ICSE Class 8 Chemistry Chapter 3 Transformation of Substances

Transformation Of Substances

In This Chapter You Will Learn

Change of state - physical changes

Chemical reactions involving energy changes

Boiling and melting points of pure substances

Chemical equations and simple calculations

Characteristics of chemical reactions

Types of chemical reactions

Distillation

Electrolysis and its applications

Introduction

Change is a universal phenomenon. Almost all substances undergo change. While some changes are easy to detect, some are so small that they are difficult to identify. In this chapter you will learn about the different types of changes and transformation that substances and their derivatives can undergo.

Physical Change, Chemical Change And Chemical Reactions

3.1 Physical Change: Change Of State

A physical change is one in which a substance alters temporarily in some or all of its physical properties, viz. state, shape, size, appearance, etc., but not in its chemical composition.

3.1.1 Characteristics Of Physical Change

(i) No New Substance Is Formed

You might have observed that when a cube of ice is taken out of a refrigerator, it melts into water. If this water is kept back in the refrigerator, it re-freezes into ice. This indicates that the properties of water and ice are the same, i.e. their chemical composition is the same. On melting of ice or on freezing of water no new substance is formed. Only the physical state of the substance changes.

(ii) The Change Is Temporary And Reversible, I.e. The Substance Returns To Its Original State

Example 1: You have noted that when a cube of ice is taken out of a refrigerator, it melts and changes into water. It turns again into ice when put back in the refrigerator. This shows that on removing the cause of change the substance returns to its original state.

Example 2: Gently heat some powdered sulphur in a hard glass test tube. It melts into a pale yellow liquid. Stop heating and allow the test tube to cool. Molten sulphur quickly changes back to the solid state.

The above examples prove that physical change is both temporary and reversible.

(iii) There Is No Change In Mass During A Physical Change

Weigh a beaker containing some solid wax on a beam balance. Melt the wax and again weigh the beaker. You will find that the two weights are identical. This shows that there is no change in the mass of the substance involved as a result of physical change.

(iv) There Is Usually No Gain Or Loss Of Energy As A Result Of Physical Change

Water changes into steam by absorbing a certain amount of heat energy. The same amount of energy is given out when steam changes back into water by giving up its heat. Therefore, we can say that there is no net gain or loss of energy as a result of physical change.

The Characteristics Of Physical Change Can Be Summarized As Follows:

1. No new substance is formed.

2. There is no change in the chemical composition of the original substance.

3. The change is temporary and it can be reversed by reversing the conditions.

4. The change is only in the state, size, shape, colour, texture or the smell of some or all of the substances that undergo physical change.

5. There is no change in the masses of the substances involved in a physical change.

6. There is usually no loss or gain of energy as a result of physical change.

Examples Of Physical Change

The formation of dew, melting of ice, sublimation of iodine, magnetisation of iron, breaking of glass, drying of wet clothes, crystallisation of salt or sugar, dissolution of sugar in water, glowing of an electric bulb, etc., are just a few common examples to name.

A diagram showing the change of states of wax is displayed, illustrating solid wax, molten wax (liquid), and wax vapour (gas).

Another diagram shows the interconversion of states of ice, displaying solid ice converting to liquid water with heat, and liquid water converting to water vapour with heat, with cooling processes in reverse.

Activity 1

Take a carrot and cut it into pieces.

A visual representation of a carrot cut into pieces is shown.

Now answer the following questions:

1. Is the change temporary?

2. Is the change reversible?

3. Why is cutting of carrot into pieces a physical change?

Teacher's Note

Physical changes happen all around us - when ice melts into water or when we tear paper, we can see these changes occur without creating new substances, making them easy to understand from everyday experiences.

3.2 Chemical Change

In a chemical change the original substance gives rise to one or more new substances that have entirely different compositions and properties compared to those of the original substance.

Characteristics Of Chemical Change:

1. New Substance Is Formed:

Take some iron powder and sulphur powder in a test tube and heat them. A grey solid is formed which is not attracted by magnet and is insoluble in carbon disulphide. That means a new substance is formed known as iron sulphide which has properties completely different from the properties of iron and sulphur. [Iron is attracted by magnet and sulphur is soluble in carbon disulphide]

Fe + S \(\rightarrow\) FeS

Iron Sulphur Iron sulphide

2. The Change Is Permanent And Irreversible:

When a piece of paper is burnt a new substance ash is produced. Even when the burning is stopped the ash cannot be changed back into paper. This shows that the formation of the ash from paper is a permanent and irreversible change.

3. There Is Usually A Change In The Mass Of The Original Substance:

Take a piece of magnesium in a crucible with a lid. Weigh it and then heat it by opening the lid after short intervals to let the air enter the crucible. When the whole of the magnesium is burnt, cool the crucible and weigh it again. You will find that the final weight is more than the initial weight. This proves that when a chemical change takes place there is a gain in the mass.

2Mg + O\(_2\) \(\rightarrow\) 2MgO

Magnesium Magnesium oxide

In the above process magnesium combines with oxygen to form magnesium oxide. Hence mass is gained.

4. Exchange Of Energy Takes Place:

When wood is burnt in air carbon dioxide and water vapour are produced but at the same time energy is also released in the form of heat and light.

The Characteristics Of Chemical Change Can Be Summarized As Follows:

1. New substance(s) is/are formed.

2. The composition of the original substance completely changes.

3. The change is permanent and irreversible.

4. There is a change in the mass of the original substance.

5. There is an exchange of energy during a chemical change which means that heat and light may be released or absorbed.

Examples Of Chemical Change

The cooking of rice, the formation of curd from milk, the digestion of food, the formation of salt from acid and base, the burning of fuel, the liberation of gases, etc., are some examples of chemical change.

The Burning Of Candle Is An Example In Which Both Physical And Chemical Changes Take Place Simultaneously.

When a candle is lighted, some of the solid wax first melts and turns into liquid, then it turns into vapour to produce a flame. New substances CO\(_2\) and H\(_2\)O vapour are formed alongwith the evolution of light and heat energy. This shows a chemical change.

When some of the molten wax drops to the floor, it again solidifies which shows a physical change. Thus the melting of candle wax is a physical change and the burning of wax to produce CO\(_2\) and H\(_2\)O is a chemical change.

3.3 Chemical Reaction

You now know that there are various types of chemical changes taking place in our surroundings.

Any chemical change in matter which involves transformation into one or more substances with entirely different properties is called a chemical reaction.

But What Happens In A Chemical Reaction?

A chemical reaction involves breaking of chemical bonds between the atoms or groups of atoms of reacting substances and rearrangement of atoms making new bonds to form new substances.

A diagram shows the breaking of bonds in methane and oxygen molecules, and the formation of new bonds to produce carbon dioxide and water molecules.

Teacher's Note

Chemical reactions are happening in our bodies constantly - when we digest food or when our cells produce energy - these fundamental processes transform one substance into another completely different substance.

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