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What is an acid?

 An acid is a chemical compound which is soluble in water, tastes sour and turns blue litmus paper pink.

Acids neutralize bases.

Acids may be divided into two groups, strong acids and weak acids.


Many weaks acids occur naturally in foods and can be eaten. A list of food acids is shown in the table below.

  Common food acids.

Food acid Food
Citric acid Citrus fruits like oranges, lemons, limes and mandarins.
Lactic acid Fermented milk products like yogurt, buttermilk and sour cream.
Malic acid Green appes and grapes
Tartaric acid Grapes and wines
Acetic acid Vinegar

Some weak acids like oxalic acid which is found in plants is poisonous in high concentrations and should not be eaten. Oxalic acid is used in some bleaching and cleaning products.


Strong acids are corrosive and should not be tasted. They react with active metals like magnesium, zinc and iron to produce hydrogen gas.

Name Chemical formula Common use of acid
Hydrochloric acid HCl Used to clean mortar of bricks and to lower the pH of swimming pools
Sulfuric acid H2SO4 Used in lead acid automobile batteries and in industry as a catalyst and a dehydrating agent.
Nitric acid HNO3 Used int eh manufacture of fertilizer and explosives.


When acids react with substances they produce salts.  Salts are composed of a metal and non-metallic component which comes from the acid.

Hydrochloric acids produce chloride salts. eg. sodium chloride, NaCl

Nitric acids produce nitrate salts. eg. potassium nitrate, KNO3

Sulfuric acid produces sulfate salts. eg. Magnesium sulfate, MgSO4


The reaction of acids can be classified into three types of general reactions.

The first reaction is called a neutralization reaction.

1. Acid + Base => Salt + Water

  Hydrochloric acid + sodium hydroxide => sodium chloride + water

  HCl + NaOH => NaCl + H2O

2. Acid + Carbonate => Salt + Carbon Dioxide + Water

  Nitric acid + sodium hydrogen carbonate => sodium nitrate + carbon dioxide + water

  HNO3 + NaHCO3 => NaNO3 + CO2 + H2O

3. Acid + Active metal => Salt + Hydrogen gas

  Magnesium + sulfuric acid => magnesium sulfate + hydrogen

  Mg + H2SO4 =>MgSO4 + H2

Acids always produce salts when they react.