Lakhmir Singh and Manjit Kaur ICSE Class 7 Chemistry Chapter 2 Physical and Chemical Changes Solution
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Board | ICSE |
Class | 7 |
Subject | Chemistry |
Chapter | 2 |
Chapter Name | Physical and Chemical Changes |
Author | Lakhmir Singh and Manjit Kaur |
Objective Type Questions
A) Choose one correct option.
1) Option A – A and B – Decaying of wood and Burning of Wood
2) Option C – Sublimation of naphthalene
3) Option C – Cooking Rice
4) Option C – Process A is a physical change but Process B is Chemical change
5) Option C – State
6) Option D – B and D – Bursting a balloon and Burning of wax
7) Option A – Desirable Change: Destruction caused by floods.
8) Option A – A and D – Rusting of an Iron vessel and Rotting of fruits.
9) Option A – Chemical Change.
10) Option B – Ripening of Fruits.
11) Option D – None of these.
12) Option A – Zinc
B) Fill in the blanks
1) Chemical change
2) Physical Change, Chemical Change
3) Chemical Change
4) Change
5) Chemical Change
6) Carbon dioxide
7) Cause
8) Physical change
9) Desirable Change
10) Periodic Change
11) Physical Change
12) Periodic Change
13) Non periodic Change
C) Write T for True and F for false statement
1) False
2) False
3) True
4) False
5) True
6) False
7) True
8) True
9) False
10) True
11) False
12) True
D) Give two examples for each of the following.
1) Desirable Change – (i) Ripening of fruits, (ii) Drying of clothes
2) Slow changes – (i) Rusting of iron, (ii) formation of fossil fuel
3) Fast changes – (i) Burning of matchstick, (ii) lightning of electric bulb
4) Irreversible physical change – (i) Cutting a piece of paper, (ii) Breaking a stone
5) Reversible physical change – (i) Inflating a balloon, (ii) Melting of wax
6) Exothermic change – (i) Adding Quicklime in water, (ii) Freezing of water
7) Chemical changes – (i) Burning of wood, (ii) Ripening of fruits
8) Endothermic changes – (i) Adding ammonium chloride in water, (ii) Melting of Ice
E) Match the following.
1) Absorption of heat – (C) Endothermic change
2) Temporary change – (E) Physical Change
3) Permanent change – (B) Chemical Change
4) Evolution of heat – (A) Exothermic Change
5) Repeats at a fixed interval of time – (D) Periodic Change
Subjective Type Questions
F) Answer the following question in short.
1) Five changes that we observe in our everyday life is change of day and night, fire by burning fuel, boiling of water, cooking food, melting of ice.
2) One example of beneficial change is ripening of fruits and one example of harmful change is pollution of environment.
3)
a) Growth of a plant – Irreversible change
b) Cooking of food – Irreversible change
c) Melting of wax – Reversible change
d) Formation of curd from milk – Irreversible change
e) Pulling of rubber string – Reversible Change
4)
Chemical Change |
Physical Change |
1) In chemical changes new substances are formed. | 1) In Physical changes there is only change in state, size, shape and color can be observed. |
2) Chemical change is a permanent change and usually irreversible. | 2) Physical change is a temporary change and can be easily reversible. |
3) Energy is absorbed or released in form of heat, light or sound in chemical changes. | 3) The amount of energy released or absorbed in physical changes is very little. |
5) The explosion of a cracker is initiated by putting it on fire. By burning the substances present in the cracker it makes the explosion. In the process the substance is turned into a new substance through a chemical reaction and cannot be easily reversed. The covering of the cracker turns into ash by burning which also cannot be reversed. So we can say that the explosion of a cracker is a chemical change.
6) When heat is applied to ice it melts and turns into water. Though ice and water looks different from each other they are made of same molecules. When ice turns into water the only thing that changes is its state. The change is also easily reversible; water can be easily turned into ice by freezing it. So we can say that melting of ice into water is reversible.
7)
a) An example of a chemical change that occurs by the action of heat is cooking.
b) An example of a physical change that occurs by the action of heat is boiling of water.
8)
a) Curdling of milk – slow change
b) Ageing – slow change
c) Drying of clothes – slow change
d) Burning of diesel – Fast change
e) Burning of matchstick – Fast change
f) Germination of Seed – slow change
9) A periodic motion is a motion that repeats itself at regular interval of time. We know that the motion of a pendulum repeats itself at a regular interval. That’s why motion of pendulum is considered as periodic motion.
10) Some undesirable changes from our daily life are rotting of rotting of fruits, rusting of iron and Pollution of our environment.
11) When we mix salt in water it gets dissolved in the water and disappears. In this process no new substance is formed only a change in physical property has occurred. The salt can also be easily separated from the water by boiling the solution. So we can say that dissolving salt in water is a physical change.
12) When we cut the surface of an apple and keep it in the open air we can see that the color of the surface turning dark. This is because of the formation of new substances by chemical reaction between oxygen and the skin of the apple. This is a chemical change because new substances are formed and the changes cannot be reversed.
13) Chemical changes are essential for our survival. All the new substances we use in our daily life are formed by any kind of chemical change. So the importance of chemical change in our life is immense.
14) In all kind of changes energy is required. In some changes energy is released and in some energy is absorbed. For example if we burn wood then energy is released in form of light and heat. When ice melts it absorbs heat energy from its surroundings. From these examples we can see that change of energy is involved in both chemical and physical changes.
G) Answer the following questions in details.
1) All the substances around us have different properties. If we observe that one or more of the properties of the objects have changed then we can say that a change has been occurred. For example when an iron nail is kept in open for some time it gets rusted and we can see visible change in the nail. When iron is kept in open air it chemically reacts with the oxygen in the air and forms iron oxide which is known as rust. In this case we detected a change by observing the change in color of the object.
2) The change in object does not occur on its own. For any change to occur there must be a cause.
For example water does not turn into ice on its own. When the temperature of its surroundings falls below the freezing temperature of water it starts releasing heat from it and turns into ice. In this case the surrounding temperature was the cause of the freezing of the water. So we can say that a physical change does not occur on its own.
3) When a candle burns there are two changes that happen simultaneously. First the wax is burned and energy is released in the form of light and heat which is a chemical change. The second change is the melting of the wax by the heat released by burning of the wax. This change is a physical change as only the physical state of the wax is changing here. So burning of wax is chemical change and melting of wax is physical change.
4)
a) In physical change no new substances are formed and most of the cases the change is reversible, for example melting of ice. However, in chemical change new substances are formed and most of the changes are irreversible, for example burning of wood.
b) Fast changes are those which happen within a few seconds or minutes. Bursting of a cracker is an example of fast change. On the other hand slow changes are those which take long period of time to complete. For example rusting of iron is a slow change.
c) Reversible changes are those changes where the changes can be reversed by removing the cause of the change. Freezing of water into ice is an example of reversible process. Irreversible changes are those where the changes cannot be reversed and the change is permanent. Cutting a piece of paper is an example of irreversible process.
d) Exothermic changes are those where energy is released in form of heat during the change. Reaction of quicklime and water is an example of exothermic change. Endothermic changes are those where heat energy is absorbed by the system during the change. Mixture of ammonium chloride and water is an example of endothermic reaction.
e) The changes that are beneficial to us are called desirable changes. Cooking is one of the desirable changes. The changes that are harmful to us are called undesirable changes. Earthquake is an undesirable change.
5) Physical Changes can be both reversible and irreversible.
A physical change like melting of ice is reversible as water can be turned into ice by freezing. On the other hand a physical change like cutting a piece of paper is an example of irreversible process as it cannot be turned into a single piece of paper again.
6) The primary characteristics of a chemical changes are
i) New substances are formed in a chemical change.
ii) Chemical changes are usually permanent and cannot be reversed.
iii) A significant amount of energy is either released or absorbed in these changes.
iv) During these changes sound may also be produced.
v) The smell of the objects may change due to chemical change.
Picture based questions
Answer the following questions based on the given pictures.
i) If naphthalene balls are kept in an open area then after some days the balls will become significantly smaller. The naphthalene balls would go through a process called sublimation. In this process solid objects turn into gases without turning into liquid first. This change is physical change.
ii) If bread is kept in an open area for some days then it will start changing colour to black or green due to growth in fungi. This will happen due to the exposure of the bread to the moisture in the air. This is a chemical change.
iii) If a half cut apple is kept in open for some days then the cut part of apple will react with air and start growing new substances. It will turn dark coloured within a few hours. The change is a chemical change.
Application based questions
1) When water is added to a vessel containing quicklime it goes through a chemical reaction and a new substance called calcium hydroxide is formed. In this chemical change excessive heat is released that’s why this chemical change is categorized as exothermic reaction.
2) Rusting of iron is a chemical change.
Rusting is not a reversible process so it cannot be reversed by applying paint or grease on the surface after rusting. However if paint or grease is applied on the iron surface before the rusting then it can be prevented. Rusting happens when iron gets in contact with oxygen and creates iron oxide or rust. It cannot be reversed one turned into rust.
Chapter 1: Matter Chapter 3: Elements, Compounds and Mixtures
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