CBSE NOTES Class 10 Science Chapter-2. Acids, Bases and Salts Types Of Acids | Indicators

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2. Acids, Bases and salts

Acids: All acids are sour in taste. 

Type of acids: 

(I)   Natural acids: This type of acid is obtained from ntural sources e.g Ascorbic acid (Amla and Guava), Lactic acid (Milk), Oxalic acid (Tomato), Citric acid (Lemon and Orange) etc. 0

(II)  Mineral acids: such type of acid is obtained from minerals e.g Hydrochlorice acid, Sulphuric acid and Nitric acid. 

Properties of acids:

(1)   Acids are sour in taste.

(2)   All acids give H⁺ hydrogen ion with water. 

(3)   It turns colour of blue litmus to red. 

(4)   Acids conduct electricity current.

(5)   It reacts with metals produces salt and hydrogen gas. 

Bases: Bases are bitter in taste and soapy touch. 

Properties of Bases:

(1)   Bases are bitter in taste.

(2)    All bases give (OH^- ) hydroxide ion with water.

(3)    It turns colour of red litmus into blue. 

(4)    Bases conduct electricity current. 

(5)    It also reacts with metals gives salt and hydrogen gas. 

Acid-Base Indicators : These are substances which indicate by chnaging in colour after testing acids and bases. 

Acid-base indicators are two types: 

(I)    Natural Indiccators : Litmus paper, turmeric and red cabbage leaves are natural indicators. 

(II)   Synthetic Indicators : Methyle orange and phenolphthalein are synthetic indicators. 

These indicators tell us whether a substance is acidic or basic by change in colour.

Olfactory Indicator: There are some substances whose odour changes in acidic or basic media. These are called olfactory indicators. 

Which of these – vanilla, onion and clove, can be used as olfactory indicators

Universal Indicator: Universal indicator shows different colour over the range of pH value from 1 to 14 for a given solution. Universal indicator is available both in the form of strips and solution. Universal indicator is the combination of many indicators, such as water, propanol, phelophthalein, sodium salt, sodium hydroxide, methyl red, bromothymol blue monosodium salt, and thymol blue monosodium salt. The colour matching chart is supplied with universal indicator which shows the different colours for different values of pH.

Chemical Properties of Acids and Bases: 

Reaction with acids and metals: 

Acids react with metals give corresponding salt and hydrogen gas; 

   Acid          +              Metal       →             Salt           +         Hydrogen Gas  

Hydrochloric acid reacts with zinc and produces zinc chloride and hydrogen gas.

 2 HCl          +                Zn           →           ZnCl₂          +              H₂

Hydrochloric acid                 Zinc                               Zinc Chloride                   Hydrogen Gas

Hydrochloric acid reacts with sodium and gives sodium chloride and hydrogen gas. 

2 HCl           +            2 Na          →     2NaCl               +                H₂

_{Hydrochlorice acid                Sodium                Sodium Chloride              Hydrogen Gas} 

Hydrogen gas and zinc sulphate are formed when metal zinc reacts with sulphuric acid.

H₂ SO₄        +            Zn             →     ZnSO4              +                H₂  

Sulphuric acid               Zinc                     Zinc Sulphate                  Hydrogen gas 

Testing of Hydrogen Gas: 

When we react any metal with an acid the reaction gives corresponding salt and produces hydrogen gas. During this period of reaction, when we place a burnt candle near by gas it produces a pop sound. The pop sound indicates that the produced gas is hydrogen. 

Reaction of Metal Carbonate/Metal Hydrogencarbonate with Acids:

Limestone, chalk and marble are different forms of calcium carbonate. All metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide and water.
Thus, the reaction can be summarised as –

Metal carbonate + Acid → Salt + Carbon dioxide + Water

Example: 

Calcium Chloride reacts with hydrochloric acid and gives calcium chloride, carbon dioxide and water. 

CaCO₃            +                2HCl              →         CaCl₂      +        CO₂        +       H₂O

(Calcium Carbonate)   (Hydrochloric Acid)   (Calcium Chloride)  (Carbon Dioxide)             (Water)  

Nitric Acid reacts with sodium carbonate and gives sodium nitrate carbon dioxide and water.

2NHO₃       +           Na₂CO₃                →                 NaNO₃        +        CO₂         +      2H₂O

(Nitric Acid)        (Sodium Carbonate)                     (Sodium Nitrate)    (Carbon dioxide)     (water)

Similarly;

Sodium Carbonate  +  Hydrochloric Acid  →  Sodium Chloride + Carbon Dioxide  +  Water

Calcium Carbonate + Sulphuric Acid  →  Calcium Sulphate + Carbon Dioxide  +  Water

Reaction of Metal hydrogencarbonate With Acids

General Formulla; 

Metal Hydrogen Carbonate (Bicarbonate)  + Acid → Salt   +   Carbondioxide   +   water 

Example: 

Sodium Bicarbonate reacts with Hydrochloric Acid produces Sodium Chloride Carbon dioxide and water.  
NaHCO₃        +      2HCl     →      NaCl      +    CO₂   +     H₂O  
(Sodium bicarbonate)   (Hydrochloric Acid)    (Sodium Chloride)   (Carbon dioxide)     (Water)

Reaction with bases and Metals: 

Bases react with metals gives corresponding salts and hydrogen gas. 

Sodium hydroxide reacts with zinc gives sodium zincate and hydrogen gas. 

2NaOH(aq)         +            Zn(s)        →           Na₂ ZnO₂(aq)       +         H₂(g)   

(Sodium hydroxide)        (Zinc)          (Sodium Zincate)          (Hydrogen Gas) 

Sodium hydroxide reacts with alluminium and water gives aodium aluminate and hydrogen  gas. 

2NaOH(aq)        +       2 Al (s)  +  2H₂O      →       2 NaAlO₂(aq)          +      2H₂(g)   

(Sodium hydroxide)   (Aluminium)  (water)        (Sodium Aluminate)          (Hydrogen Gas) 

Historical Monuments and Acid Rain:

Burning of fossil fuels releases oxides of sulphur and nitrogen. Nitrogen oxide and sulphur dioxide form nitric acid and sulphuric acid on reaction with water. When rain droplets mix with these gases; present in atmosphere because of pollution; they form acid rain.

Acid rain causes damage to the historical monuments and other buildings. For example Taj Mahal, which is made of marble, is getting damaged because of reaction with acid rain. Marble is calcium carbonate which reacts with the acid and thus gets corroded.

Use of Reaction between Acid and Metal Carbonate or Metal Bicarbonate in Fire Extinguisher:

Metal carbonate or metal hydrogen carbonate and acid are used in fire extinguisher to produce carbon dioxide gas. Acid and metal carbonate or bicarbonate are kept in separate chambers in a fire extinguisher. On emergency they are allowed to react with one another. The carbon dioxide gas so produce is poured over fire. As carbon dioxide does not support burning, it puts off the fire.