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Properties of Acids

  1. Acids are Corrosive in Nature
    If a strong acid is poured on materials such as paper, wood or cloth, it destroys them. It can even burn skin. Acids are not stored in metal containers because they gradually corrode and eat up the metal container. If you pour a drop of sulphuric acid on an iron sheet, you can see the effect immediately. Warning sign is displayed on containers containing concentrated acids and bases in order to warn people about the dangerous corrosive nature of strong acids and bases.


Strong and Weak Acids
  • Sulphuric acid, nitric acid and hydrochloric acid are known as strong acids.
  • Organic acids such as acetic acid, formic acid, citric acid and also carbonic acid are known as weak acids.

We use carbonic acid in soft drinks and soda. The organic acids are used as food ingredients.

All Acids Contain Hydrogen : In fact, acid can be defined as that which contains replaceable hydrogen atom. This is clear from the formulae of acids.

Example : HCl, H2SO4, HNO3, CH3COOH

  1. Acid Solutions conduct Electricity : Pure water is a bad conductor of electricity. But when we add a little acid to the water, it (acidulated water) conducts electricity.
  2. Acids are Soluble in Water
    Take a beaker and add 20 ml of water. Slowly add few drops of concentrated hydrochloric acid. Stir it. Touch the base of the beaker. What do you feel? It feels hot. This reaction is highly exothermic and so the beaker becomes hot. The hydrochloric acid dissolves in water producing dilute acid.
    A compound generally dissociates into positive and negative ions when dissolved in water. An acid produces hydrogen ions (H+) when it is dissolved in water.
  3. Action of Acids with Metals
    Acids react with several metals such as magnesium (Mg), zinc (Zn), sodium (Na), or potassium (K) to form a salt and liberate hydrogen gas.
    Generally metals react with acid to give salt with the liberation of hydrogen gas.
    However, some metals such as copper (Cu), silver (Ag), gold (Au) or mercury (Hg) do not react with acids to give hydrogen gas.
  4. Action of Acids with Metallic Oxides
    Take a conical flask and add a little amount of black copper oxide. Slowly add dilute hydrochloric acid stirring with a rod. What do you observe? The black colour of copper oxide has turned to bluish green. This is due to the formation of copper (II) chloride.
  5. Acids react with metal carbonates and bicarbonates to liberate Carbon dioxide Gas
    Take a hard-glass test tube having a two holed rubber cork. Through one hole insert a thistle and through the other insert a glass delivery tube. Add small amount of calcium carbonate. Add a little amount of dil. HCl through the thistle funnel. A brisk effervescence is produced. Pass this gas into a test tube containing lime water through the delivery tube. What happens to the lime water? The lime water turns milky showing that the gas which comes through the delivery tube is carbon dioxide.
    Acids react with compounds such as potassium carbonate, sodium bicarbonate or calcium carbonate to liberate carbon dioxide gas, with a brisk effervescence, leaving behind a salt solution.
    This property of acids is widely used in soda-acid type fire extinguishers.
    Metal carbonate + Acid salt + Water + Carbon dioxide
    With metal bicarbonates, acids produce salt, water and carbon di oxide. For example with sodium bicarbonate it produces sodium nitrate, water and carbon dioxide.
  6. Action with indicators
    An indicator is a substance that changes colour when brought in contact with an acid or a base. Many natural substances, such as black currant juice and red cabbage juice are good indicators. But a commonly used indicator is litmus. It is a purple dye, which comes from lichens. A paper soaked in litmus solution and dried is called litmus paper. A blue litmus paper turns red in acids. There are also other indicators that are commonly used to test acids, e.g. methyl orange and phenolphthalein. Methyl orange has orange colour in acids. Phenolphthalein turns colourless in acids.
    Acids turn blue litmus paper red. The presence of acids can also be tested by methyl orange, which has orange colour in acids. Phenolphthalein is also used as an indicator. It is colourless in acids.

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