Notes Class 7 Science Chapter 4 Acids, Bases and Salts

Notes For All Chapters Science Class 7

1. Introduction

In our daily life we use many substances like lemon, vinegar, curd, sugar, salt etc. These substances have different tastes. Based on their taste and chemical nature, they can be grouped into:

• Acids
• Bases
• Neutral substances

We should never taste unknown substances because they may be harmful.

2. Acids

• Substances that taste sour are called acids.
• Their chemical nature is acidic.
• The word acid comes from the Latin word “acere”, meaning sour.
• Acids found naturally in plants and animals are called natural acids.

Examples of Natural Acids

3. Bases

• Substances that are bitter in taste and feel soapy when touched are called bases.
• Their chemical nature is called basic.

Examples of Bases

4. Indicators

We cannot taste every substance to know whether it is acidic or basic. So, we use indicators.

Definition

Indicators are substances that change colour when added to acidic or basic solutions.

Types of Natural Indicators

• Litmus
• Turmeric
• China rose (Gudhal)

5. Litmus Indicator

Litmus is a natural dye obtained from lichens.
It is available as:

• Litmus solution
• Red litmus paper
• Blue litmus paper

Effect of Litmus

6. Turmeric as an Indicator

• Turmeric is a natural indicator.
• It remains yellow in acidic and neutral solutions.
• It turns red in basic solutions like soap solution.

Example: Turmeric stain on cloth turns red when washed with soap because soap is basic.

7. China Rose (Gudhal) Indicator

• Made by soaking China rose petals in warm water.
• The coloured water acts as an indicator.

Colour Change with China Rose

8. Neutral Substances

• Substances that do not change the colour of red or blue litmus are called neutral.
• They are neither acidic nor basic.

Examples from activities:

• Sugar solution
• Common salt solution

9. Phenolphthalein Indicator

• A special indicator used in the laboratory.
• It is colourless in acidic solution.
• It turns pink in basic solution.

10. Neutralisation

Definition

The reaction between an acid and a base is called neutralisation.

What Happens in Neutralisation?

• Acid and base cancel each other’s effect.
• A new substance called salt is formed.
• Water is produced.
• Heat is evolved (temperature increases).

General Equation

Acid+Base→Salt+Water+Heat

Example
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

HCl+NaOH→NaCl+H2​O

11. Neutralisation in Everyday Life

(a) Indigestion

• Our stomach contains hydrochloric acid for digestion.
• Too much acid causes indigestion and pain.
• We take antacids like milk of magnesia (magnesium hydroxide).
• It neutralises excess acid.

(b) Ant Bite

• Ant injects formic acid into the skin.
• Relief is given by applying:
• Moist baking soda (sodium hydrogencarbonate)
• Calamine solution (contains zinc carbonate)
  These substances neutralise the acid.

(c) Soil Treatment

• Excess fertilisers make soil too acidic.
• Acidic soil is treated with:
• Quick lime (calcium oxide)
• Slaked lime (calcium hydroxide)
• If soil is too basic, organic matter (compost) is added.
• Compost releases acids that neutralise basic soil.

(d) Factory Wastes

• Factory wastes may contain acids.
• If released directly into rivers or lakes, they can kill aquatic life.
• So, they are neutralised with bases before disposal.

12. Acid Rain

Rain containing excess acids is called acid rain.

Caused by gases like:

• Carbon dioxide
• Sulphur dioxide
• Nitrogen dioxide
• These gases dissolve in rainwater to form acids.

Effects of Acid Rain

• Damages buildings and monuments
• Harms plants
• Affects animals and aquatic life

13. Safety Precautions

• Do not taste unknown substances.
• Do not touch unknown chemicals.
• Laboratory acids and bases are corrosive and harmful to skin.