Acids, Bases and Salts Class 7 Science Notes - Chapter 5

According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 4.

Acids and Bases

Acids

• Acids are substances that taste sour and are corrosive in nature.
• It turns blue litmus paper to red.
• These substances are chemically acidic in nature. For example, orange juice, curd, vinegar, hydrochloric acid etc.

Bases

• Bases are substances that, in aqueous solution, are slippery to the touch and bitter in taste.
• It turns red litmus paper to blue.
• These substances are chemically basic in nature. For example, soap, ammonium hydroxide, calcium hydroxide, etc.

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Neutralisation Reaction

• When an acid and base react with each other to form salt, water and heat, then such a reaction is known as a neutralisation reaction.
• In this reaction, the acidic and basic nature of the acid and base, respectively, are destroyed.
• The reaction between hydrochloric acid and sodium hydroxide is a classic neutralisation reaction where sodium chloride is formed.
  $HCl+NaOH\to NaCl\left(salt\right)+H_{2}O$

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Salts

Salts

• Salt is the product formed from the neutralisation reaction of acids and bases.
• In the reaction between hydrochloric acid and sodium hydroxide, the salt formed is sodium chloride.

       $HCl+NaOH\to NaCl+H_{2}O$

• Salt can be acidic, basic or neutral in nature.

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Visual Indicators

Indicators

Substances, which are used to test whether a substance is acidic or basic, are called indicators.

Natural Indicators

• Plants or plant parts contain useful chemicals that are used for testing an acidic or basic property of a solution and are termed natural indicators. China Rose and turmeric are examples of natural indicators.

Litmus

• Litmus is a naturally occurring purple indicator which is extracted from lichens.
• When added to an acidic solution, it turns red, and when added to a basic solution, it turns blue.
• It is available in the form of a solution or as strips of paper, known as litmus paper.

Phenolphthalein

• Phenolphthalein is an acid-base indicator which is colourless in an acid solution but turns pink to red as the solution becomes alkaline.
• It is a synthetic indicator and is used for the neutralisation experiment.

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Olfactory Indicators

• Olfactory indicators are substances whose smell changes whether they are mixed with an acidic or a basic solution.
• Onion, clove oil and vanilla extract are examples of such indicators.

Visual Indicators

• Visual Indicators are substances used to show visually (as by a change in colour) the condition of a solution with respect to the presence of a particular material (as a free acid or base).
• Common examples are litmus, red cabbage, phenolphthalein, etc

Neutralization

Neutralisation in Daily Life

• During indigestion, taking milk of magnesia (magnesium hydroxide) gives us relief as it neutralizes the effect of excess acid produced inside the stomach.
• The effect of ant sting, which is caused by formic acid, can be neutralized by rubbing moist baking soda (basic in nature).
• To ensure that plants can grow well, the soil is treated with either acids or bases, depending if it’s basic or acidic in nature.
• Factory wastes, generally acidic in nature and can cause environmental damage, are treated with basic substances before discharge.

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Recap of Concepts

Safety Measures While Using Acids

• When diluting acids, pour the acid into the water, NOT water into acid, as this may cause spattering of the acid.
• Safety gloves are to be worn whenever working with acids or bases.

Uses and Applications

pH of Soil

• Excessive use of chemical fertilisers changes the pH of the soil.
• Plants do not grow well when the soil is either too acidic or too basic.
• Hence substances are added to bring the pH to a neutral level.

Uses of Acids and Bases

• Food preservation – Citric acid.
• Aerated drinks – Carbonic acid.
• Baking powder – Tartaric acid.
• Cooking – Acetic acid(vinegar)
• Manufacture of soaps – Sodium hydroxide.
• Manufacture of bleaching powder – Calcium hydroxide
• As a foaming agent in fire extinguishers – Aluminium hydroxide.