Physical and Chemical Changes

Physical changes affect the form of a chemical substance, but not its chemical composition.
Examples of physical changes include changing states of matter (e.g. melting ice into water) and dissolving sugar in water.
Physical changes are generally reversible - the original substance can be retrieved through processes such as evaporation or freezing.
Although a substance’s state or form changes physically, its molecular structure stays intact.

In chemical changes or chemical reactions, substances combine or break apart to form new substances.
Chemical changes result in the formation of substances with new properties because the arrangement of atoms is changed.
Indicators of chemical change include colour change, the formation of a precipitate, the evolution of gas, change in temperature, and light emission.
A chemical change is not easily reversible under normal conditions.
In a chemical change, the number of atoms before the reaction (reactants) and after the reaction (products) is the same, which is a reflection of the law of conservation of mass.

The key factor to differentiate between physical and chemical changes is the change in composition: In a physical change the substance retains its composition, in a chemical change a new substance is formed.
Energy, in the form of heat or light, is generally absorbed or released during chemical changes, not physical changes.
Physical changes do not alter the identity of the substance unlike chemical changes.
The reversibility of physical vs chemical changes can also be used as a contrasting point.

Chemical equations are used to describe chemical changes.
In a chemical equation, the reactants are shown on the left and the products on the right.
The arrow in a chemical equation signifies the direction of the reaction and is often read as “reacts to produce”.
Balancing chemical equations ensures that the law of conservation of mass is honoured, i.e. the same number of each type of atom is present on both sides of the equation.