Corrected Statement — Melting of butter and wax are examples of physical changes.False.
Corrected Statement — Melting of ice is an endothermic, reversible physical change.
Question 3
Match the examples of changes in List I with the correct type in List II.
| List I | List II |
|---|---|
| 1. Expansion of a steel rod on heating | A. Physical |
| 2. Rotting of eggs | B. Chemical |
| 3. Milk turning sour | |
| 4. Tarnishing of copper | |
| 5. Separation of naphthalene from a mixture of naphthalene & sodium chloride |
Answer
| List I | List II |
|---|---|
| 1. Expansion of a steel rod on heating | Physical |
| 2. Rotting of eggs | Chemical |
| 3. Milk turning sour | Chemical |
| 4. Tarnishing of copper | Chemical |
| 5. Separation of naphthalene from a mixture of naphthalene & sodium chloride | Physical |
Question 4
Fill in the blanks with the correct words for each of the statements given below:
Sublimation is ............... [considered/not considered] a change of state of matter.
Sublimation involves a ............... [chemical/physical] change.
During sublimation a solid on heating turns directly into ............... [liquid/vapour] & on cooling back to ............... [solid/liquid].
Ammonium chloride, a sublimable solid undergoes ............... [synthesis/dissociation] on heating.
Sublimation of iodine is an example of ............... [physical/chemical] change.
Answer
Sublimation is considered a change of state of matter.
Sublimation involves a physical change.
During sublimation a solid on heating turns directly into vapour & on cooling back to solid.
Ammonium chloride, a sublimable solid undergoes dissociation on heating.
Sublimation of iodine is an example of physical change.
Question 5
Name the following.
The product/s obtained during 'photosynthesis' — which is a chemical change.
The term involved in the change of state from — 'water to ice' – which is a physical change.
The final product of the physical change involved during — 'melting of a piece of wax'.
The type of change i.e., physical or chemical involved — 'during respiration in human beings.'
The type of change i.e., physical or chemical involved — 'when a substance undergoes a change in state, colour, or size'.
Answer
Glucose & Oxygen
Freezing/Solidification
Wax [CxHy]
Chemical change
Physical change