Reversible Changes

By | June 15, 2018

Question 1 Give few examples of changes which are observed in our everyday life?

Question 2 What is meant by reversible change? Explain with example?

Question 3 State some of the ways of bringing about changes in materials around us?

Question 4 Explain how iron blade of a spade is fixed to a wooden handle?

Question 5 Give example of a beneficial change?

Question 6 Give example of a harmful change?

Question 7 Explain how a metal rim is fixed around the wooden wheel of a cart?

Question 8 Define the term expansion and contraction?

Things around us have certain properties such as state (solid, liquid or gas), position, shape size, colour, temperature, composition and structure. When one or more properties of a thing become different, we say that it has changed or a change has taken place in it.

When ice melts it forms water. Now, we know that ice is a solid whereas water is liquid. So, the melting of ice involves a change in state (from solid state to liquid state).

Examples of important changes in everyday life are formation of curd from milk, burning of fuels ,drying of clothes ,rusting of iron ,Melting of ice, Boiling of water (or Evaporation of water); making of chapatis from flour; burning of fire works, germination of seeds, flowering of plants, ripening of fruits, shedding of leaves by trees, growth of human beings and animals, formation of day and night, position of sun in the sky, changes in seasons, changes in weather and rainfall etc.

Some changes are beneficial to us whereas some are harmful to us. The beneficial changes take place faster but the harmful changes are either stopped or made to go slow. For instance, the ripening of fruits is beneficial change. So, we try to make the ripening of fruits faster by using artificial methods. On the other hand the spoiling of cooked food is a harmful change, so we try to slow down this change by keeping the cooked food in refrigerator.

The change in a material (or object) does not occur on its own. There is always a ’cause’ which brings about a change in material.

Heat is the cause of the change of state of ice from solid to liquid . Some of the ways of bringing about changes in materials are by applying heat, light, electricity, or force.

Classification of Changes
A substance undergoes a change to form a ‘new substance’ only when certain agents like heat, light, electricity or force etc are applied on it. Some of the changes can be reversed whereas other changes can not be reversed. So, all the changes around us can be classified into two groups:

  1. Reversible changes
  2. Irreversible changes

Reversible Changes
A change which can be reversed to form the ‘original substance’ is called a reversible change.

Ice is a solid substance. When we heat ice, it melts to form liquid water. A change from solid to liquid takes place during the melting of ice. If we cool the water from ice to water, by heating, has been reversed by cooling.Thus, the melting of ice (to form water) is a reversible change.

   Heat
Ice      ————————————-> Water
(Solid) <————————————- (liquid)
Cool

Examples of reversible changes  are:

  • Melting of ice.
  • Boiling of water.
  • Melting of wax.
  • Stretching of a rubber band.
  • Stretching of a spring.
  • Inflation of a ballon.
  • Ironing of clothes.
  • Folding of paper.
  • Rolling a chapati(roti) from dough.
  • Dissolving salt in water.
  • Dissolving sugar in water.
  • Knitting of sweater (woollen yarn to knitted sweater).
  • Melting of ice candy.
  • Melting of ice cream (solid ice cream to molten ice cream).
  • Drying of clothes (wet clothes to dry clothes)
  • Heating of milk (cold milk to hot milk)
  • Expansion of metals on heating.
  • Straight wire to coiled wire.
  • Melting of coal tar.
  • Folding of dress (shirts etc)
  • Moulding of wet clay into pot on potter’s wheel and
  • Softening of iron on heating to red hot stage.

When we boil water by heating, then it changes into steam. Now, if we cool the steam, then water is formed again. So, the changing of water into steam has been reversed by cooling. Thus, the boiling of water (or evaporation of water) is a reversible change.

Heat
Water   ————————————-> Steam
                   (Liquid) <————————————- (Water vapour or gas)
Cool

When ice changes into water, then there is a change from solid state to liquid state. And when water changes into steam, then there is a change from liquid state to gaseous state, So in general we can say that : change of state is reversible change.

If we stretch a rubber band with force of our hands, it undergoes a change and its length increases. But, on releasing the force, the rubber band comes back to its original length. So, the stretching of rubber band is a reversible change, the stretching of a spring is also a reversible change.

Take some salt and dissolve it in beaker. A salt solution is formed. A change has occurred in salt during the formation of salt solution. Keep the beaker containing salt solution. Keep the beaker containing salt solution over a burner and evaporate it. On evaporation, water is eliminated and salt is left behind. Thus,  dissolving salt in water is a reversible change.

Coal tar is a black, solid material which is used in making and repairing roads. When coal tar is heated, it melts to form a thick black liquid. The melting of coal tar on heating, is a reversible change because when hot, molten coal tar gets cooled, it solidifies again. The  piece of iron metal is heated in a furnace till it becomes red-hot. At red-hot stage, the piece of iron becomes soft. The red-hot piece of iron tool is cooled, it becomes hard again. The softening of iron on heating to red hot stage is a reversible change. This is because when red hot iron is cooled, it becomes hard again.

Expansion (on heating) is a Reversible Change
When an object is heated, it increases in size. The increase in size on heating, is called expansion. The decrease in size of an object on cooling, is called contraction. Expansion occurs on heating whereas contraction occurs on cooling. The reversible change of expansion is used :

  1. in fixing an iron rim on the wooden wheel of a cart and
  2. in fixing the iron blade of digging tool (like a spade) to wooden handle.

Fixing of iron Rim to the Wooden Wheel of a Cart

Wheel

Wooden wheels of bullock carts and horse carts (tongas) have thin iron rims (or thin iron types) around them. The iron rims are fitted around wooden wheels by the process of expansion on heating (followed by contraction on cooling).

The iron is made slightly smaller than the wooden wheel (around which it is to be fitted). The iron rim is heated uniformly by making a fire due to which it expands and becomes somewhat bigger in size. Being bigger in size, the hot iron rim is easily put around the wooden wheel. Cold water is then poured over the hot rim to cool it. On cooling, the hot iron rim contracts (becomes slightly smaller in size) and fits tightly on the wooden wheel.

Fixing of Iron Blade of Digging Tool to a Wooden handle

The iron blade of a soil digging tool (like a spade) is fixed to a wooden handle by the process of expansion on heating.

The iron blade of spade has a ring in which the wooden handle is to be fixed. The iron ring of spade is made slightly smaller than the thickness of the wooden handle which is to be fixed in it. To fix the handle, the ring of iron blade of spade is heated over fire. On heating, the iron ring expands and becomes slightly bigger in size.

One end of the wooden handle now easily passes through the hot ring. Cold water is then poured over the hot ring of the spade blade to cool it. On cooling, the hot ring of spade blade contracts (become smaller in size) and fits tightly on the wooden handle.

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