Class seven Science Physical And Chemical Change
Chapter 6: Physical And Chemical Change
In a physical change, a substance undergoes changes only in its physical properties such as shape, size, colour, state, taste and odour and no new substance is formed. It is a temporary change. Ex. Melting of wax, tearing of paper, breaking of glass, sublimating dry ice, mixing sand and water, shredding of paper. Physical changes can be characterised by the following points.
(a) It is a reversible change.
(b) It is a temporary change.
(c) It changes only for the time being.
(d) In this change, heat involves a but very little.
“Change is the law of nature”. Some common changes we see around us are souring of milk. Rusting of iron, burning of fuels, melting of ice,
ripening of fruits, water change into vapour etc.
Some more examples of physical changes :
(i) Evaporation or condensation of water
(ii) The hammering of iron sheet
(iii) Stretching of rubber band
(iv) Glowing of an electric bulb
(v) Expansion of substances on heating
(vi) Dissolution of salt in water
(vii) Boiling of water
(viii) Tearing of paper
(ix) Melting of wax
(x) Whipping of cream
(xi) Contraction of substances on cooling
A change is termed as chemical if it is permanent, in which the composition and chemical properties of a substance get changed and one or more new substances are formed is called chemical change. A chemical change may be characterised by the following points :
(a) It is a permanent change.
(b) It is not reversible.
(c) During a chemical change, a large amount of energy is either absorbed or released.
(d) It is an irreversible change.
Some more examples of chemical changes :
(i) Digestion of food
(ii) Churning of Curd
(iii) Germination of seeds
(iv) Respiration by animals
(v) Electrolysis of water to convert hydrogen and oxygen
(vi) Making curd from milk
(vii) Photosynthesis by plants.
(viii) Burning of fuels
(ix) Cooling of food
(x) Burning of wax
A chemical reaction is a process which transforms one or more parent substances into new substances with new properties. For example: when green-coloured copper carbonate is heated strongly, a black residue of copper oxide and carbon dioxide, a new substance is formed which have altogether different properties. In the equation below the substance that is on the left side from the arrow are called reactants and the substances on the right side are called products.
Characteristics of chemical reactions :
When a chemical reaction takes place it is accompanied by one or more of the following characteristics:-
1. Change of state during the reaction :
A substance changes its physical state from solid to liquid and liquid to gas on heating and after cooling gases changes to a liquid and liquids changes to solid due to change in the forces of attraction present between the particles.
2.Energy Changes during a chemical reaction :
The chemical reaction taking place is mainly of two types :
(i) Exothermic reaction and
(ii) Endothermic reaction
(i) Exothermic reaction: The reactions.
(i) Exothermic reaction: The reactions which give off energy are termed as exothermic reactions i.e. a chemical reaction in which a product and heat are given out by reactants. For example, the burning of methane insufficient
amount of oxygen, carbon dioxide and water. Energy is produced in the form of heat and light. So, the burning of methane is an exothermic reaction.