ACIDS, BASES and SALTS , Class-7

1. ACID - BASE INDICATORS

An acid - base indicator is a substance that gives different colours in acidic and basic media.
Examples:- Litmus, Phenolphthalein , Turmeric juice, China rose juice, Red cabbage juice.
(i) Litmus turns red in acidic solution and blue in basic solutions.
(ii) Phenolphtalein turns red or pink in basic solutions and is colourless in acidic and neutral solutions.
(iii) The yellow turmeric juice turns reddish to deep brown in a basic medium and remains yellow in an acidic or neutral medium.
(iv)China rose juice turns green in a basic medium and red in an acidic solution.
(v)Red cabbage juice, purple in colour, turns green in a basic medium and red in an acidic solution.

2.Acidic household substances

(i) Bathroom acid
(ii) Vitamin C tablets
(iii) Lemon juice
(iv)Orange juice
(v) Tomato juice
(vi)Vinegar
(vii) Yoghurt
(viii)Fizzy drinks

3.Basic household substances

(i) Milk of magnesia (antacid)
(ii) Toothpaste
(iii) Soap solution or detergent solution
(iv) Solution of washing soda
(v) Slaked lime and whitewash

4.ACIDS and their behaviour

(A) Uses of acids

(B) Preparation of acids

(i) Non- metals react with oxygen to form oxides.Some non- metal oxides are acidic. When these oxides are dissolved in water,acids are formed.
CO₂ + H₂O → H₂CO₃
SO₃ + H₂O → H₂SO₄
3NO₂ + H₂O → NO + 2HNO₃

(C) General behaviour of acids

1. They have a sour taste.
2.They turn blue litmus red.
3.They corrode most metals.
     It means acids act chemically on metals and form salts.
    That is why sour things are not kept in metal pots.
    When acids react with some metals, hydrogen gas is produced.
     metal + acid → salt + hydrogen

   Zn + H₂SO₄ → ZnSO₄ + H₂
   Fe + H₂SO₄ → FeSO₄ + H₂
4. When acids react with carbonates and hydrogen carbonates (bicarbonates), salt and water is formed         along with carbon dioxide with effervescence.
    acid + metal carbonate → salt + water + carbon dioxide
    acid + metal hydrogen carbonate → salt + water + carbon dioxide
    Examples -
    Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Concentrated acids and dilute acids

(a) The solution that is saturated or contains a large amount of the acid , is called a concentrated acid
(b) The solution that contains a small amount of the acid , is called a dilute solution.

Acid rain:-

Rain with a high acid content is called acid rain.Acidic gases like sulphur dioxide, sulphur trioxide, nitrogen dioxide are dissolved in rain water and form acid.

5.BASIC SUBSTANCES

Bases are compounds that react with acids to form salts and water.The reactions between bases and acids are called neutralisation reactions.
base + acid → salt + water
CaO + 2HCl → CaCl₂

Examples of bases :-
Sodium oxide (Na₂O) , 

Calcium oxide (CaO) , 

Sodium hydroxide (NaOH) , 

Ammonium hydroxide ( NH₄OH) 

General behaviour:
(i)They have bitter taste.
(ii) Thet turn red litmus blue.
(iii) They neutralise acids.
(iv) Caustic alkalies corrode glass and some metals like Al, Zn, Sn and Pb.