CBSE 10th Science Board Exam 2021: Important MCQs from Chapter 2: Acids, Bases and Salts

 

 

Revision Notes for CBSE Class 10 Science Notes for Chapter 2 Acids, Bases and Salts:

Acid - A substance that produces hydrogen ions (H⁺) in aqueous solutions.

For Example - Sulphuric Acid (H₂SO₄), Hydrochloric Acid (HCl).

Types of Acids: Acids are divided into two types on the basis of their occurrence i.e., Natural acids and Mineral acids.
(i) Natural Acids: Acids which are obtained from natural sources are called Natural Acids or Organic Acids.
Examples:
Methanoic acid (HCOOH)
Acetic acid (CH₃COOH)
Oxalic acid (C₂H₂O₄) etc.

(ii) Mineral Acids: Acids that are prepared from minerals are known as Mineral Acids Example; Inorganic acids, man-made acids or synthetic acid are also known as Mineral Acids.
Example:
Hydrochloric acid (HCl)
Sulphuric acid (H₂SO₄)
Nitric acid (HNO₃)
Carbonic acid (H₂CO₃)
Phosphoric acid (H₃PO₄) etc.

Chemical Properties of Acid:
 

(i) Reaction of acids with metal: Acids give hydrogen gas along with respective salt when they react with a metal.
Metal + Acid → Salt + Hydrogen
Examples:
Hydrogen gas and zinc chloride are formed when hydrochloric acid reacts with zinc metal.

General properties of acids:

• They have a sour taste.
• They turn blue litmus to red.
• They conduct electricity in solution form.
• They release H⁺ ions in aqueous solution.

Reactions of Acids

(i) Reaction of Acid with Metal

Na (metal) + H₂ SO₄ (acid) → H₂ (hydrogen gas) + Na SO₄ (salt)

(ii) Reaction of Acid with Carbonates

Na₂ CO₃ (s) + 2 HCl (aq) → 2NaCl (aq) + H₂O(l) + CO₂(g)

(iii) Reaction of Acid with Bicarbonates

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

Base - A substance that produces hydroxide ions (OH^–) in aqueous solutions. 

For Example - Sodium Hydroxide (NaOH), Potassium hydroxide (KOH)

General properties of bases:

• They have a bitter taste.
• They are soapy to touch.
• They turn red litmus to blue.
• They conduct electricity in solution form.
• They release OH^– ions in aqueous solution

Reactions of Bases

(i) Reaction with Metals

2NaOH + Zn → Na₂ZnO₂ + H₂

(ii) Reaction with Non-metallic Oxides

2NaOH + CO₂ → Na₂CO₃ + H₂O

(iii) Reaction with Acids

NaOH + HCl → NaCl + H₂O        

Strong Acids An acid which completely dissociates into its ions in aqueous solution. For example: Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3)
Strong Base A base which completely dissociates into its ions in aqueous solution. For example: Sodium hydroxide (NaOH), Potassium hydroxide (KOH) Weak Acids An acid which does not completely dissociate into its ions in aqueous solutions.  For example: Acetic acid (CH3COOH), Carbonic acid (H2CO3)

For water or neutral solutions : pH = 7

For acidic solutions : pH < 7

For basic solution : pH > 7

Weak Base

A base which does not completely dissociate into its ions in aqueous solution.

For example: 

Ammonium hydroxide (NH₄OH)

Indicators: Indicators are substances which indicate the acidic or basic nature of the solution by the colour change.
Types of Indicator: There are many types of indicators. Some common types of indicators are:

1. Natural Indicators: Indicators obtained from natural sources are called Natural Indicators. Litmus, turmeric, red cabbage, China rose, etc., are some common natural indicators used widely to show the acidic or basic character of substances.
Litmus: Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colours- blue and red.
An acid turns blue litmus paper red.
A base turns red litmus paper blue.

Turmeric: Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.

Red Cabbage: The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.

 2. Olfactory Indicator: Substances which change their smell when mixed with acid or base are known as Olfactory Indicators. For example; Onion, vanilla etc.
Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.
Vanilla: The smell of vanilla vanishes with base, but its smell does not vanish with an acid.
Olfactory Indicators are used to ensure the participation of visually impaired students in the laboratory.

3. Synthetic Indicator: Indicators that are synthesized in the laboratory are known as Synthetic Indicators. For example; Phenolphthalein, methyl orange, etc.
Phenolphthalein is a colourless liquid. It remains colourless with acid but turns into pink with a base.
Methyl orange is originally orange in colour. It turns into the red with acid and turns into yellow with base.

Importance of pH in everyday life

(i) pH in our digestive system: Our stomach produces hydrochloric acid that helps in the digestion of food. During indigestion the stomach produces too much acid and this causes pain and irritation. To get rid of this pain, antacids like magnesium hydroxide [Mg(OH)₂] also known as milk of magnesia and sodium hydrogen carbonate (baking soda) are used to neutralize excess acid.

(ii) Tooth decay caused by acids: Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating. When the pH of acid formed in the mouth falls below 5.5, tooth-decaying starts. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and

prevent tooth decay.

(iii) pH of soil and plant growth: Most of the plants require a specific pH range (close to 7) for their healthy growth. If the soil is too acidic or basic, the plants grow badly or do not grow at all. pH of the soil can be adjusted by using certain chemicals. For example, if the soil is too acidic then it is treated with materials like quicklime or slaked lime. On the other hand, if the soil is too alkaline then alkalinity can be reduced by adding decaying organic matter.

Preparation and uses of important compounds

Caustic Soda (Sodium Hydroxide, NaOH)

Preparation: In the process of electrolytic decomposition of brine (aqueous solution of sodium chloride), brine decomposes to form sodium hydroxide.

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

In this process, chlorine is obtained at anode and hydrogen gas is obtained at cathode as by products. This whole process is known as Chlor – Alkali process.

Uses:

Sodium hydroxide is used for degreasing of metals, manufacturing of paper, soap, detergents, artificial fibres, etc.

Bleaching Powder (Calcium Oxychloride, CaOCl₂)

Preparation: Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂].

Ca(OH)₂ + Cl₂→ CaOCl₂ + H₂O

Uses:

Bleaching powder is used –

(i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;

(ii) as an oxidising agent in many chemical industries; and

(iii) to make drinking water free from germs

Baking Soda (Sodium Hydrogen Carbonate, NaHCO₃)

Preparation:

The chemical name of the compound is sodium hydrogen carbonate (NaHCO₃). It is produced by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process.

NaCl  +  H₂O  +  CO₂   +   NH₃   →     NH₄Cl        +    NaHCO₃

                                                       Ammonium          Sodium

                                                         chloride     hydrogen carbonate

Uses:

(i) Baking soda is used in making of baking powder, which is used in cooking.

Baking powder is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place –

NaHCO₃   +    H⁺     →     CO₂ + H₂O + Sodium salt of acid

            (From any acid)

Carbon dioxide produced during the reaction can cause bread or cake to rise making them soft and spongy.

(ii) Baking soda (sodium hydrogen carbonate) is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.

(iii) It is also used in soda-acid fire extinguishers.

Washing Soda (Sodium Carbonate, Na₂CO₃.10H₂O )

Preparation:

Sodium carbonate is manufactured by the thermal decomposition of sodium hydrogen carbonate obtained by Solvay process.

NaCl + NH₃ + H₂O + CO₂ →  NaHCO₃ + NH₄Cl

NaHCO₃   →   Na₂CO₃ + CO₂ + H₂O

               Heat

Na₂CO₃ + 10 H₂O → Na₂CO₃.10H₂O

Uses:

(i) Sodium carbonate (washing soda) is used in glass, soap and paper industries.

(ii) It is used in the manufacture of sodium compounds such as borax.

(iii) Sodium carbonate can be used as a cleaning agent for domestic purposes.

(iv) It is used for removing permanent hardness of water

Plaster of Paris (Calcium Sulphate Hemihydrate, CaSO₄. ½ H₂O)

Preparation:

On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate(CaSO₄. ½H₂O) which is called Plaster of

Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.

CaSO₄. ½H₂O + 1½H₂O  → CaSO4.2H₂O

       (POP)                              (Gypsum)