Chemical and Physical Change

There are two kinds of change matter may undergo: These are chemical and physical change.

Chemical change involves the alteration in the chemical constitution of the substance which undergoes it, while physical change does not involve the alteration of the chemical composition of the substance but the alteration of its physical state. Based on this underlying difference, both changes exhibit certain characteristics by which you can easily identify them. These characteristics are expressed in the table below:

CHEMICAL CHANGE	PHYSICAL CHANGE
1. Always produces new kind of matter with a different mass from the original substance.	Produces no new kind of matter, mass of matter remains the same.
2. Usually accompanied by considerable heat change.	Is not accompanied by great heat change.
3. Is generally not reversible.	Is generally reversible.

Examples of Chemical and Physical Change:

Chemical and physical change include the following in the table below:

EXAMPLES OF CHEMICAL CHANGE	EXAMPLES OF PHYSICAL CHANGE
1. The burning of any substance in air, including candle..	1. The heating of a metal wire by electricity.
2. The addition of water to calcium oxide.	2. The dissolution of sodium chloride in water.
3. All explosions, including that of natural gas or hydrogen with air; dynamites and bombs.	3. All cases of melting of a solid to a liquid (or the freezing of liquid to solid).
4. The rusting of iron.	4. All cases of vaporization of a liquid (or the condensation of a gas to liquid).
	5. Magnetization of iron, as well as the demagnetization of iron.

Change of State

When substances undergo physical change, there are 3 distinct states upon which they can transform – solid, liquid and gaseous. When solid substances gain heat, the tendency is for them to change to the liquid state. This change is known as fusion or melting.

However, some solid substances when heated change directly to the gaseous state, without passing through the liquid state. This change is called sublimation. Liquid substances when heated change their state to gaseous. This is called vaporization.

Remember that these changes are reversible: when heat is taken from a gaseous substance, it eventually becomes liquid. This is known as condensation. The same goes for a solid substance that had sublimed - when the gas is cooled, it changes back to solid. A liquid substance changes to solid when heat is taken from it. This is known as solidification, freezing or crystallization.

Note: when a substance undergoes a change of state, its temperature remains the same. This is because the heat required for the change is latent heat; the substance will contain equilibrium mixture of both states (i.e. initial state and final state).