Physical and Chemical Changes

Difference Between Physical and Chemical Changes

A physical change is a change in which no new substances are formed. Examples include changes of state such as melting, freezing, evaporation, and condensation.
A chemical change, on the other hand, is a change in which one or more new substances are formed. Examples include burning, rusting, and the reaction between vinegar and baking soda to produce carbon dioxide.

Observations that indicate a chemical change might have occurred include the change in colour, formation of a gas (often seen as bubbles in a liquid), formation of a precipitate (a solid that forms and settles out of a liquid mixture), change in temperature (either getting hotter or colder), or light being given off.
Other common indicators of chemical reactions include changes in energy such as the release of heat or light, or the change in smell or taste.

Reversible changes are changes that can be undone or reversed. For instance, water can be frozen into ice and then be returned to a liquid state by heating.
Irreversible changes are permanent changes that cannot be undone. For example, once a piece of paper is burned, it cannot be turned back into a paper. This is an example of chemical change.

In a chemical reaction, the mass of the products is equal to the mass of the reactants. This is known as the law of conservation of mass.
If the mass seems to change during a reaction it may be due to a gas being produced and escaping into the air or a gas from the air being used up in the reaction.

Chemical reactions can either absorb energy from the surroundings (endothermic reaction) or release energy to the surroundings (exothermic reaction).
In an endothermic reaction, the surrounding area will feel colder. Examples of endothermic reactions include dissolving ammonium nitrate in water.
In an exothermic reaction, the surrounding area will feel warmer. Examples of exothermic reactions include the mixing of vinegar and baking soda.