Acids Bases and Salts Notes || Class 10 Chapter 2 Science ||

Chapter 2, Class 10

ACIDS, BASES AND SALTS

Acids

1.       Those substances which has sour (खट्टा) taste, is called acids.

2.       The chemical nature of such substances is acidic.

3.       The acids present in plant materials and animals are called organic acids.

4.       Organic acids (or naturally occurring acids) are weak acids.

5.       E.g. Curd, lemon juice, orange juice and vinegar (सिरका) taste sour, it means acidic in nature.

Important name of the Organic acids

1.       Acetic acid contains in vinegar

2.       Formic acid (Methanoic acid) in Ant’s sting (डंक मारना), Nettle sting

3.       Citric acid – Citrus fruits such as oranges, lemons, etc.

4.       Lactic acid in Curd (दही)

5.       Oxalic acid in Spinach (पालक), tomatoes

6.       Ascorbic acid (Vitamin C) in Amla, Citrus fruits

7.       Tartaric acid in Tamarind (इमली), grapes, unripe (कच्चा) mangoes, etc.

Mineral Acids

1.       The acids prepared from the minerals of the earth are called mineral acids.

2.       Mineral acids are manmade acids.

3.       The three most common mineral acids are: Hydrochloric acid (used in dye-stuffs (रंगनेवाला पदार्थ), textile, food and leather industries), Sulphuric acid (used in fertilisers (खाद), paints, dyes, chemicals, detergents, explosives (बारूद) and car batteries) and Nitric acid (used in fertilisers, explosives (like TNT: Tri-Nitro Toluene), dyes and plastics).

4.       All the mineral acids are strong acids.

5.       Only one mineral acid, carbonic acid, is a weak acid.

Concentrated and Dilute Acids

1.       A concentrated acid is one which contains the minimum possible amount of water in it.

2.       The concentration of an acid is decreased by adding more water to it.

3.       When water is added to a concentrated acid, then a dilute acid is formed.

4.       Thus, a dilute acid is one which contains much more of water in it.

Properties of Acids

1.       Acids have a sour taste

2.       Acids turn blue litmus to red

3.       Acid solutions (acid is dissolved in water) conduct electricity (They are electrolytes)

4.       Acids react with metals to form hydrogen gas – When an acid reacts with a metal, then a salt and hydrogen gas are formed. i.e.

Metal + Acid      →   Salt + Hydrogen gas

E.g. when dilute Sulphuric acid reacts with zinc metal, then zinc sulphate and hydrogen gas are formed:

Zn (s)       +         H₂SO₄ (aq)        →                 ZnSO₄ (aq)          +            H₂ (g)

Zinc (metal)        (Sulphuric acid – Dilute)     (Zinc sulphate – salt)      Hydrogen

In this reaction, zinc metal displaces hydrogen from sulphuric acid. And this hydrogen is evolved as hydrogen gas. Most of the acids react with metals to form salts and evolve hydrogen gas. This shows that hydrogen is common to all acids.

5.       Acids react with metal carbonates (and metal hydrogen carbonates) to form carbon dioxide gas

                    Metal carbonate      +        Acid         →          Salt        +         Carbon dioxide       +      Water

                    Metal hydrogen carbonate     +    Acid      →       Salt     +       Carbon dioxide      +       Water

E.g. (i) When dilute hydrochloric acid reacts with sodium carbonate, then sodium chloride, carbon dioxide and water are formed:

Na₂CO₃ (s)         +           2HCl (aq)             →            2NaCl (aq)          +       CO₂ (g)      +      H₂O (l)

Sodium carbonate            Hydrochloric acid               Sodium chloride        Carbon dioxide       Water

(ii) When dilute hydrochloric acid reacts with sodium hydrogen carbonate, then sodium chloride, carbon dioxide and water are formed:

NaHCO₃ (s)    +         HCl (aq)         →              NaCl (aq)        +           CO₂ (g)       +          H₂O (l)

Sodium hydrogen-      Hydrochloric acid            Sodium chloride              Carbon                   Water

      Carbonate                                                                                                dioxide

On passing the carbon dioxide gas evolved through lime water,

When carbon dioxide gas is passed through lime water, the lime water turns milky due to the formation of a white precipitate of calcium carbonate

   Ca (OH)₂ (aq)             +              CO₂ (g)             →              CaCO₃ (s)           +              H₂O (l)

Calcium hydroxide               Carbon dioxide                  Calcium carbonate                        Water

   (Lime water)                                                                       (White ppt.)

                                                                                        (Makes lime water milky)

6.       Acids react with bases (or alkalis) to form salt and water

              When an acid reacts with a base, then a salt and water are formed. That is

                            Acid        +           Base       →          Salt           +       Water

Actually, when an acid is treated with a base, the base neutralises the acid and destroys its acidity. Since an acid and a base neutralise each other’s effect, so the reaction between an acid and a base to form salt and water is called a neutralisation reaction.

E.g.   NaOH (aq)         +            HCl (aq)          →          NaCl (aq)            +          H₂O (l)

    Sodium hydroxide              Hydrochloric acid         Sodium chloride              Water

           (Base)                                   (Acid)                              (Salt)                  

         

When hydrochloric acid reacts with sodium hydroxide solution, then a neutralisation reaction takes place to form sodium chloride and water

7.       Acids react with metal oxides to form salt and water

Acids react with metal oxides to form salt and water

Metal oxide      +          Acid        →          Salt     +          Water

Copper (II) oxide is a metal oxide. Dilute hydrochloric acid reacts with copper (II) oxide to form copper (II) chloride and water

CuO (s)        +           2HCl (aq)       →           CuCl₂ (aq)           +           H₂O (l)

Copper (II) oxide    Hydrochloric acid        Copper (II) chloride     Water

      (Black)                                                             (Blue-green)

WHAT DO ALL ACIDS HAVE IN COMMON

An acid is a substance which dissociates (अलग करना) (or ionises) on dissolving in water to produce hydrogen ions [H⁺ (aq) ions]

E.g. an aqueous solution of hydrochloric acid dissociates (or ionises) to form hydrogen ions (along with chloride ions):

HCl (aq)          →                   H⁺ (aq)         +         Cl^– (aq)

Hydrochloric acid          Hydrogen ions            Chloride ions