Changes Around Us
1. How do you know that a change has taken place?
Ans We can know that a change has taken place if there is a change in color, odor, position, new substance is made etc.
2. What are the factors that cause a change?
Ans The factors that cause a change are-
1. passing light and electricity
2. by changing temperature
3. by applying force
4. by mixing two substance.
3. Write differences between physical changes and chemical change.
Ans Physical change -
1. The change of shape, size, position, state of matter are called physical change.
2. They are temporary.
3. Some of them are reversible.
4.New substance is not formed
Chemical change-
1. The change when a new substance is formed is called chemical change.
2. They are permanent.
3. They are not reversible
4.Write four examples each of physical and chemical change.
Ans Physical change- cutting fruits, stretching a rubber band, melting of ice, melting of wax
Chemical change- rusting of iron, burning of paper, boiling of an egg, frying and egg.
5. Write 10 examples of Reversible and Irreversible changes.
Ans Reversible change- stretching a rubber band, folding a paper, modeling clay, inflating a balloon, drawing a line with a pencil, bending of straw, opening a plastic lid, melting of ice, evaporation of water, water into ice.
Irreversible change- growth of a child, aging, baking a cake, cutting of vegetable,breaking a chair, slicing a bread, burning of paper, curdling of milk, breaking a glass , breaking a magnet
6. Define reversible and irreversible change.
Ans Reversible change- changes that can be reversed by reversing the cause to get the original substance is called a reversible change. eg inflating a balloon, etc
Irreversible change- Changes that can't be reversed by reversing the cause to get the original substance is called and irreversible change. eg rusting of iron.
7. Define physical change and chemical change.
Ans Physical change- A change in which a substance undergoes a change in its physical properties is called physical change.
Chemical change-A change in which one or more new substance are formed is called chemical change.
8. Write some examples of changes caused by heating.
Ans The changes caused by heating are-
1.Some objects get simply heated but no other changes take place.
2.Some objects expand on heating.
3.Some objects burn on heating.
4.Some objects change their state.
9. Classify the following changes into physical and chemical changes-
a) Melting of was- melting of wax is a physical change as there is a change in state and shape which is a physical properties.
b) Burning of paper – burning of paper is a chemical change as a new substance is formed (ash) and the original substance cannot be retained.
10. How does a blacksmith change a piece of iron into different tools?
Ans A blacksmith first heats a piece of iron till it becomes red hot. Then it becomes soft and can be beaten into desired shape.
11. How is a wooden handle fixed to an iron tool?
Ans The iron blade of the tools has a ring to which the wooden handle is fixed. Normally the ring is slightly smaller than the wooden handle. To fix the handle, the ring is heated and it expands, now the handle easily fix to the ring. When the ring cools down it contracts and fix tightly to the handle.
12. How is a metal ring fixed on a wooden wheel of a cart?
Ans The metal ring is made slightly smaller than the wheel. On heating the ring expands and fixed on the wheel. Cold water is then poured over the rim, which contracts and fits tightly to the wheel.
13. Why is the shaping of wet clay into a clay pot a reversible change, whereas baking a clay pot is irreversible change?
Ans The shaping of wet clay into a clay pot is a reversible change because it can be change to wet clay (get the original substance back) . And baking of a clay pot is irreversible as the pot cannot be changed to wet clay.
14. You must have seen that construction workers heat a black thing called Coal Tar to repair roads. State whether the heating of coal tar is reversible or irreversible.
Ans When coal tar is heated it melts to form a thick dark liquid (change of state). Melting of coal tar is a reversible change as it solidifies on cooling.
15. When water is mixed with plaster of Paris and allowed to dry. It sets into a hard mass. State whether it is reversible or irreversible.
Ans This is an irreversible change as when water is added to Plaster of Paris it sets to its hard form and can't be converted back.
8. Write some examples of changes caused by heating.
Ans The changes caused by heating are-
1.Some objects get simply heated but no other changes take place.
2.Some objects expand on heating.
3.Some objects burn on heating.
4.Some objects change their state.
9. Classify the following changes into physical and chemical changes-
a) Melting of was- melting of wax is a physical change as there is a change in state and shape which is a physical properties.
b) Burning of paper – burning of paper is a chemical change as a new substance is formed (ash) and the original substance cannot be retained.
10. How does a blacksmith change a piece of iron into different tools?
Ans A blacksmith first heats a piece of iron till it becomes red hot. Then it becomes soft and can be beaten into desired shape.
11. How is a wooden handle fixed to an iron tool?
Ans The iron blade of the tools has a ring to which the wooden handle is fixed. Normally the ring is slightly smaller than the wooden handle. To fix the handle, the ring is heated and it expands, now the handle easily fix to the ring. When the ring cools down it contracts and fix tightly to the handle.
12. How is a metal ring fixed on a wooden wheel of a cart?
Ans The metal ring is made slightly smaller than the wheel. On heating the ring expands and fixed on the wheel. Cold water is then poured over the rim, which contracts and fits tightly to the wheel.
13. Why is the shaping of wet clay into a clay pot a reversible change, whereas baking a clay pot is irreversible change?
Ans The shaping of wet clay into a clay pot is a reversible change because it can be change to wet clay (get the original substance back) . And baking of a clay pot is irreversible as the pot cannot be changed to wet clay.
14. You must have seen that construction workers heat a black thing called Coal Tar to repair roads. State whether the heating of coal tar is reversible or irreversible.
Ans When coal tar is heated it melts to form a thick dark liquid (change of state). Melting of coal tar is a reversible change as it solidifies on cooling.
15. When water is mixed with plaster of Paris and allowed to dry. It sets into a hard mass. State whether it is reversible or irreversible.
Ans This is an irreversible change as when water is added to Plaster of Paris it sets to its hard form and can't be converted back.
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