PHYSICAL AND CHEMICAL CHANGES

PHYSICAL AND CHEMICAL CHANGES

All the changes taking place around us can be classified into the following two types:

• Physical changes and
• Chemical changes

PHYSICAL CHANGES:

Definition: A change which alters some specific physical property of the matter, like its state, texture, magnetic or electrical conditions or its colour, without causing any change in the composition of its molecules, is called physical change, provided it get reversed, if the cause producing the change is removed.

Following points need special consideration:

(i) Now new or different product is formed : The composition of molecules of the substance remains unaltered.

e.g. Melting of ice and freezing of water to form ice: When ice is heated, it melts to form liquid water, conversely, when water is cooled in a refrigerator, it freezes to form ice. Since both ice and liquid water are made up of same water molecules (H2O), no new substance is formed during melting of ice and freezing of water. Hence melting of ice and freezing of water to form ice are physical changes.

(ii) The change is temporary and reversible : It means the change can be reversed by altering the causes which produce the change.

e.g. The water formed from ice can be changed back to ice by placing it in a freezing mixture (a mixture of ice and common salt).

On altering the experimental conditions, the change which gets reversed, is a physical change.

(iii) There is no net gain or loss of energy : The amount of energy required to bring about a physical change is generally equal to the amount of energy required to reverse the change. Thus, there is not net energy change involved.

e.g. If 1 g of water of 100 o C on changing into steam needs 2260 J of heat energy, then 1 g of steam at 100 o C on changing into water at 100 o C, gives out 2260 J of h eat energy. Thus, the net energy change is zero.

(iv) There is no change in the weight of substance: During a physical change it is only the energy which is added or removed. No matter is added during a physical change. Similarly, no matter is removed during a physical change. Therefore, mass of the substance remains same.

Some Examples Involving Physical Changes:

Some Common Examples of Physical Changes:

• Formation of dew.
• Evaporation of water.
• Crystallisation of sugar from its solution.
• Ringing of an electric bell.
• Breaking of a glass pane.
• Making of ice cream.
• A rock rolling down a hill.
• Bending of a glass tube by heating.
• Melting of wax.
• Sublimation of camphor.

CHEMICAL CHANGES:

Those changes in which new substances are formed are called chemical changes. In a chemical change, the original substances loses their chemical identity and get converted into new substances. The new substances thus formed cannot be converted into original substances by any physical process. Thus, chemical changes are permanent and hence it is irreversible.

Following points needs special consideration:

(i) A chemical change results in the formation of one or more new products : The products formed have different properties than the original substance. Thus, the composition of the molecules of products is different from the original substance.

e.g. Heating of sugar

When sugar is gently heated in a test tube, it melts. It gradually changes to brown colour, giving a large amount of steamy fumes. In the end a black mass is left which consists of carbon. Thus, new substances, viz. carbon and water (steam) are formed. In this change, the arrangement between the molecules of carbon, hydrogen and oxygen breaks. The hydrogen and oxygen atoms separate from carbon atoms and join together to form water. The carbon atoms are set free and are left as black residue.

Sugar carbon + Steam

(ii) The weight of the substance undergoing chemical change usually changes:

e.g. During the heating of sugar, the weight of the black residue is far less than the actual weight of the sugar. However, this is an apparent change in weight. If we take the weight of steam into account and add to it the weight of carbon, then total weight will be equal to the weight of sugar crystals. Thus, total weight of substances taking part in a chemical change remains constant.

(iii) The chemical change is permanent and irreversible : It means the change will not reverse by altering the experimental conditions.

e.g. The sugar, which has decomposed on heating to form carbon and steam will not change to sugar on cooling.

(iv) During chemical change energy is either absorbed or given out : The various atoms in a chemical compound are joined by attractive forces commonly called bonds. The making or breaking of the bonds always requires exchange of energy. Thus, some amount of heat is either absorbed or given out during a chemical change.

Some Examples Involving Chemical Changes:

Some Common Examples  of Chemical Changes:

• Burning of wood or charcoal
• Burning of candle
• Digestion of food
• Curdling of milk
• Formation of biogas (Gobar gas)
• Burning of petrol or diesel
• Smoking of cigarette
• Drying of paint
• Rusting of iron
• Ripening of fruit
• Clotting of blood
• Fading of the colour of a dyed cloth
• Baking of cake
• Photosynthesis
• Formation of wine
• Butter turning rancid
• Decomposition of water into hydrogen and oxygen.
• Formation of water from hydrogen and oxygen

(c) Difference Between Physical and Chemical Changes