Precipitation - Water droplets falling as snow, rain, mist, hail or fog.

Acid Rain - precipitation that is more acidic that normal.

Acid - a hydrogen compound, turning Universal Indicator yellow, orange or red

Alkali - a soluble base, turning Universal Indicator blue or purple

Neutralisation - a reaction of an acid with a alkali forming a salt and water

Indicator - a dye that has different colours in acid and alkaline solutions

pH - a number that shows the strength of the acid or alkali. Also indicates neutral

Salt - a compound formed when a metal replaces the hydrogen in an acid