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

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.E.g.:-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. Eg:- soap, ammonium hydroxide, calcium hydroxide, etc.

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Neutralisation Reaction

  • When an acid and base react with each other to form a salt, water and heat then such reaction is known as 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+NaOHNaCl(salt)+H2O

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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+NaOHNaCl+H2O

  • 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.

Acidic Solution Basic Solution
Red litmus No change Turns blue
Blue litmus Turns red No change

 

Natural Indicators

  • Plants or plant parts contain useful chemicals that are used for testing an acidic or basic property of a solution are termed as 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.
(A) Red litmus turns blue indicating a basic solution
(B) Blue litmus turns red indicating an acidic solution

Phenolphthalein

  • Phenolphthalein is an acid-base indicator which is colourless in 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 being acidic in nature can cause environmental damage, are treated with basic substances before discharge.

To know more about “Neutralisation in Daily Life”, visit here.

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 at 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.

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