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Acids

Acidic, Basic and Neutral materials of Class 7

Acid Defination

The word acid is derived from the Latin word acidus which means sour. Orange, Lemons, and grapes taste sour because they contain citric acid. Tamarind contain tartaric acid, vinegar contains acetic acid.

An acid is a compound which contains one or more hydrogen atoms replaceable partially or completely by a metal or a positive radical to produce a salt.” e.g. H2SO4 contains two replaceable hydrogen atoms thus, it is an acid.

Types of acids

  1. Mineral acids: All the Acids which are obtained from rocks or from minerals are called mineral acids. For example HCl, H2SO4, HNO3 all these acid are obtained from minerals.
  2. Organic acids:Acids that are found in animals and plants are known as organic acids. All most all organic acid are carbon containing acid like acetic acid, formic acid.

Some of the example are

  1. Lactic acid which is present in sour milk
  2. citric acid which is present in citrus fruits
  3. Tamarind which contains tartaric acid
  4. Vinegar which we use in different foods and as a preservative and it contains acetic acid.
  5. Formic acid is present in ants. The sharp pain caused by sting of an ant, beatles and bees is due to formic acid, which they push into the body or spray on the skin.

PHYSICAL PROPERTIES OF ACIDS

  • Sour taste: Almost all acidic substances have a sour taste.
  • Action on litmus solution: Acids turn blue litmus solution red.
  • Action on methyl orange: Acids turn methyl orange pink.
  • Action on phenolphthalein: Phenolphthalein remains colourless in acid.
  • Conduction of electricity: The aqueous solution of acid conducts electricity.
  • Corrosive nature: Most acids are corrosive in nature. They produce a burning sensation on the skin and make holes on surfaces on which they fall.

CHEMICAL PROPERTIES OF ACIDS

REACTION OF ACIDS WITH METALS: When acid reacts with a metal, then a salt and hydrogen gas are formed.

For example Metal + Acid → Salt + Hydrogen gas

  • Reaction of dil H2SO4 with zinc metal.

Experiment: zinc granules are taken in glass with 5 ml dilute sulphuric acid,due to reaction of acid with metal hydrogen gas forms and it produce bubbles

Test for gas: Bring a burning candle near the gas filled soap bubble.

Observation: The gas present in soap bubble burns with pop sound which shows the gas evolved during reaction is hydrogen gas. Figure-Reaction of Zn granules with dil. H2SO4 and test for hydrogen gas by burning The reaction involved is In this reaction more active metal zinc displaces less active hydrogen from H2SO4 and this hydrogen is evolved as gas. Thus it is an example of displacement reaction. Some more examples of reaction of different metals with a particular acid:

  • Example 1. Mg(s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) Magnesium hydrochloric acid magnesium chloride hydrogen (a metal) (dil) (a salt) gas
  • Example 2. Zn (s)+2HCl(aq) → ZnCl2 (aq) +H2(g) zinc hydrochloric acidzinc chloridehydrogen (a metal) (dil) (a salt) gas
  • Example 3. Fe(s)+2HCl(aq) → FeCl2 (aq) +H2(g) ironhydrochloric acidiron(II) chloride hydrogen (a metal) (dil) (a salt) gas
  • Example4. Cu +2HCl(aq) → no reaction copperhydrochloric acid (a metal) (dil)

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