IMPORTANT TERMS

Evaporation -               Changing from a liquid form to gaseous form is called evaporation.

Condensation -             Changing from a gaseous form to liquid form is called condensation.

Freezing -                    Changing from a liquid form to a solid form is called freezing.

Melting -                      Changing from a solid form to a liquid form is called melting.

Sublimation-                 Changing from solid form directly to gaseous form is called sublimation.

CLASSIFICATION OF CHANGES

Physical Change-         Changes in which no new substances are formed are called physical changes. They are generally reversible. E.g. Melting of wax, solution of sugar and water, Boiling of water.

Chemical Change-        Changes in which new substances with different properties are formed are called chemical changes. They are generally irreversible. E.g. Burning of paper, growth of nail.

Reversible Change-      Changes that can be carried out in reverse direction. E.g. Water changes into ice on cooling, wax change into liquid on heating, blowing of balloon.

Irreversible Change-     Changes that cannot be carried out in reverse direction. E.g. Cooking of food, Growth of plant and animals.

Exothermic Changes-   Change in which heat is given out is called exothermic changes.

Example-----Carbon + oxygen (On burning) -----> Carbon dioxide + Heat

Example-----Quick lime + Water   ------>Calcium hydroxide + Heat

 

Endothermic Changes- The changes takes place by absorption of heat is called Endothermic Changes.

Example------Iron + sulphur      (On Heating) ----->     Iron sulphide

Example-----Milk + sugar        (on Heating)  ----->  Sweet milk

Fast changes-              Changes that occur quickly is called Fast changes. E.g. burning of match stick, burning of fire cracker, Evaporation of petrol.

Slow Changes-             Changes that occur slowly is called slow change. E.g. growth of nail, rusting of iron.

Periodic change-          A change that occurs during a definite time interval is known as periodic change. E.g. phases of moon, heart-beat.

Non-periodic change-   A change that does not repeat again and again after a regular interval of time is called non-periodic change. E.g. earthquake, flood.

Desirable Changes-      Those changes which are desired to occur that is we want them to take place are known as desirable changes. E.g. ripening of fruits, germination of seeds.

Undesirable change-     Those changes which we do not want to take place are called undesirable changes. E.g. souring of milk, rotting of fruits.

 

CAUSES THAT BRING CHANGES

Change in Shape - e.g. Change in shape of balloon by blowing air in it.

Change in Size - e.g. Balloon kept in hot water expands.

Change in Physical State - e.g. Wax melts on heating.

Change in Position - e.g. Throwing a small piece of rock from one place to another.

Change in Colour - e.g. Heating a piece of iron and it becomes red, exploding of fire crackers.

Change in Internal Structure - e.g. Cooking of food changes the internal structure.

 

EXPANSION and CONTRACTION

Contraction -                The decrease in size of matter on cooling is called contraction.

Expansion -                  The increase in size of matter on heating is called expansion.

Advantage and Disadvantage of Expansion

One advantage of expanding metal is used for fitting Iron rim over a wooden wheel. The rim is made slightly smaller in size than wooden wheel. It is heated so that it expands and is fitted over wooden wheel. Thereafter, cold water is poured over it so that it contracts and fits the wooden wheel tightly.

One disadvantage of expansion is that roads crack during the cold season because they contract and expand during the hot summer. This expansion can make road surfaces rough. Metal and steel structures used in bridges also expand when they heat up, causing cracks in the bridge.

TABLE FOR EXPANSION AND CONTRACTION OF SUBSTANCES

Expansion In Solids	Expansion In Liquids	Expansion in Gases
Railway tracks consist of two parallel metal rails joined together. Small gaps, called expansion gaps are deliberately left in between the rails as there is expansion of the rails in hot weather.	Water expands on heating. Take a glass filled with water up to the brim and pour its water in a frying pan for heating. If we pour the water back in same glass, the water overflows.	If we keep an inflated balloon in the sun for some time, It will grow in size as the air inside it expands on taking heat from the surrounding areas.
Contraction in Solids	Contraction in Liquids	Contraction in Gases
If we hold a very hot glass tumbler under cold water, it cracks. This is because the outer surface of the glass comes in direct contact with cold water and contracts more as compared to the inner surface.	Water contracts on cooling. Take a beaker filled with water up to a certain level and cool it to 50 C. If we pour the water back and check the level of the water in same beaker, it shows a decreased level.	If an inflated Balloon is tied at the mouth of the bottle and the bottle is placed in ice cold water, the balloon will in size, as the air inside the balloon contracts on cooling.

 

Applications of Expansion and Contraction in Day-to-Day life

• The jammed metal lid of jam jar can be opened by heating. The jar is inverted and the lid is dipped in hot water. After some time, the metal lid can be opened easily as it will get slightly expanded.
• The mercury in thermometer rises due to its expansion on heating and thus bulb of the thermometer containing mercury comes in contact with hot object its level raises and indicate us the temperature.
• Electric Wires sag in summer due to expansion and thus hang loose from the poles in summer. However, they themselves contract in cold weather and become taut in winter.

INTRESTING FACTS

• Water contracts when cooled upto 4 degrees Celsius and then expands to form ice.
• When water freezes it expands by 9%.
• Gold melts at 1,064 degrees Celsius and boils at 2,900 degrees Celsius.
• Sponges hold more cold water than hot.
• Room temperature is defined as between 20 to 25 degree Celsius.
• Mercury is the only metal that is liquid at room temperature.