An introduction to chemical reactions

In a nutshell

Chemical reactions involve reactants undergoing chemical change to produce new products. In this lesson you will learn about the importance of chemical reactions, conservation of mass and some of the signs which show that a chemical reaction is taking place.

Definitions

Key word	Definition
Reactants	Starting materials in a chemical reaction.
Products	Substances formed at the end of chemical reaction.
Precipitate	Solid substance which settles at the bottom of a solution.
Physical change	When a substance changes form or state of matter.

Chemical reactions

A chemical reaction involves reactants undergoing chemical change to produce different chemicals called products.

In a chemical reaction, chemical bonds in the reactants break, the atoms rearrange themselves and new bonds form to produce products.

Conservation of mass

The number of atoms in the reactants is the same as the number of atoms in the products, therefore there will be no change in mass. This is referred to as a conservation of mass.

Evidence for chemical reactions

When a chemical reaction is taking place, many changes can be observed. Some of these changes are listed in the table below.

Chemical reaction observations

Observation	Description
Change in colour	If the products formed have a different colour to the reactants, then a colour change will be seen.
Bubbles	If gas products are formed in a chemical reaction, they may escape in the form of bubbles.
Temperature change	Most chemical reactions release energy and temperature increases are observed. A few chemical reactions absorb energy from the surrounding and a drop in temperature is observed.
Precipitates form	If the product formed in a chemical reaction is insoluble in the solvent/solution, it will precipitate out.
Change in odour	Some substances have a distinctive odour. When new products form in a reaction, the odour can change.

Importance of chemical reactions

Chemical reactions are important because they make useful new products.

Example

Chemical reactions are required to produce plastics and drugs.

Some chemical reactions are important because they release energy. The energy released can be used to generate electricity, heat buildings and power planes and cars.

Example

Burning fossil fuels.

Science; Chemical reactions; KS3 Year 7; An introduction to chemical reactions

Chemical reactions are not always large-scale.

Example

Cooking involves chemical reactions.

Physical and chemical changes

Melting, boiling and dissolving do not produce new products and are therefore not chemical reactions; these are simply physical changes and, in most cases, can be reversed.

Example

The evaporation of water is a physical change and can be reversed by condensation.

Science; Chemical reactions; KS3 Year 7; An introduction to chemical reactions

1.	Water
2.	Beaker
3.	Bunsen burner