Class 6 Science Chapter 6 Important Questions

Many changes are taking place around us on their own. In the fields, the crop changes from time to time. Sometimes, leaves fall from trees, changes color and dry out. The flowers bloom and then wither away.

Changes around us can be grouped as those that can be reversed and those that cannot be reversed.
Reversible changes a change in which the original substance can be recovered. As examples, folding of a paper, rolling out of a chapati from a ball of dough, stretching of rubber to its normal size, melting of ice.

The iron blade of these tools has a ring in which wooden handle is fixed. Normally, the ring is slightly smaller in size than the wooden handle. To fix the handle, the ring is heated and it expands to become larger in size and now the handle easily fits into the expanded ring. On cooling the ring, it contracts and fits tightly onto the handle.

The melting of coal tar is a reversible change because when coal tar is heated, it melts and is converted into thick dark liquid which again solidified on cooling.

The heating of iron is a reversible change. When a blacksmith first he heats a piece of iron it becomes red hot. It then becomes soft and can be beaten into desired shape.

Salt dissolving in water is a physical change. The chemical composition of the salt is not changed after dissolution.

Some of the important changes happens in our everyday life are: formation of curd from milk, cooking of food, burning of fuels, drying of clothes, rusting of iron, melting of ice, germination of seeds, ripening of fruits etc.

A change which can be reversed to form the original substance is called a “Reversible change”. As an example, when we heat ice, it melts to form liquid water. A change from solid to liquid takes place during the melting of ice. Now, if we cool the water by keeping in the freezer of a refrigerator, it again changes into solid ice. So, the change from ice to water, by heating, has been reversed by cooling. Thus, the melting of ice (to form water) is a reversible change.

When ice changes into water, then there is a change from solid state to liquid state. And when water changes into steam, then there is a change from liquid state to gaseous state. In all of the three state, the water does not change in a new substance. Only water makes a different form of state. Form of water can be reversed easily. So, in general we can say that change of state is a reversible change.

If we stretch a rubber band with the force of our hands, it undergoes a change and its length increases. But on releasing the force, the rubber band comes back to its original length. This means that the change which occurred in rubber band on stretching has been reversed on releasing. So, the stretching of rubber band is a reversible change. Similarly, the stretching of a spring is also a reversible change.

Take some salt and dissolve it in water taken in a beaker. A salt solution is formed. A change has occurred in salt during the formation of salt solution (because salt has disappeared into water). This change can, however, be reversed as follows: keep the beaker containing salt solution over a burner and evaporate it. On evaporation, water is eliminated and salt is left behind. Thus, dissolving salt in water is a reversible change.

When water is added to cement, it sets into a hard mass after sometime. If a bag of cement lying in the open gets wet due to rain during the night, some changes occur in cement due to which it gradually sets into a hard mass. Even if, we keep the wet cement in bright sunshine the next day, the changes which have taken place in cement on getting wet, cannot be reversed. Thus, the setting of cement on mixing with water is an “irreversible change”.

Curd is made from milk. This is done as, a very small quantity of previously made curd is added to warm milk. The milk is then stirred and keep aside for a few hours at warm place. During this time, milk changes into curd. This curd, however cannot be changed back into milk by any means. The taste of curd is quite sour while milk does not. So, the formation of curd from milk is an in irreversible change which cannot be reversed.

If we drop a toy accidentally, it breaks into a number of pieces. We cannot get back original toy again from the broken pieces. This means that the change which takes place during the breaking of a toy, cannot be reversed. In other words, the breaking of a toy is an “Irreversible change”.

Plaster of Paris is a white, powdery substance which immediately sets to a hard mass on adding water to it. In hospitals, a thick coat of the paste of plaster of paris is applied over the bandage on the fractured bone of a person. POP sets and becomes hard on drying and keeps the fractured bone in place to get joined properly. The change which occurs in POP during setting cannot be reversed. Thus, the setting of plaster of paris on mixture water is an “Irreversible change”.

A candle is made of wax. We take a small wax candle and measure its length with a scale. Now, lighted it with a burning matchstick. Allow the candle to burn for some time (15 to 20 minutes). Now, blow out the burning candle and measure its length again. We will find that the length of this remaining candle is much less than its original length. This happens as, when we light the candle, then the wax present in it starts burning to produce heat and light. The burning of wax also produces carbon dioxide gas, water vapour and soot (unburned carbon particles), which all go into air. We cannot combine all the products of burning of wax to get back the original candle. So, the burning of wax is an “Irreversible change”. In other words, burning of wax is a change which cannot be reversed.

Yes, by unfolding the dress this change can be reversed.

The softening of iron on heating to red hot stage is a reversible change. And when the hot iron is cooled, it becomes hard again.

It is all depends on situations:
i)By raising the drawing we can undo the change. For example, if we draw with the help of pencil, we can erase the drawing by using an eraser. Then, it is a reversible change.
ii)If we draw using a pen, we cannot erase the drawing. Then this change cannot be reversed, so it is irreversible change.

No, it cannot be reversed, so it is an “Irreversible change”.

These two acts are of different kinds. Making a lump of clay into a pot is a reversible change. This is because the wet clay pot can be converted back into the original clay. The baking of clay pot in an oven, is an irreversible change. This is because the baked clay pot cannot be changed back into the original clay.