Lesson: Physical and Chemical Changes

## Type 1 $–$ Very Short Answer Questions (5 Q.)

Question: 1

Tearing of paper is an example of physical reversible change. Is this statement true?

Solution:

No. This change is irreversible.

Question: 2

What happens when an article of iron is kept in the open?

Solution:

Any article left is open gets rusted due to formation of iron oxide.

Question: 3

What is the product formed on burning of magnesium?

Solution:

Magnesium oxide

Question: 4

Give the general name of copper sulphate.

Solution:

Blue vitriol or neela thotha.

Question: 5

How is rusting of a bicycle prevented?

Solution:

By applying paint or grease.

## Type 2 $–$Short Answer Questions (5 Q.)

Question: 1

Explain why boiling of water is a physical change although heat is involved.

Solution:

Boiling of water is a physical change because when we boil water it only changes to another state, which is steam but its chemical composition remains the same.

Question: 2

Write the equation of conversion of magnesium oxide into magnesium hydroxide.

Solution:

Magnesium oxide (MgO) + Water (H2O) $\to$

Magnesium hydroxide [Mg(OH)2]

Question: 3

Write the equations of the reaction between carbon dioxide and lime water.

Solution:

Carbon dioxide (CO2) + Lime water [Ca(OH)2] $\to$

Calcium carbonate (CaCO3) + Water (H2O)

Question: 4

Shruti cut an apple and placed half of it in the open. After a few hours she observed the following change.

She said that this is a physical change but Saksham differed and said that this is a chemical change. Can you tell who is correct? Also, give an explanation to support your answer.

Solution:

Saksham is correct, as the change of colour, in this case, is due to the formation of a new substance which is formed by the reaction of the exposed apple with oxygen present in the air.

Question: 5

Why is it that ships suffer a lot of damage from rusting in spite of being painted?

Solution:

A ship is made of iron and it is always in contact of water, partly inside and the remaining outside water. Also, the salt content of sea water is very high. Thus, the process of rust formation is even faster than it normally happens on land.

## Type 3 $–$Long Answer Questions (4 Q.)

Question: 1

What is rust? Explain why rusting is a chemical change.

Solution:

The deposition of a layer of iron oxide on an iron article, when it reacts with oxygen in the presence of water or water vapour, is called rusting.

When an iron article is left in open, iron reacts with oxygen in the presence of water or water vapour in the environment and forms iron oxide. This reddish-brown chemical compound is called rust. So rust is hydrated iron oxide.

In the process of rusting a new substance (iron oxide compound) is formed. So rusting is a chemical change. The chemical reaction of the process is:

Iron (Fe) + Oxygen (O2, from air) + Water (H2O) $\to$ Rust (Iron oxide, Fe2O3)

Question: 2

How is a physical change different from a chemical change?

Solution:

In physical change, the state, appearance, form, shape and size of a substance changes but not the substance itself. When a sheet of paper is folded, its shape changes and when it is torn, its size changes. These are examples of physical change. These changes are generally reversible.

On the other hand, in chemical change, the substance itself changes because its internal properties alter to form one or more new substance/s. When a strip of magnesium is burned in the presence of oxygen, it burns with a dazzling light. In the process magnesium oxide is produced which is a new substance, and from this newly formed compound, the original substances cannot be retrieved by simple physical processes.

Question: 3

What is crystallisation? Give an example of the use of the process of crystallisation. Explain, why it is a physical change?

Solution:

Crystallization is the process of separating dissolved solid from a solution by formation of crystals. It is also used for removing impurities of a solid.

For removing impurity, a solid substance is mixed in a suitable solvent and is stirred till it gets dissolved. The solvent is heated so that more solute can be dissolved in it. When more and more solid is added to the liquid, a point comes beyond which no more solid can be dissolved in the liquid. Then, the solution is left undisturbed in an open container to cool down slowly. When it cools, the large-sized pure crystals settle at the bottom of the vessel whereas the impurity remains in the solution.

The process of crystallization is used in the purification of sea water. The technique of crystallisation is a physical change because the crystals can be dissolved again in the solvent to form a solution.

Question: 4

Is preparation of curd from milk a physical change? Explain with suitable reasons.

Solution:

Preparation of curd from milk is not a physical change. It is a chemical change because when milk undergoes the process of curdling, a new is substance formed, which has different properties. Also, milk and curd differ in taste. Milk is sweet to taste whereas curd is sour. They differ in composition as well. Milk has more lactose content while curd contains lactic acid. Also, conversion of milk to curd is irreversible. Milk cannot be obtained from curd.