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ACIDS, BASES AND SALTS

Curd, lemon juice, orange juice and vinegar taste sour because they contain acids. The chemical nature of These substances is acidic.

The word acid comes from the Latin word acere which means sour. The acids in these substances are natural acids.

Which substances are bitter in taste and feel soapy on touching are known as bases. The nature of such substances is said to be basic.

Special type of substances are used to test whether a substance is acidic or basic. These substances are known as indicators.

The indicators change their colour when added to a solution containing an acidic or a basic substance.

Turmeric, litmus, China rose petals (Gudhal), etc., are some of the naturally occurring indicators.

NATURAL INDICATORS AROUND US

Litmus: A natural dye

The most commonly used natural indicator is litmus. Litmus is extracted from lichens.

Litmus has a mauve (purple) colour in distilled water.

When added to an acidic solution in Litmus, Litmus turns red and when added to a basic solution in Litmus, Litmus turns blue.

Litmus is available in the form of a solution, or in the form of strips of paper, known as litmus paper.

Litmus is available as red and blue litmus paper.

The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.

Turmeric is another natural indicator

Turmeric is a naturally occurring yellow colour.

Turmeric is an acid-base indicator. When it reacts with bases, it changes color to deep red. This red form of the indicator can change back to yellow when acids are added.

When it reacts with acid, it does not changes color it remains yellow.

China Rose as Indicator

China rose (Gudhal) petals and place them in a beaker. Add some warm water. Keep the mixture for some time till water becomes coloured.

China rose indicator added to acidic solutions it changes color to dark pink (magenta) and basic acidic solutions it changes color to green.

NEUTRALISATION

Acids turn blue litmus red and bases turn red litmus blue. Let us see what happens when an acid is mixed with a base.

When the solution is basic, phenolphthalein gives a pink colour.

When the solution is acidic, it remains colourless.

When an acidic solution is mixed with a basic solution, both the solutions neutralise the effect of each other.

When an acid solution and a base solution are mixed in suitable amounts, both the acidic nature of the acid and the basic nature of the base are destroyed.

The resulting solution is neither acidic nor basic.

In neutralisation reaction, heat is always produced, or evolved. The evolved heat raises the temperature of the reaction mixture.

In neutralisation reaction a new substance is formed. This is called salt.

Salt may be acidic, basic or neutral in nature.

The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat.

Acid+Base → Salt+Water (Heat is evolved)

Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H2O)

NEUTRALISATION IN EVERYDAY LIFE

Indigestion

Our stomach contains hydrochloric acid. Hhydrochloric acid helps us to digest food.

Too much of acid in the stomach causes indigestion. Sometimes indigestion is painful.

To relieve indigestion, we take an antacid (milk of magnesia).

Milk of magnesia contains magnesium hydroxide.

Milk of magnesia neutralises the effect of excessive acid.

Ant bite

When an ant bites, it injects the acidic liquid (formic acid) into the skin. The effect of the acid can be neutralised by rubbing moist baking soda (sodium hydrogencarbonate) or calamine solution, which contains zinc carbonate.

Soil treatment

Excessive use of chemical fertilisers makes the soil acidic. Plants do not grow well when the soil is either too acidic or too basic.

When the soil is too acidic, it is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide).

The soil is basic, organic matter (compost) is added to it. Organic matter releases acids which neutralises the basic nature of the soil.

Factory wastes

The wastes of many factories contain acids. They are allowed to flow into the water bodies, the acids will kill fish and other organisms.

The factory wastes are neutralised by adding basic substances.

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