PHYSICAL AND CHEMICAL CHANGES
All the changes taking place around us can be classified into the following two types:
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Physical changes and
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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:
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Physical Change
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Observation
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Change in Physical Property
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1. Switching on of an electric bulb
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The bulb glows and gives out heat and light energy.
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The physical appearance of the bulb changes.
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2. Rubbing a permanent magnet on a steel rod.
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The steel rod gets magnetized. If it is brought near iron nails, they get attracted.
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The steel rod acquired the property of attracting pieces of iron.
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3. Action of heat on iodine
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The brownish grey crystals of iodine change to form violet vapours. On cooling the vapours condense on cooler parts on the test tube to form crystals.
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Change in state and colour.
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4. Dissolving of common salt in water
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The white crystalline salt disappears in water. However, the water tested exactly like common salt. Moreover, common salt can be recovered by evaporation.
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Change of state.
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Some Common Examples of Physical Changes:
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Formation of dew.
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Evaporation of water.
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Crystallisation of sugar from its solution.
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Ringing of an electric bell.
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Breaking of a glass pane.
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Making of ice cream.
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A rock rolling down a hill.
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Bending of a glass tube by heating.
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Melting of wax.
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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:
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Chemical Change
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Observation
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Equation
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1. Burning of magnesium in air
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When a magnesium ribbon is heated in a flame of Bunsen burner, it catches fire and burns with a dazzling white flame to form white ash.
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Magnesium + Oxygen
Magnesium oxide
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2. Rusting of iron
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When iron (silver grey) is left exposed to moist air for a few days, reddish brown powdery mass (rust) is found on its surface
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Iron + Oxygen (from air) + Water vapour
Rust
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3. Burning of LPG
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When LPG (Liquified Petroleum Gas) is burnt, it burns with a pale blue flame and liberates colourless gas carbon dioxide along with steam.
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Butane (LPG) + Oxygen
Carbon dioxide + Water
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Some Common Examples of Chemical Changes:
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Burning of wood or charcoal
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Burning of candle
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Digestion of food
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Curdling of milk
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Formation of biogas (Gobar gas)
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Burning of petrol or diesel
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Smoking of cigarette
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Drying of paint
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Rusting of iron
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Ripening of fruit
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Clotting of blood
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Fading of the colour of a dyed cloth
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Baking of cake
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Photosynthesis
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Formation of wine
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Butter turning rancid
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Decomposition of water into hydrogen and oxygen.
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Formation of water from hydrogen and oxygen
(c) Difference Between Physical and Chemical Changes
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Physical change
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Chemical Change
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1. The change takes place only in the state, texture, colour, electrical or magnetic properties of solubility, etc. However, molecular properties (composition) do not change.
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1. The change takes place in the state, texture, colour, electrical or magnetic properties along with the change in its molecular properties (i.e. its molecular arrangement changes).
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2. The specific properties of the substance remain unaltered after the physical change.
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2. The specific properties of the substance change completely after the chemical change.
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3. Now new substances are produced.
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3. Always new substances are produced.
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4. There is no change in weight, if a substance is undergoing a physical change.
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4. There is always a change in apparent weight, when a substance undergoes a chemical change.
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5. There is no net absorption or release of energy (such as heat or light energy) during a physical change.
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5. There is always a net absorption or release of energy during a chemical change.
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6. It is temporary change and is usually reversed by removing the cause of the change.
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6. It is a permanent change and cannot be reversed by removing the cause of the change.
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