Acids are invisible so this property of solutions can be revealed by using an . When a substance in water, it forms an aqueous solution which may be acidic, alkaline or . Pure water is . Indicators show whether a solution is , alkaline or neutral by the way their colours change. Indicators are often made from coloured compounds found in . The scale was developed to show how acidic or alkaline a solution is.
Three common indicators are , and phenolphthalein. follows the colours of the rainbow.
    Colour                 pH                 Description                 Examples
         1 or 2                  Hydrochloric acid
         3 or 4         Weak acid                       Vinegar
         5 or 6         (Weak acid)                     Rain water
         7                                  Deionised water
         8 or 9         (Weak alkali)                   Baking powder (in water)
         10 or 11              Ammonia solution
         12 or 13          Sodium hydroxide

is red in acidic solutions and blue in alkali. Penolphthalein is in alkaline solutions and in acid.
An acid reacts with a base to produce salt and water. This reaction is called neutralisation. Metal and metal hydroxides act as bases. An alkali is a base which in . The reaction can be summarised as a equation:

acid + alkali salt + water

The type of salt produced in a neutralisation reaction depends both on the metal in the base and on the acid used. Neutralising hydrochloric acid produces , neutralising nitric acid produces and neutralising sulphuric acid produces .
When an acid reacts with a , a gas called and a salt are produced. A simple test for is that when the mouth of a test tube is held to a flame, the gas burns in air with a squeaky pop.
When an acid reacts with a , a salt, water and carbon dioxide are produced. A simple test for carbon dioxide is that it turns .

Example word equations

calcium oxide + calcium sulphate +

+ hydrochloric magnesium chloride + hydrogen

ammonium hydroxide + hydrochloric acid +

copper oxide + sulphuric acid +

iron + nitric acid iron(III) + hydrogen

oxide + sulphuric acid zinc sulphate +

potassium hydroxide + nitric acid + water

+ sulphuric acid nickel + carbon dioxide +

+ sulphuric acid zinc sulphate +

carbonate + acid iron(II) nitrate + + water

sodium hydroxide + hydrochloric acid +

+ zinc nitrate + carbon dioxide + water
 

There are very many important applications of these reactions.
Burning fossil fuels produces acidic oxides, notably dioxide, carbon dioxide and others. These oxides cause rain water to be significantly . Consequently, in developed countries, all water supplies require using the cheapest available alkali, calcium hydroxide – with the common name, . Farmers neutralise fields using either , or sometimes limestone, which contains mainly the chemical .
Indigestion in the stomach is caused by an excess of naturally occurring acid. The symptoms can be relieved by taking Milk of Magnesia which is a mixture of hydroxide and carbonate.
Aluminium ions can be made soluble in alkali and most other metal ions cannot. Consequently this provides a very convenient way of purifying aluminium compounds on an industrial scale. is achieved by blowing carbon dioxide through the alkaline solution of aluminium ions; when the solution becomes acidic, aluminium hydroxide precipitates and can be filtered off in a highly purified state. When dried, the product is known as alumina.