the following questions :
1. A sharpener loses its sharpening power, if used continuously. Is the loss of sharpening
power a reversible change or an irreversible change?
2. Is ironing of clothes an irreversible change? Give a reason for your answer.
3. Is converting wood into saw dust a reversible change? Give one reason for your answer
4. State three ways by which a change can be brought about.
5. Differentiate between a reversible and an irreversible change.
6. How the expansion of metals, which is a reversible change, put to good use? Support you
answer by an example.
7. How does the solubility of a substance change with:
(a) rise in temperature
(b) fall in temperature?
8. Why is it difficult to dissolve sugar in ice cold water?
9. You have accidently dropped a glass tumbler, which breaks. Is the change reversible? Give
reason.
10. Give four examples of the change which :
(a) cannot be reversed
(b) can be reversed.
11. A thick coating of plaster of Paris paste is applied on a bandage on a fractured bone. The
paste becomes hard on drying. Can this change be reversed?
12. You have boiled some milk on a hot summer day. On cooling it you forgot to place iti
the fridge. What kind of change will take place in the milk after 12 hours and why?
Answers
Explanation:
1) Yes, it is a reversible change because the edges of sharpener blade can be grinded by a suitable apparatus to make it sharp again.
2) Ironing is reversible change as while ironing we provide heat to our clothes and after ironing, this given heat is radiated slowly by our clothes into atmosphere. ... so, thus it attains equilibrium by radiating heat.
3) No, conversion of wood to sawdust is an irreversible change. Explanation: A physical change occurs in a substance when it changes to a different form of matter without changing its composition; like ice melting into water, wood changing to sawdust, dissolving sugar in water etc.
4) It can be physical,chemical or temporary.
5)
Reversible change :- A change which can happen backward, that is, can be reversed is called a reversible change
Irreversible change :- A change which cannot happen backward, that is, it cannot be reversed is called an irreversible change.
Reversible change :- A substance can return to its original state.
Irreversible change :-A substance cannot return to its original state.
Reversible change :- The chemical properties of the substance do not change.
Irreversible change :-The chemical properties of the substance change.
Reversible change :- Most physical changes are reversible changes.
Irreversible change :- All chemical changes are irreversible changes.
6) The expansion of metals, which is a reversible change, can be put to good use for certain industrial applications, such as thermostats.
7) For many solids dissolved in liquid water, the solubility increases with temperature. The increase in kinetic energy that comes with higher temperatures allows the solvent molecules to more effectively break apart the solute molecules that are held together by intermolecular attractions.
8) Higher temperature means that molecules move faster. So molecules in cold water move slower than molecules in hot water. ... If the water molecules are moving fast, they will bounce off of sugar molecules faster, so the sugar will dissolve faster in warm water than cold, where water molecules are moving more slowly
9) No, the change is not reversible because reversible change means to get the substance back in its original state as it was previously. When a glass tumbler gets broken, it can't be regained back in its original state
10) Cutting of an apple . Tearing of paper. Melting of ice . Freezing of water into ice .
11) A thick coating of a paste of Plaster of Paris (POP) is applied over the bandage on a fractured bone. It becomes hard on drying to keep the fractured bone immobilised. ... No, the change in plaster of Paris cannot be reversed. When water is added to POP, it undergoes a chemical change.
12) The bacteria enterss the milk and spoils it. It will be a Chemical change as it cannot be reversed
1) A sharpener loses its sharpening power, if used continuously .Is the loss of sharpening
power reversible change? Or an Irreversible change?
Ans: Yes, it is a reversible change because the edge of the sharpener blade can be grinded
by suitable apparatus to make it sharp again.
2) Is ironing of clothes an Irreversible change? Give reason for your answer.
Ans: No, it is not an Irreversible change. This is because after we iron clothes and then
stamp, wash or even wear it while sleeping makes the clothes crumbled.
3) Is converting wood into sawdust a reversible change? Give one reason for your answer.
Ans: No, it is an Irreversible change. This is because during the conversion wood gets
converted into very fine particles which cannot be joined again.
4) State three ways by which a change can be brought about?
Ans: Three ways by which a change can be brought about are:
a) By heating.
b) By pressure or force
c) By dissolving
5) Differentiate between a reversible and an irreversible change.
Ans: Irreversible changes:
A change is called irreversible if it cannot be changed back again. For example you cannot
change a cake back into its ingredients again.
Irreversible changes are permanent. They cannot be undone.
Reversible changes:
A change that can be reversed is called reversible change.
Reversible changes are temporary they can be undone by removing the cause of the change.
6) How the expansion of metals, which is a reversible change, put to good use? Support your
answer with an example.
Ans: The expansion of metals is a reversible change and its good use can be expressed by an
example below:
Iron rims are mounted on wooden cart wheels. To start with the Iron rims are slightly
smaller than the wooden wheel. The iron rim is heated when its size becomes larger than
the wooden wheel. At this moment, it is slipped on the wooden wheel and cold water is
poured over it. On cooling the iron rim contracts and holds the wooden wheel tightly.
7) How does the solubility of a substance change with:
a) Rise in temperature
b) Fall in temperature.
Ans: a) Certain substances are more soluble in water than other substances.
With the rise in temperature of water more amount of substance dissolve in it that is its
solubility increases.
b) With the fall in temperature the dissolve substance comes out in the form of crystals and
settles at the base of the solution.
8) Why is it difficult to dissolve sugar in Ice cold water?
Ans: It is difficult to dissolve sugar in ice cold water because the solubility of sugar is very
less in cold water.
9) You have accidentally dropped a glass tumbler, which breaks. Is the change reversible?
Give reasons.
Ans: No, the change is not reversible because reversible change means to get the substance
back in its original state as it was previously. When a glass tumbler gets broken, it can't be
regained back in its original state.
10) Give four examples of the change which:
a) Cannot be reversed
b) Can be reversed.
Ans: a) Ripening of fruits, kneading dough, burning of paper and cooking of food.
b) Melting of ice, glowing of bulb, magnetization of steel and evaporation of water.
11) A thick coating of plaster of Paris paste is applied on a bandage on a fractured bone. The
paste becomes hard on drying. Can this change be reversed?
Ans: No, this change cannot be reversed. This is because, when water is added to plaster of
Paris (POP), it changes to another substance and on drying it hardens. Once the POP has
hardened, its shape cannot be changed. Therefore, the change in POP cannot be reversed.
12) You have boiled some milk on a hot summer day. On cooling it you forgot to place it in
the fridge. What kind of change will take place in the milk after 12 hours and why?
Ans: It is a kind of Irreversible change. It is because the milk curdles as it was not kept in the fridge.