- Curd, lemon juice, orange juice and vinegar taste sour. These substances taste sour because they contain acids. The chemical nature of such substances is acidic. The word acid comes from the Latin word acere which means sour. The acids in these substances are natural acids.
- On the other hand Generally, substances like baking Soda are bitter in taste and feel soapy on touching are known as bases. The nature of such substances is said to be basic.
- In nature Acetic acid found in Vinegar, Formic acid in Ant’s sting, Citric acid in Citrus fruits such as oranges, lemons, etc. Lactic acid Founds in Curd , Oxalic acid in Spinach, Ascorbic acid in Amla, Citrus fruits (Vitamin C). Tartaric acid in Tamarind, grapes, unripe mangoes, etc.
- Similarly in base Calcium hydroxide found in Lime water, Ammonium hydroxide in Window cleaner, Sodium hydroxide/ Potassium hydroxide in Soap and Magnesium hydroxide in Milk of magnesia.
- 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.
NATURAL INDICATORS AROUND US
- Turmeric, litmus, China rose petals (Gudhal), etc., are some of the naturally occurring indicators.
- A natural dye The most commonly used natural indicator is litmus. It is extracted from lichens. It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. 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. Acid turns blue litmus red. Bases turn red litmus blue.
- 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. A turmeric stain turns to red when it is washed with soap.
- China rose indicator turns acidic solutions to dark pink (magenta) and basic solutions to green.
- 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. 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.
Acid+Base → Salt+Water (Heat is evolved)
example: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H2O)
NEUTRALISATION IN EVERYDAY LIFE
Our 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 neutralises the effect of excessive acid.
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.
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). If the soil is basic, organic matter (compost) is added to it. Organic matter releases acids which neutralises 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, neutralised by adding basic substances.
[Source : NCERT]