According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 4.
Acids and Bases
- 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 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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- 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.
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- 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.
- Salt can be acidic, basic or neutral in nature.
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Substances, which are used to test whether a substance is acidic or basic, are called indicators.
|Acidic Solution||Basic Solution|
|Red litmus||No change||Turns blue|
|Blue litmus||Turns red||No change|
- 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 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 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 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 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
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.
Frequently Asked Questions on CBSE Class 7 Science Notes Chapter 5 Acids and Bases
What is the neutralisation reaction?
A neutralization reaction is a chemical reaction where an acid and a base are combined with the intent of producing a neutral pH level.
What is litmus?
Litmus paper is colouring matter from lichens that turn red in acid solutions and blue in alkaline solutions and is used as an acid-base indicator.
What is an olfactory indicator?
An olfactory indicator is a substance whose smell varies depending on whether it is mixed with an acidic or basic solution.