A process which proceeds of its own accord without any outside assistance is termed as spontaneous or natural process. The reverse process which does not proceed on its own is referred to as a non-spontaneous or unnatural process.
In general, the tendency of a process to occur naturally is called the spontaneity.
(i) Rolling ball
A ball rolls down hill spontaneously but it will not roll uphill unless work is done on it.
(ii) Heat flow
When two balls of metal, one hot and one cold are connected, heat flows spontaneously from the hot ball to the cold one, never from cold to hot. It takes work to transfer heat from one piece to the other, say by means of refrigeration pump.
(iii) Gas mixing
When two gases are mixed, they spontaneously form a gaseous mixture. Gas mixtures do not mix themselves out so that pure gases concentrate to opposite ends of the container.
(iv) Gas flow
When a vessel containing gas is connected to another evacuated vessel, the gas spreads throughout spontaneously unless the pressure is the same in both the vessels. The reverse process of compressing the gas into the original vessel does not occur unless work is done on it.
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