Acids, Bases and Salts

Chemicals can be broadly classified into three categories, acids, bases and salts.

Acids

• Acids are substances which taste sour.
• They maybe corrosive or non-corrosive.

Examples:

Hydrochloric acid and sulphuric acid are corrosive acids. Carbonic acid is a non-corrosive acid.

• Acids turn blue litmus red.
• There are two types of acids:

Organic acids: Naturally occurring acids obtained from plants and animals. Mineral acids: They are obtained from minerals present in the Earth’s crust.

Bases

• Bases are substances which are bitter in taste and soapy to touch.

Examples: Calcium hydroxide and zinc hydroxide

• Water-soluble bases are called alkalis.
• Example: Sodium hydroxide
• Bases turn red litmus blue.

Indicators

An indicator is a substance which illustrates the presence of a chemical substance by changing colour. Indicators can be natural or synthetic. For example, litmus, turmeric and China rose are naturally occurring indicators, while methyl orange and phenolphthalein are synthetically produced indicators.

Natural Indicators

Litmus Paper:

Acids turn blue litmus red and bases turn red litmus blue. The solutions which do not change the colour of either red or blue litmus are known as neutral solutions.

Turmeric:

It is yellow in an acidic medium and turns reddish brown in a basic medium.

China Rose:

It turns the colour of acidic solutions to dark pink (magenta) and that of basic solutions to green.

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

Activity of Indicators

Mix some water with lemon juice in a plastic cup/tumbler/test tube. Then Put a drop of the above solution on a strip of the red litmus paper with the help of a dropper. Is there any change in color? Repeat the same exercise with the blue litmus paper. Note down if there is any change in color. Perform the same activity with the following substances: Tap water, detergent solution, aerated drink, soap solution, shampoo, common salt solution, sugar solution, vinegar, baking soda solution, milk of magnesia, washing soda solution, lime water. If possible make solutions in distilled water. Record your observations and make a Table of your result, than check are there any substances on which litmus had no effect? Name those substances. 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.

Synthetic Indicators

Phenolphthalein:

It remains colourless in an acidic medium and turns pink in a basic medium.

Acid Rain

• Rain which contains a higher level of acid than normal is called acid rain.
• Rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released into the air as pollutants) dissolve in the rain drops to form carbonic acid, sulphuric acid and nitric acid respectively.

Salts

Salts are produced by the neutralisation of acids with bases. During this process, water is also produced along with the evolution of heat.

Example: Hydrochloric acid on reaction with sodium hydroxide gives sodium chloride salt along with water.

HCl + NaOH → NaCl + H₂O

USES OF SALTS:

• As a table salt.
• In the manufacture of butter and cheese.
• In leather industry.
• In the manufacturing of washing soda and baking soda.
• For the preparation of sodium hydroxide by electrolysis of brine.
• In cold countries rock salt is spread on ice to melt it.

USES OF SALTS IN DAILY LIFE

Common salt: It is used in large amounts as table salt. It is used for the preservation of food in pickling. It is used to preserve raw hides. It is used to manufacture chemicals such as chlorine and caustic soda (NaOH).

Baking soda: It is used for making baking powder. It is used for preparing antacid tablets, for controlling the acidity of the stomach.

Washing soda: It is used by washermen to wash clothes. It is used in manufacture of glass and caustic soda. It is used in fire extinguishers. It is used in the manufacture of detergents.

Chile saltpetre: It is used in the manufacture of nitric acid, gun powder and fire works.

Nitre: It is used as a fertiliser. It is also used in the manufacture of gun powder, fire works and nitric acid.

Smelling salt: It provides relief from common cold.

Epsom salt: It is used as a laxative by patients suffering from constipation.

Green vitriol: It is used in making blue black inks and in curing leather.

Blue vitriol: It is used as fungicide in agriculture. It is used in dyeing and printing industry. It is used in electroplating copper metal.

Plaster of Paris: It is used for making statues. It is used for setting broken bones.

Potash alum: It helps in rapid settling of suspended impurities in water. It is used for the purification of water.

Neutralisation

The reaction in which an acid reacts with a base to form a salt and water is called a neutralisation reaction.

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.

Neutralisation in Everyday Life

Indigestion

Our stomach contains hydrochloric acid. However, presence of a large quantity of acid in the stomach causes indigestion. To relieve this, we take an antacid such as milk of magnesia, which contains magnesium hydroxide base to neutralise the effect of excessive acid.

Ant Bite

The sting of an ant contains formic acid. When an ant bites, the effect of the sting can be neutralised by rubbing a base like moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate.

Soil Treatment

When 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 neutralise the basic nature of the soil.

Factory Wastes

• The wastes of many factories contain acids.
• If the waste is allowed to enter the water bodies, the acid will kill the fish and other aquatic organisms.
• So, factory wastes are neutralised with basic substances before releasing them into water bodies.

MIND MAP