Chapter - 5 Acids Bases and Salts

ACIDS

Acids are substances that taste sour. Curd, lemon juice, orange juice and vinegar taste sour. The chemical nature of such substances is acidic. The c word acid comes from the Latin word acere which means sour. The acids in these substances are natural acids.

SOURCES OF ACIDS

1.    Acetic acid - Vinegar

2.    Formic acid- Ant’s sting

3.    Citric acid - Citrus fruits such as oranges, lemons, etc.

4.    Lactic acid- Curd

5.    Oxalic acid - Spinach

6.    Ascorbic acid- Amla, Citrus fruits (Vitamin C)

7.    Tartaric acid- Tamarind, grapes, unripe mangoes, etc.

All the acids mentioned above occur in nature.

BASES

Substances like baking soda which are bitter in taste and feel soapy on touching are known as bases. The nature of such substances is said to be basic.

1.    Calcium hydroxide- Lime water

2.    Ammonium hydroxide- Window cleaner

3.    Magnesium hydroxide- Milk of magnesia

NEUTRAL

Substances which are neither acidic nor basic are called neutral.

INDICATOR

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

LITMUS: A NATURAL DYE

The most commonly used natural indicator is litmus. It is extracted from lichens. It has a mauve (purple) color in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. The solutions which do not change the color of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper

CHINAROSE INDICATOR

China rose indicator turns acidic solutions to dark pink (magenta) and basic solutions to green.

PHENOLPHTHALEIN

When the solution is basic, phenolphthalein gives a pink color. On the other hand, when the solution is acidic, it remains colorless.

NEUTRALISATION

·      When an acidic solution is mixed with a basic solution, both the solutions neutralize 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 neutralization reaction, heat is always produced, or evolved. The evolved heat raises the temperature of the reaction mixture.

·      In neutralization 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 neutralization. Salt and water are produced in this process with the evolution of heat.

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

NEUTRALISATIONS IN EVERYDAY LIFE

·      Stomach contains hydrochloric acid. It helps us to digest food, but too much of acid in the stomach causes indigestion. Sometimes indigestion is painful. To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralizes the effect of excessive acid.

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

·      Excessive use of chemical fertilizers 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 basic is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is basic, organic matter is added to it. Organic matter releases acids which neutralizes the basic nature of the soil.

·      The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralized by adding basic substances.