All the elements in nature fall into three classes: metals, non-metals and gases. Non-metals are also sometimes called metalloids.

The compounds formed by combination of the elements can also be classified as organic and inorganic compounds.

Organic compounds are formed from a combination of carbon and hydrogen; they are also sometimes known as hydrocarbons.

In addition to this, all these compounds taste sour, bitter or salty.

The sour tasting substances are known as acids. Bitter tasting compounds are generally soapy to feel also; they are known as bases or alkalis.

What is an acid?

When a substance dissolves in water, the solution may be acidic, neutral or alkaline. An acid is any substance which produces H+ ions or H3O+ ions in water. H+ ions are called hydrogen ions; H3O+ ions are called hydroxonium ions. You will mostly see acids in reactions as forming H+ ions. In reality, H+ is a single proton, and does not exist on its own.

It always attaches to something; in water it joins to H2O to form H3O+ ions. All acids taste sour and are mostly derived from oxides of non-metals dissolved in water.


Measure of acidity pH.

PH is a measure of how acidic or how alkaline a solution in water is.

The pH scale goes from 1 to 14, with 1 being very strongly acidic, and 14 being very strongly alkaline.

A pH of 7 is neutral. You can measure the pH of a solution using universal indicator.

Just as litmus paper will be red for an acid and blue for an alkali, so universal indicator is a mixture of indicators which will give different colours for a different pH.

Any acid will have a pH of less than 7. Any alkali will have a pH of more than 7.

A strong acid (HCl or H2SO4 or HNO3 )will have a pH of 1 (red).


A weak acid will have a pH of 3 to 4 (orange). Examples of weak acids are ethanoic acid (vinegar), citric acid (lemon juice) and rain water.

Rain water has a natural pH of 5•5 (see carbonic acid). Water and salts are neutral, pH 7 (green).

A weak alkali (ammonia) will have a pH of 11 to 12 (blue). A strong alkali (Ca(OH)2 or NaOH) will have a pH of 14 (purple).

Examples of Acids.

The three common acids you will find in the laboratory are

1) Hydrochloric acid – HCl(aq)

2) Nitric acid – HNO3(aq)

3) Sulphuric acid – H2SO4(aq)

They are all strong acids – see pH. They all ionise in water to form hydrogen ions (H+ ions).

1) HCl(aq) H+(aq) + Cl-(aq)

2) HNO3(aq) H+(aq) + NO3(aq)

3) H2SO4(aq) H+(aq) + HSO4(aq)

They are all examples of hydrogen compounds with non-metals. Hydrochloric acid is hydrogen chloride (in water).

Nitric acid is hydrogen nitrate (in water). Sulphuric acid is hydrogen sulphate (in water).

Sulphuric acid is made using the contact process. They are called Mineral Acids because they were originally obtained from minerals in rocks.Oxides of non-metals are acidic, such as CO2, NO, SO2.

Hydrogen oxide (H2O) is Water – it is neutral, see Water. Lots of everyday substances contains acids.

Acids are found in:

  • citrus fruits (le mon juice, orange juice)
  • vinegar
  • car batteries (sulphuric acid)
  • your stomach (hydrochloric acid)

Rainwater is a little acidic, but pollution (e.g. sulphur dioxide) from burning fossil fuels may make it even more acidic, forming acid rain.

When acids are present in food, they usually taste sour (think of the taste of lemon juice or vinegar).

Strong acids are very dangerous.

Properties of Acids.

They have a pH less than 7, see pH. They will turn blue litmus paper red. Hydrochloric acid and sulphuric acid will react with.

  • Any alkali or base, see neutralisation.
  • Any metal above hydrogen in the reactivity series. The metal will fizz, giving off hydrogen gas, and leaving the metal salt in solution.
  • It is not safe to put a metal into an acid which is above magnesium in the reactivity series.
  • Any chloride or sulphate can be safely made by reacting the appropriate metal (from lead to magnesium in the reactivity series) with hydrochloric acid to make the chloride or sulphuric acid to make the sulphate.
  • Any metal carbonate or metal hydrogen carbonate. The metal carbonate or metal hydrogen carbonate will bubble giving off carbon dioxide gas, leaving the metal salt and water.

Any chloride or sulphate can be made by reacting the appropriate metal carbonate or hydrogen carbonate with hydrochloric acid to make the chloride or sulphuric acid to make the sulphate.

Strong and Weak Acids – Strength and Concentration.

Acids and alkalis can be described as strong or weak. This does not mean the same as concentrated or dilute.

The strength of an acid or alkali depends on how ionised it is in water.

A strong acid or alkali is completely (100%) ionised. For hydrochloric acid

hydrogen chloride (in water) hydrogen ion + chloride ion HCl<=”” sub=””> H+(aq) + Cl(aq) All of the hydrogen chloride molecules become hydrogen ions and chloride ions in water (see examples for other strong acids).

For sodium hydroxide

sodium hydroxide (in water) sodium ion + hydroxide ion

NaOH(aq) Na+(aq) + OH(aq)

Sodium hydroxide exists as ions both in water and in the solid.

(see examples for other strong alkalis).

A weak acid or alkali is only partly (less than 100%) ionised. For ethanoic acid

ethanoic acid (in water) hydrogen ion + ethanoic ion

CH3CO2H(aq) H+(aq) + CH3CO2(aq)

Some of the ethanoic acid molecules become ions in water but most of them stay as molecules. The reaction is reversible (shown by the arrow).


Ammonia + water ammonium ion + hydroxide ion NH3(g) + H2O(l) NH4+(aq) + OH(aq)

L Some of the ammonia molecules become ions in water but most of them stay as molecules. See also Concentration and Differences between Strong and Weak Acids.

Common uses of Acids – see also uses of Sulphuric Acid.

Outside of their uses in the chemical industry,

Common uses of acids are

  • Steel used in construction is acid treated before painting. Dilute sulphuric or hydrochloric acid will remove any surface rust which would otherwise spread under the painted surface. ‘Rust remover’ used to repair cars is dilute phosphoric acid – H3PO4.
  • Baking powder contains tartaric acid.
  • ‘Lime scale’ removers contain dilute acids. Try using lemon juice or vinegar (weak acids). Lime scale is calcium carbonate (also called furring).
  • A wasp sting is alkaline. It may be neutralised with a weak acid (lemon juice or vinegar).
  • A bee sting is acidic. It may be neutralized by an alkali

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