Indicators

Acid base and salt of Class 8

What is INDICATORS

Indicator indicates the nature of particular solution whether acidic, basic or neutral. Apart from this, indicator also represents the change in nature of the solution from acidic to basic and vice versa. Indicators are basically coloured organic substances extracted from different plants. A few common acid base indicators are

  • Litmus: Litmus is a purple dye which is extracted from 'lichen' a plant belonging to variety Thallophyta. It can also be applied on paper in the form of strips and is available as blue and red strips. A blue litmus strip, when dipped in an acid solution acquires red colour. Similarly a red strip when dipped in a base solution becomes blue.
  • Phenolphthalein: It is also an organic dye and acidic in nature. In neutral or acidic solution; it remains colourless while in the basic solution, the colour of indicator changes to pink.
  • Methyl Orange: Methyl orange is an orange coloured dye(yellow) and basic in nature. In the acidic medium the colour of indicator becomes red.
  • Red cabbage juice: It is yellowin colour and remains as such in the neutral and acidic medium. In the basic medium its colour becomes reddish or deep brown.NCERT solutions for class 8 Science prepared by pw will help you to solve your NCERT text book exercise. 

NEUTRALISATION:

It may be defined as a reaction between acid and base present in aqueous solution to f form salt and water.

HCl(aq) + NaOH (aq) indicatorsNaCl(aq) + H2O(l)

Basically neutralisation is the combination between H+ ions of the acid with OH- ions of the base to form H2O.

e.g. H+(aq) + Cl(aq) + Na+(aq) + OH(aq) Na+(aq) + Cl(aq) + H2O(l)

H+(aq) + OH(aq) indicatorsH2O(l)

Neutralisation reaction involving an acid and base is of exothermic nature. Heat is evolved in all neutralisation reactions. If both acid and base are strong, the value of heat energy evolved remains same irrespective of their nature.

e.g. indicators

 

Strong acids and strong bases are completely ionised of their own in the solution, NC energy is needed for their ionisation. Since the cation of base and anion of acid on both sides of the equation cancels out completely, the heat evolved is given by the following reaction 

H+(aq) + OH(aq) indicatorsH2O(l) + 57.1 kJ

If either the acid or base is weak, some energy is needed for its ionization. As a result, heat evolved is comparatively less. 

e.g. indicators

In this case heat energy needed for the ionization of per mole of NH4OH is (57.1 − 51.5) = 5.6 kJ

NH4OH (aq) →

Applications of neutralisation: 

  • People particularly of old age suffer from acidity problems in the stomach which is caused mainly due to release of excessive gastric juices containing HCI. The acidity is neutralised by antacid tablets which contain sodium hydrogen carbonate (baking soda), magnesium hydroxide etc.
  • The stings of bees and ants contain formic acid. Its corrosive and poisonous effect can be neutralised by rubbing soap which contains NaOH (an alkali).
  • The stings of wasps contain an alkali and its poisonous effect can be neutralised by an acid like acetic acid (present in vinegar).
  • Farmers generally neutralize the effect of acidity in the soil caused by acid rain by adding slaked lime (Calcium hydroxide) to the soil.
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