Contents
Stoichiometry is a fundamental concept in Chemistry Topics that deals with the quantitative relationships between reactants and products in chemical reactions.
What is a Reversible Change?
We will now discuss reversible changes and irreversible changes around us in detail, one by one. Let us start with reversible changes.
A change which can be reversed to form the ‘original substance’ is called a reversible change. This will become clear from the following example. 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. Now, if we cool the water (formed from ice) by keeping in the freezer of a refirgerator, it again changes into solid ice. So, the change from ice to water, by heating, has been reversed by cooling. Thus, the melting of ice (to form water) is a reversible change. This reversible change can be represented as follows :
We can also represent this reversible change with the help of a diagram as follows :
When we keep some ice at room temperature, it appears to melt on its own to form water. In this case, the heat required to melt ice is supplied by the surrounding air (which is at a higher temperature than ice).
There are a very large number of reversible changes around us. Some of the examples of reversible changes (or changes which can be reversed) are : Melting of ice ; Boiling of water ; Melting of wax ; Stretching of a rubber band ; Stretching of a spring; Inflating a balloon; Ironing of clothes; Folding of paper; Rolling a cha patti (roti) from dough; Dissolving salt in water ; Dissolving sugar in water ; Knitting of sweater (Woollen yam 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 (trousers, etc.) to pass through waterlogged area ; Moulding of wet clay into pot on a potter’s wheel; and Softening of iron on heating to red-hot stage. We will discuss some of these changes in somewhat detail.
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. This can be shown as follows :
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 a reversible change.
If we stretch a rubber band with the 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. This means that the change which occurred in rubber band on stretching, has been reversed on releasing. So, the stretching of rubber band is a reversible change. Similarly, the stretching of a spring is also a reversible change.
Take some salt and dissolve it in water taken in a beaker. A salt solution is formed. A change has occurred in salt during the formation of salt solution (because salt has disappeared into water). This change can, however, be reversed as follows : 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. This is because when hot, molten coal tar gets cooled, it solidifies again.
A blacksmith (lohar) changes the pieces of iron into different tools like hammer, axe, spade, etc. This is done as follows : 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 (being soft) can be hammered easily into the desired shape to make a tool. And when the hot iron tool is cooled, it becomes hard again. Thus, 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.
In order to pass through a waterlogged area (as during rains), we usually shorten the length of our dress (like trousers, etc.) temporarily, by folding it up. The folding up of a dress (trousers, etc.) to pass through a waterlogged area is a reversible change. This is because it can be reversed by unfolding the dress.
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 expansion of an object on heating, is a reversible change. This is because when the hot object is cooled, it decreases in size and comes back to the original size. The decrease in size of an object on cooling, is called contraction. Thus, in most simple terms : expansion means increase in size and contraction means decrease in size. Expansion occurs on heating whereas contraction occurs on cooling. The reversible change of expansion is used :
- in fixing an iron rim on the wooden wheel of a cart, and
- in fixing the iron blade of a digging tool (like a spade) to a wooden handle.
1. Fixing of Iron Rim to the Wooden Wheel of a Cart
All of us have seen the wooden wheels of bullock carts and horse carts (tongas) having thin iron rims (or thin iron tyres) around them. The iron rims are fitted around wooden wheels by the process of expansion on heating (followed by contraction on cooling). This is done as follows.
The iron rim 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
[see Figure (A)], 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 to the wooden wheel [see Figure (b)],
2. Fixing of Iron Blade of a 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 (followed by contraction on cooling). This is done as follows.
The iron blade of spade has a ring in which the wooden handle is to be fixed [see Figure (A)]. The iron ring of spade blade 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 a 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 (becomes smaller in size) and fits tightly on to the wooden handle [see Figure (b)].