ICSE Class 7 Chemistry Chapter 3 Physical and Chemical Changes Chemical Equations

Physical And Chemical Changes: Chemical Equations

In This Chapter You Will Learn

Physical and chemical changes

Chemical reactions

How a chemical equation represents a chemical reaction

How to balance a chemical equation

The various types of chemical reactions.

Changes Around Us: Physical And Chemical Changes

Almost all substances undergo change. These changes have been taking place around us continuously, such as day and night, change of season, ripening of fruits, burning of fuel, cooking of food, etc.

There are different kinds of changes - natural and man-made, slow and fast, periodic and non periodic, desirable and undesirable, small and large, reversible and irreversible, temporary and permanent, etc.

All these changes are exciting and each one of them has a reason behind it. Since most of the changes are different in nature it is necessary to study them in detail. (You have studied about various kinds of changes in detail in Class VI).

Physical And Chemical Changes

In science, all kinds of changes taking place in substances are broadly classified into two types. They are (i) Physical change and (ii) Chemical change.

Physical Change

A physical change is a temporary change, in which no new substance is formed and the chemical composition of the original substance remains the same, even though some of its physical properties, like colour, state, shape, size, etc., might change.

Examples: Water changes into ice on freezing which again melts into water when kept at room temperature. This indicates that changing of water into ice and ice into water is a temporary change and there is no change in the chemical composition, only the state changes.

Similarly, when a carrot is cut into pieces its shape and size changes but there is no change in its chemical composition and taste. In both the above examples no new substances are formed. Hence they are physical changes.

Characteristics Of Physical Change

1. No new substance is formed.

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

3. The chemical properties of the original substance remain the same.

4. The change is temporary and can be reversed by changing the conditions in most of the cases.

5. There may or may not be any change in the state, size, shape, colour and smell of the substance that undergoes physical change.

6. As a result of the change, there may or may not be any change in the energy possessed by the original substance.

Examples Of Physical Change

Melting of ice, glowing of an electric bulb, cutting of glass, heating of an electric iron, boiling or freezing of water, making of a sugar or salt solution, beating of metals into sheets, formation of clouds, sublimation of iodine, crystallisation of sugar, etc.

The case of carrot cut into pieces cannot be reversed.

Chemical Change

A chemical change is a permanent change, in which new substances are formed whose chemical composition, physical and chemical properties are different from those of the original or parent substance(s).

Examples: When a piece of paper is burnt, it turns into ash. This is a new substance whose properties are completely different from those of paper. Ash cannot be changed back into paper.

When a piece of iron is left for a few days in air, containing moisture, it slowly forms a reddish brown substance called rust. Rust is a new substance completely different from iron in its properties and cannot be changed back into iron.

Characteristics Of Chemical Change

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

2. The composition and the properties of the parent substance(s) change(s).

3. The change is permanent and cannot be reversed by changing the conditions or through ordinary physical processes.

4. There is an exchange of energy during a chemical change, which means that heat and light may be given out or absorbed.

5. There is usually a change in the mass of the parent substance(s), as well.

Examples Of Chemical Change

Cooking of food, formation of curd from milk, ripening of fruits, digestion of food, formation of gases, burning of coal, wood and cooking gas, rusting of iron, respiration in living beings, etc.

An Example Showing Simultaneous Physical And Chemical Change

When a candle is lighted, some of the solid wax melts and turns into a liquid. If this liquid drops to the floor or on a table, it re-solidifies. This is a physical change, involving only a change in state, which is reversible.

But most of the molten wax rises up in the wick, turns into vapour and burns with a flame. This forms two new substances, water vapour and carbon dioxide, and the size of the candle decreases. This is a chemical change, and it cannot be reversed.

Thus, we see that the melting of wax is a physical change but the burning of a candle is a chemical change. Both the changes take place at the same time.

Differences Between Physical And Chemical Change

Teacher's Note

Physical changes like boiling water for tea and chemical changes like milk turning sour happen daily in our kitchens - understanding these helps us appreciate the science behind cooking.

Chemical Reactions

Any chemical change in matter which involves its transformation into one or more new substances is called a chemical reaction.

The substances that react with one another are called reactants, and the new substances thus formed are called products.

Reactants - Products

Both reactants and products are pure substances, i.e., elements and compounds.

Example 1

Hydrogen + Oxygen - Water

In the above reaction we see that hydrogen and oxygen are the elements that undergo chemical change to form a new substance, water, a compound which is completely different from hydrogen and oxygen. Hydrogen and oxygen are the reactants and water is the product.

Example 2

Potassium chlorate with heat and MnO2 as catalyst produces Potassium chloride + Oxygen

In this reaction, potassium chlorate a compound, on heating, changes into two new compounds; potassium chloride and oxygen, which are compound and element respectively.

Thus a chemical reaction may involve combination of two or more elements or compounds reacting with each other to form compounds or it may involve splitting of a compound into two or more elements or compounds.

We have already learnt about physical and chemical changes of matter. So whenever a chemical change occurs, we can say that a chemical reaction has taken place. Let us perform the following activity to know how a chemical reaction takes place.

Activity 1

Caution: This activity should be shown by the teachers to the students. It would be better if students wear eye protection.

Clean a magnesium ribbon about 2 cm long by rubbing it with sand paper.

Hold it with a pair of tongs. Burn it using a spirit lamp or burner and collect the ash so formed in a watch-glass. Burn the magnesium ribbon keeping it as far away as possible from your eyes.

What do you observe?

You must have observed that magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen present in the air.

Magnesium + Oxygen - Magnesium oxide

2Mg + O2 - 2MgO

Conditions Necessary For Chemical Reactions

A chemical reaction takes place when one or more of the following conditions are fulfilled.

1. Close contact: For a chemical reaction to take place the reactants should be brought into close contact, i.e., they should be mixed.

For example: Sodium reacts with water violently when they come in contact with each other to produce two new substances, sodium hydroxide and hydrogen.

Sodium + Water - Sodium hydroxide + Hydrogen

2. Solution form: Some substances react with each other only when they are mixed in the solution form.

For example: When sodium chloride solution is added to silver nitrate solution a white precipitate of silver chloride and a soluble sodium nitrate are formed.

Silver nitrate (aqueous solution) + Sodium chloride - Silver chloride (white ppt.) + Sodium nitrate (solution)

Precipitate: An insoluble solid formed on reaction of two solutions is called a precipitate.

Teacher's Note

When we mix baking soda with vinegar, the fizzing reaction demonstrates how solutions must meet to create new substances - this same principle makes cleaning your house chemistry in action.

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