Acids, Bases and Salts Class 10 Notes

CBSE Class 10 Science Chapter 2

Acids, Bases and Salts

Topics covered in this chapter

  1. Introduction
  2. Chemical properties of acids and bases
  3. Common properties of acids and bases
  4. Strength of acids and bases
  5. Importance of pH in everyday life
  6. Salts and their properties

Introduction

The sour and bitter tastes of food are due to acids and bases. There are various examples that we see in our daily life defines the properties of acids and bases. Students must have noticed that, stain of curry on a white cloth become reddish-brown when soap is scrubbed on it. This happens because soap is basic is nature. In this chapter, students will study the reactions of acids and bases, how acids and bases cancel out each other effects and many more things that we observe in day to day life.

Chemical Properties of Acids and Bases

1) Acid and Bases in Laboratory

Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases by change of colour. Some substances whose odour changes in acidic and basic media are called olfactory indicators. The acidic nature of a substance is due to formation of H+ (aq) ions in solution whereas the basic nature is due to formation of OH(aq) ions in the solutions.

2) How do Acids and Bases React with Metals?

When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed.

Acid + Metal → Salt + Hydrogen gas

Example:

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

When a base reacts with a metal, hydrogen gas is evolved and salt is formed which has a negative ion composed of the metal and oxygen.

3) How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?

When an acid react with a metal carbonate or metal hydrogencarbonate, it gives salt, carbon dioxide gas and water.

Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water

Examples:

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

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

Limestone, chalk and marble are different forms of calcium carbonate. It can be illustrated from following reaction.

When we pass carbon dioxide gas through lime water then calcium carbonate is formed.

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

(Lime water) (White precipitate)

On passing the excess carbon dioxide we get the calcium bicarbonate

CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)

(Soluble in water)

4) How do Acids and Bases React with each Other?

Acid and base react with each other to form salt and water. This is also known as neutralisation reaction as the effect of base is nullified by an acid and vice versa.

Acid + Base → Salt + Water

Example:

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

5) Reaction of Metallic Oxides with Acids

The reaction between a metal oxide and acid can be written as:

Metal Oxide + Acid → Salt + Water

6) Reaction of a non-metallic Oxide with Base

Metallic oxide are acidic in nature. The reaction can be summarised as:

Non-metallic Oxide + Base → Salt + Water

Example:

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

Similar Properties of Acid and Bases

Acids produce hydrogen ions, H+(aq), in solution which is responsible for their acidic properties.

HCl + H2O → H3O+ + Cl

Hydrogen ions cannot exist alone, but they exist after combining with water molecules, Thus, hydrogen ions must always be shown as H+(aq) or hydronium ion (H3O+)

Base generate hydroxide (OH) ions in water. Bases which are soluble in water are called alkalis.

NaOH(s) → Na+(aq) + OH(aq)

The neutralisation reaction can also be defined as;

H+(aq) + OH(aq) → H2O(l)

Acid and basic solutions in water conduct electricity because of H+ and OH ions respectively. The process of dissolving an acid or a base in water is a high exothermic reaction. It result in decrease in concentration of ions (H3O+/OH) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.

Strength of Acids and Bases

Acid-base indicators are used to distinguish between an acid and a base. The strength of an acid or an alkali can be tested by using universal indicator or pH scale (0-14). Universal indicator is a mixture of several indicators which shows different colours at the different concentration of hydrogen ions in a solution. Similarly, pH scale also give the measure of hydrogen ion concentration in a solution. The p in pH stands for “potenz” which means power. We can measure pH from 0 (very acidic) to 14 (very alkaline) in a pH scale.

A neutral solution has a pH of exactly equal to 7.

An acidic solution has pH < 7.

An basic solution has pH > 7.

Importance of pH in Everyday Life

Living organism carry out their metabolic activities within an optimal pH range.

1) Plants and animals are pH sensitive

Human body works within the pH range of 7.0 to 7.8. The rain water become acid rain when pH of rain water is less than 5.6. When this acid rain flows into the rivers in lowers the pH of river water and effect the survival of aquatic life.

2) pH of soil for plant growth

Plants require a specific pH range for their healthy growth which is normally from 5.5 to 7 pH range.

3) pH in digestive system

Human stomach produces hydrochloric acid (HCl) which help in the digestion of food. Some time we suffer from the acidity problem. This is because stomach produces too much acid during indigestion and it causes pain and irritation. To get rid of this pain, people use bases called antacids.

4) pH change as the cause of tooth decay

Tooth are made up of calcium hydroxyapatite. It does not dissolve in water but is corroded when the pH in the mouth is below 5.5. So, tooth decay starts when the pH is lower than 5.5. Using toothpastes for cleaning the teeth after food can neutralize the excess acid and prevent the tooth decay.

5) Self defence by animals and plants through chemical warfare

Honey-bee produces methanoic acid which is also known as formic acid when it stung on human body. Similarly, nettle leaves also produces methanoic acid when someone touches its leaves.

Salts and Their Properties

Here we will learn about the preparation, properties and uses of salts.

1) Family of Salts

Salts having same positive and negative radicals are said to belong to a family.

Example: NaCl and Na2SO4 belong to the family of sodium salts.

NaCl and KCl belong to chloride salts.

2) pH of Salts

We can identify whether a salt is acidic, basic or neutral in nature.

  • Salts of a strong acid and a strong base are neutral with pH value of 7.
  • Salts of a strong acid and a weak base are acidic with pH value less than 7.
  • Strong base and weak acid are basic in nature with pH value more than 7.

3) Chemicals from Common Salt

Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. The large crystals of solid salts which are brown in colour are called rock salt. Beds of rock salt are formed when the seas of bygone ages dried up. Rock salt is mined like coal.

The common salt is used for various purposes in daily use such as sodium hydroxide, baking soda, washing soda, bleaching powder and many more.

Sodium hydroxide

Chlorine is produced during the electrolysis of aqueous sodium chloride (brine). This process is known as chlor-alkali process because the product formed chor is for chlorine and alkali for sodium hydroxide.

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

Bleaching Powder

The chlorine gas produced in the process of chlor-alkalib is used for manufacturing of bleaching powder.

Ca(OH)2 + Cl2 → CaOCl2 + H2O

(Dry slaked lime) (Bleaching powder)

Uses of Bleaching Powder

  1. Bleaching cotton and linen in textile industry
  2. Bleaching wood pulp in paper factory
  3. Beaching washed clothes in laundry
  4. For making drinking water free from germs
  5. Used as an oxidizing agent in many chemical industries

Baking Soda

Baking soda is commonly used in kitchen for making recipe. It is also used for faster cooking. Its chemical name is sodium hydrogencarbonate (NaHCO3).

NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3

Baking soda is a mild non-corrosive basic salt. When it is heated for cooking from chemical reaction take place;

2NaHCO3 → Na2CO3 + H2O + CO2

Uses of Baking Soda

  1. It is used for making baking powder which is a mixture of banking soda and a mild edible acid such as tartaric acid. Following reaction take place when baking soda is heated.NaHCO3 + H+ → CO2 + H2O + Sodium salt of acid [H+ is taken from any acid]
  2. Used in soda acid fire extinguishers.
  3. Use for neutralizing excess acid in the stomach as it is an ingredient in antacids.

Washing Soda

Recrystallisation of sodium carbonate provides washing soda. It is a basic salt and its chemical formula is Na2CO3.10H2O

Na2CO3 + 10 H2O → Na2CO3.10H2O

(Sodium carbonate)

Uses of Washing Soda

  1. For removing permanent hardness of water
  2. Manufacturing of compounds such as borax
  3. Used in glass, soap and paper industry
  4. Cleaning agent for domestic purposes

4) Are the Crystal of Salts really Dry?

Copper sulphate crystals seem to be dry but they contain water of crystallization. When we heat the crystals, water is removed and the salt turns white. If we moisten the crystals again with water, then blue colour of crystals reappears.

Water of crystallization is the fixed number of water molecules present in one formula unit of a salt. Chemical formula for hydrated copper sulphate is CuSO4.5H2O. There is one other salt which possesses water of crystallization is gypsum. Its chemical formula is CaSO4.2H2O.

Plaster of Paris

When gypsum is heated at 373 K then it comes calcium sulphate hemihydrate which is known as Plaster of Paris. Its chemical formula is CaSO4.H2O. Doctors use it as a plaster for supporting fractures bones. Plaster of Paris can be change again to gypsum on mixing water to it.

CaSO4.H2O + H2O → CaSO4.2H2O

(Plaster of Paris) (Gypsum)

In the above equation, half a water molecule shown is attached as water of crystallisation.

Uses of Plaster of Paris

  1. Making the toys
  2. Making the decoration material
  3. For making surfaces smooth
  4. By doctor for plastering the facture bones

Important Questions

Q1) Name the natural source of the following acid.

  1. Lactic Acid
  2. Oxalic Acid
  3. Tartaric Acid
  4. Citric Acid

Q2) Describe the preparation of washing soda from baking soda. Also write a balance chemical reaction for it.

Q3) What is a neutralisation reaction. Explain with suitable examples.

Q4) What happen when chlorine is passed the through slaked lime at 313K? State 2 uses of the product obtained.

Q5) Why curd is not kept in copper or brass utensils?

Q6) Name the product formed in following cases.

  1. Carbon dioxide is passed into lime water.
  2. Hydrochloric acid reacts with caustic soda.
  3. Granulated zinc reacts with caustic soda.

Q7) Why does 1 M HCL solutions have a higher concentration of H+ ions than 1M CH3COOH solution? 

Q8) Arrange: NaOH, blood, lemon juice in increasing order of pH values.

Q9) Explain how antacid works?

Q10) What happens when acid reacts with nonmetallic oxide?

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