Class 7 Science

Acid Base Salt

Learning Goals

Substances can be divided into three types: Acid, Base and Salt.

Acid: Taste of acid is sour. There are many substances which contain acid and so taste sour. For example: lemon, curd, pickles, orange juice, vinegar, etc. Substances which taste sour are called acidic. The chemical nature of such substance is known as ACIDIC. The word acid comes from Latin ACERE which means sour.

Base: Taste of base is bitter. Substances which contain base taste bitter. For example: soap or soap solution, baking soda, washing soda, etc. The chemical nature of substances that contain base is known as BASIC.

Acid Base Indicator

A substance which detects the acidic or basic nature of another substance by change in colour is called acid-base indicator. It is not advisable to taste any substance in laboratory because it can be harmful. Hence, acid-base indicators are used to check if a given substance is acid or base.

Types of Indicator

Indicators can be divided into two types.

Natural Indicator: Indicators that are obtained from naturally occurring substance are called NATURAL INDICATORS. Example: litmus, turmeric, China rose, etc.

Synthetic Indicator: Indicators that are made in laboratory are called SYNTHETIC INDICATORS. Example: phenolphthalein, methyl orange, etc.

Natural Indicator


Litmus is extracted from Lichens. In the chapter on Nutrition in Plants, you have read that lichen is a composite organism. Lichens consist of fungi and algae living in symbiotic relationship.

Litmus is a purple coloured liquid in distilled water. Litmus comes in the form of strips of two colours. One is called blue litmus paper and another is called red litmus paper. Litmus liquid and litmus paper are used to detect the acidic or basic nature of a substance.

Colour of litmus paper in acid: Blue litmus paper turns into red when dipped in acidic solution.

Colour of litmus paper in base: Red litmus paper turns into blue when dipped in basic solution.


Turmeric is also used as natural indicator. Turmeric is of yellow colour. Turmeric paper turns into red when it is dipped into basic solution. Turmeric paper does not change its colour with acid.

China Rose

China rose is another natural indicator. China rose solution gives dark pink (magenta) colour with acid and green colour with base.

Acid Rain

Carbon dioxide, sulphur dioxide and nitrogen dioxide which are released from vehicles and chimneys mix with droplets of rain and turn the rain water acidic. When this acidic rain water falls over earth, it is known as acid rain. Acid rain damages the buildings and is harmful for plants and animals.

The Taj Mahal, which is made of marble, is under threat because of acid rain. Many parts of Taj Mahal and many other historical buildings and monuments have got damaged due to acid rain.

Characteristics of Acid

AcidFound in
Acetic acidVinegar
Formic acidAnt's sting
Citric acidOranges, lemons, grapefruit, etc.
Lactic acidCurd
Oxalic acidSpinach, guava
Ascorbic acid (vitamin C)Amla (Indian gooseberry), citrus fruits
Tartaric acidTamarind, grapes, unripe mango, etc.

Characteristics of Base:

BaseFound in
Calcium hydroxideLime water
Ammonium hydroxideWindow cleaner
Sodium hydroxide
Potassium hydroxide
Magnesium hydroxideMilk of magnesia