Every substance in this universe undergoes some change or the other. These changes can be classified be slow or fast, reversible or irreversible, periodic or non-periodic, desirable or undesirable, and physical or chemical changes. All changes can be broadly classified into physical and chemical changes.
Physical Change
Properties like shape, size, colour and state of a substance
are called physical properties and any changes in these properties of a
substance are called physical changes.
Press the switch of an electric bulb. The bulb glows and
gives out light and heat. Now switch it off. The bulb stops glowing. After some time, if you touch the bulb it is no longer hot.
Take some water in a beaker add a teaspoon of salt to it
and stir well. The salt dissolves completely. Taste the solution. It will have
a salty taste. Now heat the solution, until all water evaporates. A white
substance, salt will be left behind in the beaker.
The changes that take
place in these examples are:
No new substance is formed.
The change is temporary and reversible.
In both cases, the change affects only the physical
properties of the substance.
Chemical change
A change in which one or more new substances are formed and the change is irreversible is a chemical change.
Consider the following
examples
Burn a piece of wood using a match stick. Ash is left behind
as a residue.
Take some milk in a vessel and add a few drops of curd into
it. Leave it undisturbed for a few hours. The milk changes into curd.
Using a pair of tongs, take a piece of magnesium ribbon and
heat it over a burner. It starts burning with a dazzling light and produces
dense white fumes. The white fumes, on cooling, leave behind a white residue of
magnesium oxide.
New chemical substances
having a composition and properties different from those of the original
substances are formed:
Wood to ash
Milk changes to curd.
Magnesium changes to magnesium oxide.
The changes are permanent and irreversible; we cannot get
back the original substance either by physical or chemical methods.
We cannot get back wood from ash.
We cannot get back milk from cur.
We cannot get back magnesium from magnesium oxide.
A considerable amount of energy is evolved or absorbed. All
these changes are chemical changes. A chemical change is a permanent change in
which the original substance gives rise to one or more new substances, with a
different composition and different properties.
Differences between
physical and chemical change
|
Physical change |
Chemical change |
|
It is a temporary
change. The original substance can be obtained back by simple means like
heating, cooling etc., |
It is a permanent
change. The original substance cannot be brought back. |
|
No new substance is
formed. The original substance does not lose its composition. |
A new substance is
formed. The composition of these substances is different. |
|
Change may be
reversible. |
Change is
irreversible. |
|
Only the physical
properties like shape, colour, size etc., get changed. |
Both the physical and
chemical properties of the substance get changed. |
|
Energy may or may
not be absorbed or evolved. |
Energy is either
absorbed or evolved. |
Rusting of iron is a chemical change that can be prevented by depositing a layer of zinc on iron. This process is called galvanization. Water pipes are galvanized to prevent rusting. Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust. The formation of large crystals of pure substance from their solution is called crystallization. It is a physical change.