Acid, Bases and Salts

ACIDS AND BASES 

Curd, lemon juice, orange juice and vinegar like 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.

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.

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. Water and human blood are great examples of neutral solutions.

Indicators:

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

1. Litmus: 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.

2. Turmeric:

• Adding a base to turmeric solution changes the colour of solution from yellow to red. 
• This is why a turmeric stain on white shirt is turned to red when it is washed with soap. It is because the soap solution is basic.

3. China Rose:

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

Acid Rain:

 Rain containing excess of acids is called an acid rain. 

The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released into the air as pollutants) dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid respectively. Acid rain can cause damage to buildings, historical monuments, plants and animals.

 Neutralisation:

Activity:

We are going to use another indicator, called phenolphthalein. When the solution is basic, phenolphthalein gives a pink colour. On the other hand, when the solution is acidic, it remains colourless.

Process of Neutralisation

• 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 a new substance is formed. This is called salt. Salt may be acidic, basic or neutral in nature. Thus, neutralisation can be defined as follows:

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) 

The following reaction is an example: 

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

Neutralisation in everyday life: 

1. Indigestion:

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.

2. 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 hydrogen carbonate) or calamine solution, which contains zinc carbonate.

3. 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). If the soil is basic, organic matter is added to it. Organic matter releases acids which neutralises the basic nature of the soil.

4. Factory wastes:

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. 

Do You Know  ?

Each cell in our body contains an acid, the deoxyribonucleic acid or DNA. It controls every feature of the body such as our looks, colour of our eyes, our height etc. Proteins that build part of our cells are also made of amino acids. The fats in our body contain fatty acids.