Class Seven Science Chapter 5 Acid-base and salt

Chapter 5: Acid-base and salt

Acids, Bases and Salts

Acids and Bases

A substance that reacts with a base to form salt and water is called acid. It is derived from the Latin word acere which means sour. The various food items like curd, lemon juice, grapes, pears, vinegar and orange juice are sour in taste because they contain acids. All of these substances are acidic in nature. The acids present in these substances are natural acids and they are not harmful to eat or drink. Some naturally occurring acids are given below in the table:

A substance that can neutralize an acid to form salt and water is called the base. The various substances like baking soda, washing soda and caustic soda are bitter in taste and feel soapy when we rub their solutions between our fingers. All of these substances are basic in nature and are collectively called as bases.

To test whether a substance is acidic or basic in nature we use special types of substances known as indicators. It detects the acidic or basic nature of substances by the change in colour when these substances are added to a solution containing acids or bases. Litmus, turmeric, China rose, etc. are some indicators that are obtained naturally.

A pH scale is used by scientists to measure how acidic or basic a liquid solution is. A pH scale ranges from 0 to 14. The solutions with a pH between 0 to 7 are acids, with 0 being the strongest and the solutions with a pH between 7 to 14 are bases, with 14 being the strongest base. If a liquid solution has a pH of 7, it is called neutral.

Natural Indicators Around Us Litmus: A natural dye

• It is a purple-coloured dye extracted from various species of lichen that changes colour around the neutral range. It is available in the form of red litmus and blue litmus.

• The acidic solution changes blue litmus red while there is no change in the colour of blue litmus in the basic solution.

• The basic solution changes red litmus blue while there is no change in the colour of blue litmus in acidic solution.

• Those substances that do not change the colour of either blue or red litmus are called neutral solutions. Turmeric is another natural indicator.

• It is yellow in colour and remains as such both in acidic and neutral solutions.

• Turmeric paper turns into red or deep brown when it is dipped into a basic solution.

China rose as an Indicator

• China rose indicator is a light pink coloured solution that is extracted from the red flowers of China rose plant with water.

• In an acidic medium, it exhibits a dark pink colour (magenta).

• In a basic medium, it exhibits a green colour.

Phenolphthalein as an Indicator

• It is a synthetic indicator.

• This indicator is colourless in a neutral or acidic solution while in the basic solution the colour changes to pink.

The rain which contains a higher level of acid than normal rain is called acid rain. It is created when pollutants in the air, such as carbon dioxide, sulphur dioxide and nitrogen dioxide that are released from vehicles and chimneys dissolve in rainwater and form acids like carbonic acid, sulphuric acid and nitric acid. Acid rain can cause damage to plants, animals, buildings and historical monuments.