Chapter 2: Acids, Bases and Salts
Class X Science
Focus Areas
- Acids: Substances which turn blue litmus solution red are called acids. Acids are sour in taste.
- Bases: Substances which change red litmus solution blue are called bases. They are bitter in taste.
- Mineral Acids: Acids which are obtained from minerals like sulphates, nitrates, chlorides etc. are called mineral acids. Example: \( H_2SO_4 \) (Sulphuric acid), \( HNO_3 \) (Nitric acid) and \( HCl \) (Hydrochloric acid).
- Organic Acids: Acids which are obtained from plants and animals are called organic acids. Example: Citric acid, ascorbic acid, tartaric acid, lactic acid, acetic acid.
- Hydronium Ions (\( H_3O^+ \)): They are formed by reaction of \( H^+ \) (from acid) and \( H_2O \). It is because \( H^+ \) is unstable.
- Strong Acids: Acids which dissociate into ions completely are called strong acids. Example: \( H_2SO_4 \), \( HCl \).
- Weak Acids: Acids which do not dissociate into ions completely are called weak acids. Example: Citric acid, acetic acid.
Indicators
Indicators are substances which indicate the acidic or basic nature of the solution by their colour change.
Types of Indicators
- Natural Indicators: Litmus, Turmeric.
- Synthetic Indicators: Methyl Orange, Phenolphthalein.
- Olfactory Indicators: These are chemical substances whose odour changes in acidic or basic medium. Example: Onion, vanilla, and clove oil.
- Universal Indicator: A mixture of indicators which shows a gradual but well-marked series of colour changes over a very wide range of change in concentration of \( H^+ \) ion.
Chemical Reactions of Acids & Bases
1. Reaction of Acid or Base with Metal
General Formula: \( \text{Acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen gas} \)
- Note: Hydrogen gas is not evolved when a metal reacts with nitric acid (\( HNO_3 \)) because it is a strong oxidizing agent. It oxidises the \( H_2 \) produced to water and itself gets reduced to any of the nitrogen oxides (\( N_2O, NO, NO_2 \)).
- The metal Copper (\( Cu \)) does not liberate Hydrogen gas with acids.
- A few metals like zinc, lead and aluminium react with bases to give off hydrogen.
Example:
\[ 2NaOH + Zn \rightarrow Na_2ZnO_2 \text{ (Sodium Zincate)} + H_2 \]
Test for Hydrogen gas: POP test (Burns with a pop sound).
2. Reaction of Acids with Metal Carbonates and Hydrogen Carbonates
General Formulas:
\[ \text{Metal Carbonate} + \text{Acid} \rightarrow \text{Salt} + H_2O + CO_2 \]
\[ \text{Metal Hydrogen Carbonate} + \text{Acid} \rightarrow \text{Salt} + H_2O + CO_2 \]
Examples:
- \( Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2 \)
- \( NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2 \)
The released \( CO_2 \) gas turns lime water milky due to the formation of \( CaCO_3 \).
\[ Ca(OH)_2 \text{ (Lime water)} + CO_2 \rightarrow CaCO_3 \text{ (Milky white)} + H_2O \]
If \( CO_2 \) is passed in excess, milkiness disappears due to the formation of soluble calcium bicarbonate:
\[ CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2 \text{ (Soluble)} \]
3. Neutralisation Reaction
A chemical reaction between an acid and a base to give a salt and water.
\[ \text{Base} + \text{Acid} \rightarrow \text{Salt} + \text{Water} \]
Example: \( NaOH + HCl \rightarrow NaCl + H_2O \)
4. Reaction of Metal Oxides with Acids
\[ \text{Metal Oxide} + \text{Acid} \rightarrow \text{Salt} + \text{Water} \]
Example: \( CuO \text{ (Copper oxide)} + 2HCl \rightarrow CuCl_2 \text{ (Copper Chloride)} + H_2O \)
5. Reaction of Non-metallic Oxide with Base
\[ \text{Non-metallic oxide} + \text{Base} \rightarrow \text{Salt} + \text{Water} \]
Example: \( CO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O \)
Chlor-Alkali Process
When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode.
Equation:
\[ 2NaCl(aq) + 2H_2O(l) \rightarrow 2NaOH(aq) + Cl_2(g) + H_2(g) \]
Uses of Products:
- Sodium Hydroxide (\( NaOH \)): De-greasing metals, soaps and detergents, paper making, artificial fibres.
- Hydrochloric Acid (\( HCl \)): Manufacturing medicines, cosmetics, ammonium chloride, cleaning steel.
- Hydrogen (\( H_2 \)): Fuels, margarine, manufacturing ammonia for fertilisers.
- Chlorine (\( Cl_2 \)): Water treatment, swimming pools, PVC, CFCs, pesticides, disinfectants.
Important Compounds and Their Uses
| Common Name | Chemical Name / Formula | Preparation Equation | Uses |
|---|---|---|---|
| Washing Soda | Sodium carbonate decahydrate \( Na_2CO_3 \cdot 10H_2O \) |
\( Na_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O \) | Manufacture of borax, caustic soda, softening of hard water. |
| Baking Soda | Sodium hydrogen carbonate \( NaHCO_3 \) |
\( NaCl + NH_3 + CO_2 + H_2O \rightarrow NaHCO_3 + NH_4Cl \) | Used as antacid, ingredient of baking powder. |
| Bleaching Powder | Calcium oxychloride \( CaOCl_2 \) |
Action of chlorine on dry slaked lime: \( Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O \) |
Bleaching clothes, oxidizing agent, disinfecting water, manufacture of chloroform. |
| Plaster of Paris | Calcium sulphate hemihydrate \( CaSO_4 \cdot \frac{1}{2}H_2O \) |
Heating Gypsum at 373K: \( CaSO_4 \cdot 2H_2O \xrightarrow{\text{Heat}} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \) |
Plastering fractured bones, making toys, decorative materials, statues. |
Importance of pH in Everyday Life
- Acid Rain: If pH of rainwater is less than 5.6, it is called acid rain.
- Digestive System: Stomach produces \( HCl \) for digestion. Excess acid causes indigestion/pain. Antacids (like Magnesium hydroxide) neutralize excess acid.
- Tooth Decay: Starts when pH of mouth is lower than 5.5. Tooth enamel (calcium phosphate) corrodes. Basic toothpaste prevents decay.
- Self Defense: Bee-sting leaves an acid (pain/irritation); relief obtained by baking soda. Nettle leaves inject methanoic acid; relief by rubbing dock plant leaf.
- Body pH Range: Human body works between pH 7.3 to 7.5.
- Soil: Plants need specific pH. Acidic soil is treated with quick lime or slaked lime.
- Copper Vessels: Tarnished copper oxide layer (basic) is cleaned by lemon (acidic).
Question Bank
Multiple Choice Questions
- Which one of the following salts does not contain water of crystallisation?
Ans: (b) Baking soda - In terms of acidic strength, which one of the following is in the correct increasing order?
Ans: (a) Water < Acetic acid < Hydrochloric acid - Sodium hydroxide turns phenolphthalein solution:
Ans: (a) pink - Nettle sting is a natural source of which acid?
Ans: (a) Methanoic acid - An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?
Ans: (d) Hydrochloric acid - Lime water reacts with chlorine to give:
Ans: (a) bleaching powder - Alkalis are:
Ans: (d) bases, which are soluble in water - Sodium carbonate is a basic salt because it is a salt of a:
Ans: (d) weak acid and strong base - Brine is an:
Ans: (c) aqueous solution of sodium chloride - What is formed when zinc reacts with sodium hydroxide?
Ans: (b) Sodium zincate and hydrogen gas
Very Short Answer Type Questions
Q1. Write a balanced chemical equation for the reaction between sodium carbonate and hydrochloric acid.
Ans: \( Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + CO_2(g) + H_2O(l) \)
Q2. Name the gas usually liberated when a dilute acid reacts with a metal.
Ans: \( H_2 \) gas is liberated. It burns with a pop sound when a burning candle is brought near it.
Q6. A white powder is added while baking breads and cakes to make them soft and fluffy. Name it.
Ans: Baking powder. It consists of sodium hydrogencarbonate and tartaric acid. \( NaHCO_3 \) gives \( CO_2 \) which makes cake soft. Tartaric acid neutralizes bitterness.
Q9. Why do HCl, HNO3, etc. show acidic characters in aqueous solution while solutions of compounds like alcohol and glucose do not?
Ans: \( HCl \) and \( HNO_3 \) release hydrogen ions (\( H^+ \)) in aqueous solution, signaling acidic character. Alcohol and glucose do not release hydrogen ions.
Long Answer Questions
Q1. (a) Write the chemical name and formula of marble. (b) Marbles of Taj are getting corroded. Explain.
Ans:
(a) Calcium carbonate, \( CaCO_3 \).
(b) Acid rain containing sulphuric acid reacts with marble: \( CaCO_3(s) + H_2SO_4(aq) \rightarrow CaSO_4(aq) + H_2O(l) + CO_2(g) \).
Q2. Sugandha prepares HCl gas in her school laboratory. She puts both dry and wet blue litmus papers in contact with the gas. State the colour changes.
Ans:
Dry blue litmus: No change.
Wet blue litmus: Turns red.
Reason: Dry HCl gas does not produce \( H^+ \) ions. In the presence of water, it forms \( H_3O^+ \) ions which show acidic behaviour.
Source Based Question: Dilution
Mixing an acid or base with water results in decrease in the concentration of ions (\( H_3O^+/OH^- \)) per unit volume. Such a process is called dilution.
Q: Identify the error in adding water to acid.
Ans: Water should not be poured into a beaker containing concentrated acid. It causes an explosive exothermic reaction (splashing). Acid must be added slowly to water with stirring.