Class 7 Acids Bases and Salts Notes for exams

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Class 7 Science – Chapter 5: Acids, Bases and Salts (Smart Notes)

Substances may be classified as:

1. acids
2. bases
3. salts

Acids

1. Taste sour.
2. Contain replaceable hydrogen.
3. May be organic or mineral.
4. Turn blue litmus red.
5. Some commonly found acids are hydrochloric acid, nitric acid, sulphuric acid, acetic acid, lactic acid and citric acid.

Mineral acids and organic acids

Mineral acids are obtained from minerals found in nature, in rocks. They are strong acids. Common mineral acids are hydrochloric acid, nitric acid and sulphuric acid.

Note: Acids must NOT be tasted to identify them.

Organic acids are found in plants and animals. They are weak acids. Common organic acids are:

Bases

1. Are bitter to taste.
2. Are soapy to touch.
3. Contain replaceable hydroxide.
4. Turn red litmus blue.
5. Turn phenolphthalein pink.

Common bases are:

Indicators

Acids and bases must not be tasted to test them. Indicators are chemical substances that change colour in an acidic or a basic medium. They are used to identify whether a substance is acidic, basic or neutral. Some common indicators are litmus (obtained from lichens), phenolphthalein, methyl orange, turmeric and China rose.

Formation of an acid
A non-metal reacts with oxygen to form an oxide. When this oxide is dissolved in water, it forms an acid.

Formation of a base
A metal reacts with oxygen to form a basic oxide. When this oxide is dissolved in water, it forms a base.

Bases which dissolve in water are called alkalis. All alkalis are bases, but not all bases are alkalis, because some bases do not dissolve in water.

Neutralisation
An acid reacts with a base to form a salt and water. This reaction is called a neutralisation reaction. Heat is released during a neutralisation reaction.

Acid + Base → Salt + Water

Neutral substance

1. A salt is a neutral substance.
2. It does not show a colour change with indicators.

Uses of neutralisation

1. Curing acidity in the stomach.
2. Neutralising the acidity or basicity of soil.
3. Curing an ant sting.
4. Cleaning surfaces soiled with acids.