Chapter – 2

Acids, Bases and Salts

In this post we have given the detailed notes of class 10 Science Chapter 2 (Acids, Bases and Salts) in English. These notes are useful for the students who are going to appear in class 10 board exams.

Chapter 2: Acids, Bases and Salts

Indicators 🧪

• Substances that change their colour when they behave in an acidic or basic manner with other substances are called indicators.

Types of Indicators

There are many types of indicators, but the common types are as follows:

(i) Natural Indicators: Indicators obtained from natural sources are called natural indicators. For example — litmus, turmeric, China rose, red cabbage, etc.

Litmus: Litmus solution is a purple-coloured dye that is extracted from a group of plants called lichens (Thallophyta). When litmus solution is neither acidic nor basic, its colour is purple. Litmus paper comes in two colours — blue and red. An acid turns blue litmus paper red, while a base turns red litmus paper blue. 🔵🔴

Turmeric: Turmeric is another type of natural indicator. It is yellow in colour. You may have noticed that when a vegetable curry stain gets on white clothes and is washed with soap (which is basic in nature), the stain turns reddish-brown. 🟡

• There is no change in the colour of turmeric with an acid.
• Its colour turns reddish-brown with a base.

(ii) Synthetic Indicators: These are indicators that are not natural but are prepared using chemical substances, such as methyl orange and phenolphthalein. They are used to test acids and bases.

(iii) Olfactory Indicators: There are some substances whose smell changes in an acidic or basic medium. Such substances are called olfactory indicators, for example — vanilla, onion, and clove.

(iv) Universal Indicator: A universal indicator is a mixture of many indicators. Indicators like litmus, methyl orange, and phenolphthalein can only tell us whether a solution is acidic or basic in nature, but a universal indicator also tells us the measure of the strength of the acid or base, along with its nature.

Chemical Properties of Acids and Bases

Reaction of Acids with Metals: An acid reacts with a metal to form the corresponding metal salt and hydrogen gas.

Acid + Metal → Salt + Hydrogen gas

The reaction of hydrochloric acid with zinc forms zinc chloride and hydrogen gas.

2HCl + Zn → ZnCl₂ + H₂

(Hydrochloric acid) (Zinc)   (Zinc chloride)   (Hydrogen gas)

The reaction of hydrochloric acid with sodium forms sodium chloride and hydrogen gas.

2HCl + 2Na → 2NaCl + H₂

(Hydrochloric acid) (Sodium)   (Sodium chloride)   (Hydrogen gas)

The reaction of the metal zinc with sulphuric acid forms zinc sulphate and hydrogen gas.

Zn + H₂SO₄ → ZnSO₄ + H₂

(Sulphuric acid) (Zinc)   (Zinc sulphate)   (Hydrogen gas)

Testing for Hydrogen Gas

• When we react a metal with an acid, it produces the corresponding salt and hydrogen gas. During this reaction, when we bring a burning candle near this gas, it produces a popping sound. The popping sound confirms that the gas produced is hydrogen. 🔥

Reaction of Acids with Metal Carbonates / Metal Hydrogencarbonates

Limestone, chalk, and marble are different forms of calcium carbonate. All metal carbonates and hydrogencarbonates react with an acid to give the corresponding salt, carbon dioxide, and water. The general form of this reaction is as follows:

Metal carbonate + Acid → Salt + Carbon dioxide + Water

Example:

Calcium carbonate reacts with hydrochloric acid to give 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 to form sodium nitrate, carbon dioxide, and water.

2HNO₃ + Na₂CO₃ → 2NaNO₃ + CO₂ + H₂O

(Nitric acid) (Sodium carbonate) (Sodium nitrate) (Carbon dioxide) (Water)

Similarly, the following reactions also take place:

Sodium carbonate + Hydrochloric acid → Sodium chloride + Carbon dioxide + Water

Calcium carbonate + Sulphuric acid → Calcium sulphate + Carbon dioxide + Water

• Reaction of metal hydrogencarbonate with acid: General formula

Metal hydrogencarbonate (bicarbonate) + Acid → Salt + Carbon dioxide + Water

Example:

Sodium bicarbonate reacts with hydrochloric acid to form sodium chloride, carbon dioxide, and water.

NaHCO₃ + HCl → NaCl + CO₂ + H₂O

(Sodium bicarbonate) (Hydrochloric acid) (Sodium chloride) (Carbon dioxide) (Water)

• Reaction of Metal with Base

Bases react with metals to form the corresponding metal salt and hydrogen gas. Sodium hydroxide reacts with zinc to give sodium zincate and hydrogen gas.

2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)

(Sodium hydroxide) (Zinc) (Sodium zincate) (Hydrogen gas)

Sodium hydroxide reacts with aluminium to give sodium aluminate and hydrogen gas.

2NaOH(aq) + 2Al(s) + 2H₂O → 2NaAlO₂(aq) + 3H₂(g)

(Sodium hydroxide) (Aluminium) (Water) (Sodium aluminate) (Hydrogen gas)

Neutralisation Reaction ⚖️

The reaction between an acid and a base to form a salt and water is called a neutralisation reaction.

The general formula for a neutralisation reaction is written as:

Base + Acid → Salt + Water

Sodium hydroxide reacts with hydrochloric acid to form common salt and water.

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O

(Sodium hydroxide) (Hydrochloric acid) (Sodium chloride) (Water)

Sodium hydroxide reacts with nitric acid to form sodium nitrate and water.

NaOH(aq) + HNO₃(aq) → NaNO₃(aq) + H₂O

(Sodium hydroxide) (Nitric acid) (Sodium nitrate) (Water)

Sodium hydroxide reacts with sulphuric acid to form sodium sulphate and water.

2NaOH(aq) + H₂SO₄ → Na₂SO₄(aq) + 2H₂O

(Sodium hydroxide) (Sulphuric acid) (Sodium sulphate) (Water)

Reaction of Metal Oxides with Acids

All metal oxides are basic in nature, so they react with acids to form salt and water. This reaction is exactly like a neutralisation reaction.

Iron (III) oxide reacts with sulphuric acid to form iron sulphate and water.

Fe₂O₃ + 3H₂SO₄ → Fe₂(SO₄)₃ + 3H₂O

(Ferric oxide) (Sulphuric acid) (Ferric sulphate) (Water)

Copper oxide reacts with hydrochloric acid to give copper chloride and water.

CuO + 2HCl → CuCl₂ + H₂O

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

Calcium oxide reacts with hydrochloric acid to give calcium chloride and water.

CaO(s) + 2HCl(aq) → CaCl₂(aq) + H₂O

(Calcium oxide) (Hydrochloric acid) (Calcium chloride) (Water)

Reaction of Bases with Non-metal Oxides

Non-metals are acidic in nature, so they react with a base to form salt and water; this reaction is similar to a neutralisation reaction.

Base + Non-metallic oxide → Salt + Water

Sodium hydroxide reacts with carbon dioxide to give sodium carbonate and water.

2NaOH(aq) + CO₂(g) → Na₂CO₃(s) + H₂O

(Sodium hydroxide) (Carbon dioxide) (Sodium carbonate) (Water)

Salts 🧂

• A salt is the ionic product of a neutralisation reaction between an acid and a base.

(i) Acidic Salts: Acidic salts are obtained as a result of the reaction between a strong acid and a weak base.

Acidic salt: NH₄Cl

HCl + NH₄OH → NH₄Cl + H₂O

(Strong acid)   (Weak base)   (Acidic salt)

(ii) Neutral Salts: Neutral salts are obtained from the reaction between a strong acid and a strong base.

Neutral salt: NaCl

HCl + NaOH → NaCl + H₂O

(Strong acid)   (Strong base)   (Neutral salt)

(iii) Basic Salts: Basic salts are obtained from the reaction between a strong base and a weak acid.

Basic salt: NaC₂H₃O₂

HC₂H₃O₂ + NaOH → NaC₂H₃O₂ + H₂O

(Weak acid)   (Strong base)   (Basic salt)

Dilution

• Adding an acid or a base to water decreases the concentration of ions (H₃O⁺/OH^–) per unit volume. This process is called dilution. Acids and bases are diluted by adding water to them.
• pH Scale: A scale has been developed to find the concentration of hydrogen ions present in a solution, called the pH scale. This scale carries numbers from 1 to 14, which tell us the strength or weakness of an acid or base along with its value. It is a type of universal indicator.
  • The higher the concentration of hydronium ions, the lower the pH.
  • The pH value of any neutral solution is 7.
  • If the pH value of a solution is less than 7, it is acidic. As the value goes below 7, the concentration of H⁺ ions increases, i.e., the strength of the acid increases.
  • If the pH value is greater than 7, the solution is basic. As the value goes above 7, the concentration of OH^– ions increases, i.e., the strength of the base increases.
• Strong Acid: Acids that produce a large number of H⁺ ions in solution are called strong acids.
• Weak Acid: Acids that produce fewer H⁺ ions are called weak acids. A solution having a large number of OH^– ions is called a strong base.
• Weak Base: A solution having a lower number of OH^– ions is called a weak base.
  • Our blood functions within a pH range of 7.35 – 7.45, with an average pH value of 7.4.
  • If the pH of blood rises above 7.45, the condition is called alkalosis, and if the pH of blood falls below 7.35, the condition is called acidosis.

Importance of pH in Everyday Life

(i) Blood and our body: Our body functions within a pH range of 7.0 to 7.8. Living organisms can survive only within a narrow range of pH. When the pH of rainwater falls below 5.6, it is called acid rain.

Harmful effects of acid rain: When acid rain water flows into a river, the pH of the river water decreases. It then becomes difficult for aquatic life to survive in such a river.

(ii) Soil acidity: Sometimes, due to various reasons or because of acid rain, the pH of the soil decreases, resulting in poor crop yield, since plants need a specific pH range for a good yield. When the acidic nature of the soil increases, it harms the plants and the crop does not grow well.

Remedy for correcting soil pH: To remove the acidity of soil, chalk powder or lime is added to the soil so that its acidity is neutralised and the nature of the soil becomes basic.

(iii) Digestion of food in an acidic medium: pH is also important because of the hydrochloric acid (HCl) produced by our stomach. This acid helps in the digestion of food at a specific pH, without harming the stomach. Normally, the pH range of our stomach is between about 1.5 and 3.5. Within this, two conditions can occur:

• Hypoacidity: In some people, the secretion of HCl is very low, due to which their food does not get digested properly or gets digested less. This condition is called hypoacidity (indigestion). Such a person needs to take acidic substances like lemon or vinegar with their food, or take medicines that produce digestive juices.
• Hyperacidity: When excess acid is produced in the stomach, a person experiences pain and a burning sensation in the stomach. To get relief from this pain or burning sensation, one needs to take an antacid.

(Antacid medicine): Antacids are weak bases that reduce the effect of acid, such as milk of magnesia (magnesium hydroxide), aluminium hydroxide, and sodium hydroxide, which are weak bases that form part of an antacid. They neutralise the acidic effect.

Tooth decay: Normally, the pH of the mouth is 5.5. If it falls below 5.5, tooth decay starts. Tooth enamel is made of calcium phosphate, which is the hardest substance in the human body. It protects the teeth from the outside. When the pH of the mouth falls below 5.5, it slowly starts getting corroded.

Reason for lowering of mouth pH: When we eat food or something sweet, leftover sugar and food particles remain in the mouth after eating, which bacteria present in the mouth break down and produce acid. This acid destroys the enamel, which is the main cause of tooth decay.

Prevention of tooth decay: This can be prevented by cleaning the mouth after eating. Using a basic toothpaste for cleaning the mouth neutralises the excess acid, which as a result prevents tooth decay.

Chlor-Alkali Process ⚡

When electricity is passed through an aqueous solution of sodium chloride (common salt), it decomposes to give sodium hydroxide, chlorine gas, and hydrogen gas. This process is called the Chlor-alkali process.

The chemical equation for this process is as follows:

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

Electrolysis of Sodium Chloride

When electricity is passed through an aqueous solution of sodium chloride, chlorine gas is produced at the anode and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode.

Products of the Chlor-Alkali Process

• Sodium hydroxide
• Chlorine gas
• Hydrogen gas

Uses of Sodium Hydroxide

• It is used to remove grease from metals.
• It is used in the making of soaps and detergents.
• It is also used in making paper.
• It is used in making artificial fibres.

Uses of Chlorine Gas

• Chlorine gas is used for the purification of water.
• Used in swimming pools.
• Used in making PVC, CFCs, and pesticides.
• It is also used in making disinfectants.

Uses of Hydrogen Gas

• It is used as a fuel.
• It is used in the manufacture of margarine.
• It is also used to prepare ammonia for use as fertiliser.

Production of Hydrochloric Acid

Chlorine and hydrogen are important products of the Chlor-alkali process, which are used in the production of hydrochloric acid. Hydrochloric acid is an important chemical which is used in the production of the following substances:

• In the manufacture of medicines,
• In the manufacture of cosmetics,
• In the manufacture of ammonium chloride, and
• Used for cleaning steel.

Production of Bleaching Powder

• Bleaching powder is formed when chlorine obtained from the Chlor-alkali process reacts with dry slaked lime.
• The chemical equation for this process is as follows:

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Uses of Bleaching Powder

• In the textile industry for bleaching cotton and linen, in paper factories for bleaching wood pulp, and in laundries for bleaching clothes.
• As an oxidising agent in many chemical industries, and
• As a disinfectant to make drinking water free of germs.

Production of Baking Soda

• The chemical name of this compound is sodium hydrogencarbonate (NaHCO₃). It is manufactured using sodium chloride as a raw material.
• Its chemical equation is as follows:

NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃

(Ammonium chloride)   (Sodium hydrogencarbonate)

This process gives two important products: (i) ammonium chloride, and (ii) baking soda.

Uses of Baking Soda

• Baking soda is commonly used in the kitchen to make tasty, crispy items like pakoras.
• It is sometimes used to cook food quickly.
• It is also a mild base, which is sometimes used to relieve hyperacidity. It is also an ingredient of antacids.
• It is used in soda-acid fire extinguishers.
• It is used in the manufacture of baking powder.
• When it is heated during cooking, the following reaction takes place:

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Preparation of Baking Powder

• Baking powder is prepared from a mixture of baking soda and a mild edible acid, such as tartaric acid.
• When baking powder is mixed with water and heated, it gives carbon dioxide, water, and the sodium salt of the acid, through the following reaction:
• NaHCO₃ + H⁺ → CO₂ + H₂O + sodium salt of the acid. This reaction produces carbon dioxide, which makes bread or cakes rise and become soft and spongy.

Key Points to Remember

• Indicators change colour to show whether a substance is acidic or basic; common examples are litmus, turmeric, methyl orange, and phenolphthalein. 🧪
• Acids react with active metals to give a salt and hydrogen gas; the hydrogen is tested with a burning candle, which gives a popping sound. 🔥
• Metal carbonates and hydrogencarbonates react with acids to give salt, carbon dioxide, and water.
• The reaction between an acid and a base to form a salt and water is called a neutralisation reaction. ⚖️
• The pH scale (1–14) measures the concentration of H⁺ ions in a solution; pH 7 is neutral, below 7 is acidic, and above 7 is basic.
• Our blood works in the pH range 7.35–7.45; the stomach’s HCl aids digestion at a pH of about 1.5–3.5.
• The Chlor-alkali process (electrolysis of NaCl solution) produces sodium hydroxide, chlorine gas, and hydrogen gas, which have many industrial uses. ⚡
• Bleaching powder (CaOCl₂) is made from chlorine and slaked lime; baking soda (NaHCO₃) and baking powder are important household chemicals. 🧂

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