Class 10 Science • Complete Chapter Notes
Acids and bases are common substances found in food, household materials, laboratories and many industrial processes. Their properties can be identified using indicators and through their chemical reactions.
Acids and bases can react with each other and cancel their characteristic effects. The products of such reactions are generally salt and water.
Acids are substances that show acidic properties in aqueous solution. Many naturally occurring acids are present in foods and biological materials.
Bases are substances that show basic properties. Bases are often described as bitter and soapy to touch, but substances should never be tasted or touched for identification.
Indicators are substances that change colour or other observable properties depending on whether a solution is acidic or basic.
Examples include litmus, turmeric and coloured petals of certain flowers.
Examples include phenolphthalein and methyl orange.
Some substances change their odour in acidic or basic media. Onion, vanilla essence and clove oil can be studied as examples of olfactory indicators.
Many metals react with acids to produce a salt and hydrogen gas.
The hydrogen gas evolved can be tested using a burning splint; hydrogen burns with a characteristic 'pop' sound.
Metal carbonates and metal hydrogencarbonates react with acids to produce a corresponding salt, carbon dioxide and water.
Lime water turns milky because insoluble calcium carbonate is formed.
The reaction between an acid and a base that produces salt and water is called a neutralisation reaction.
Metallic oxides react with acids to produce salt and water. Therefore, many metallic oxides show basic character.
Non-metallic oxides can react with bases to produce salt and water. This supports their acidic nature.
Acids show their characteristic acidic properties in aqueous solution because they produce hydrogen ions, represented in water as hydronium ions.
Hydrogen ions do not exist independently in water; they associate with water molecules to form hydronium ions.
Bases produce hydroxide ions in aqueous solution. Bases that dissolve in water are called alkalis.
Aqueous solutions of acids and bases conduct electricity because they contain ions that can carry electric charge.
Compounds such as glucose and alcohol do not produce ions in water in the same way as acids and bases and therefore do not show the same electrical conductivity in the standard test described in the textbook.
Dilution is the process of reducing the concentration of ions per unit volume by adding water to an acid or base solution.
The pH scale is used to indicate the acidic or basic nature of a solution by relating to its hydrogen/hydronium ion concentration.
| pH | Nature |
|---|---|
| Less than 7 | Acidic |
| 7 | Neutral |
| Greater than 7 | Basic / Alkaline |
A lower pH generally indicates a higher hydronium ion concentration. As pH increases above 7, basic character increases.
The strength of an acid or base is related to the amount of ions it produces in aqueous solution.
Strong acids produce a relatively larger concentration of hydrogen/hydronium ions in aqueous solution.
Weak acids produce a relatively smaller concentration of hydrogen/hydronium ions at the same concentration.
The same concept applies to strong and weak bases in terms of hydroxide ion production.
Plants require a suitable pH range for healthy growth. Farmers may test soil pH and use suitable substances to correct excessively acidic soil.
The stomach produces hydrochloric acid, which assists digestion. Excess acidity may cause discomfort, and antacids containing mild bases can neutralise excess acid.
Tooth decay becomes a concern when the pH in the mouth falls sufficiently low. The textbook notes that tooth decay starts when mouth pH is below about 5.5.
Rain water with pH below 5.6 is described as acid rain. Acidification of water bodies can make survival difficult for aquatic organisms.
Bee stings introduce an acidic substance. A mild base such as baking soda can help neutralise the acid and provide relief.
Salts are formed in many acid-base and other reactions. Salts containing the same positive or negative ion can be grouped into families.
Different salts can give aqueous solutions with different pH values depending on the ions involved and how the salt was formed.
Students should understand that the word “salt” does not automatically mean that the resulting solution has a pH of exactly 7.
Sodium chloride (NaCl), commonly known as common salt, is an important raw material used to prepare several useful substances.
These include sodium hydroxide, chlorine, hydrogen, bleaching powder, baking soda and washing soda.
Passing electricity through an aqueous solution of sodium chloride, called brine, produces sodium hydroxide along with chlorine and hydrogen.
The process is called the chlor-alkali process because chlorine and an alkali, sodium hydroxide, are among its useful products.
Bleaching powder is prepared by the action of chlorine on dry slaked lime. It is commonly represented as Ca(ClO)₂, although the actual composition is more complex.
Baking soda is sodium hydrogencarbonate, with the chemical formula NaHCO₃.
Baking powder contains baking soda and an edible weak acid. Carbon dioxide produced during the reaction helps cakes and bread rise.
Washing soda is sodium carbonate decahydrate.
Sodium carbonate can be obtained by heating sodium hydrogencarbonate, and crystallisation gives washing soda.
Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.
Hydrated copper sulphate crystals are blue. On heating, water of crystallisation is removed and the salt becomes white. Adding water again can restore the blue colour.
Gypsum is heated to form Plaster of Paris by losing part of its water of crystallisation.
Plaster of Paris can react with water and set to form gypsum again.
| Substance | Formula |
|---|---|
| Hydrochloric Acid | HCl |
| Sulphuric Acid | H₂SO₄ |
| Nitric Acid | HNO₃ |
| Sodium Hydroxide | NaOH |
| Baking Soda | NaHCO₃ |
| Washing Soda | Na₂CO₃·10H₂O |
| Bleaching Powder | Ca(ClO)₂ |
| Gypsum | CaSO₄·2H₂O |
| Plaster of Paris | CaSO₄·½H₂O |
These notes are aligned with the official NCERT Class 10 Science chapter. Students should also refer to the original textbook and current CBSE curriculum.
NCERT – Acids, Bases and Salts Official PDF → NCERT Official Textbooks → CBSE Academic Curriculum 2026-27 →