When physical change occurs substances change their state. For example, water (H₂O) can exist as a solid, a liquid or a gas. The transition between each state is dependent on the kinetic energy of the water molecules. However, the composition of the water molecules themselves remains the same – one oxygen atom is chemically bonded to two hydrogen atoms.

[INSERT CHEM_CI_01_ima]

A chemical reaction, on the other hand, is a rearrangement of atoms. Hydrogen gas (H₂) can be reacted with oxygen (O₂) to form water. Here chemical bonds are broken between the hydrogen atoms in the molecules of hydrogen gas. Similarly between oxygen atoms.

Then new chemical bonds are formed between oxygen and hydrogen atoms (reactants) to form water (product).

[INSERT CHEM_CI_02_ani]

This reaction yields a lot of heat energy and, depending on the conditions, can be explosive. Watch the short clip of the Hindenburg disaster to see just how explosive the uncontrolled reaction between hydrogen and oxygen can become.

[INSERT CHEM_CI_03_vid]

The next video clip shows a more controlled explosion of a hydrogen-air mixture which can be carried out in the school laboratory.

[INSERT CHEM_CI_04_vid]

The chemical product of this change also has different physical and chemical properties compared with the reactants.