CBSE Class 10 Science Notes: Acids Bases and Salts

**Definitions by Ion Production**

This section explores the fundamental definitions of acids and bases based on their behavior in water.

• Acids: Substances that produce hydrogen ions ($H^+$) when dissolved in water. These ions are responsible for the acidic properties.
• Bases: Substances that produce hydroxide ions ($OH^−$) when dissolved in water. Hydroxide ions give bases their characteristic properties.

Key Idea: The presence of $H^+$ ions defines acidity, while $OH^−$ ions define basicity.

**Using Indicators**

Indicators are substances that change color in the presence of acids or bases, allowing us to distinguish between them.

• Litmus Paper: A common indicator. Blue litmus paper turns red in acidic solutions, and red litmus paper turns blue in basic solutions.
• Universal Indicator: A mixture of indicators that shows a range of colors corresponding to different pH values. Provides a more detailed indication of acidity or basicity.

Neutral Substances: Neither acidic nor basic. They do not change the color of litmus paper. A universal indicator shows a green color, corresponding to a pH of 7.

**Chemical Properties and Reactions**

Acids and bases react in predictable ways. This section outlines some important chemical reactions.

• Acid + Base → Salt + Water: This is a neutralization reaction. The acid and base react to form a salt and water. Example: $HCl (acid) + NaOH (base) → NaCl (salt) + H_2O (water)$
• Acid + Metal → Salt + Hydrogen Gas: Acids react with certain metals to produce a salt and hydrogen gas. Example: $2HCl (acid) + Zn (metal) → ZnCl_2 (salt) + H_2 (gas)$
• Base + Ammonium Salt → Salt + Water + Ammonia Gas: Bases react with ammonium salts releasing ammonia gas. Example: $NaOH (base) + NH_4Cl (ammonium \ salt) → NaCl (salt) + H_2O (water) + NH_3 (gas)$

**pH Scale**

The pH scale measures the acidity or basicity of a solution. It ranges from 0 to 14.

• pH < 7: Acidic (e.g., lemon juice).
• pH = 7: Neutral (e.g., pure water).
• pH > 7: Basic (e.g., baking soda solution).

Importance: The pH scale is crucial in health (blood pH regulation), soil (plant growth), and everyday life (cleaning products). A balanced pH is essential for various processes.

**Preparation and Common Uses of Selected Compounds**

This section explores the preparation and uses of several important compounds.

• Sodium Hydroxide (NaOH):
  • Preparation: Produced by the electrolysis of sodium chloride (brine).
  • Uses: Soap making, textile industry, and paper manufacturing.
• Bleaching Powder (CaOCl₂):
  • Preparation: Produced by the action of chlorine gas on dry slaked lime (calcium hydroxide).
  • Uses: Bleaching cotton and linen in the textile industry, disinfectant, and oxidizing agent.
• Baking Soda (Sodium Hydrogen Carbonate – NaHCO₃):
  • Preparation: Produced by the reaction of sodium chloride, water, carbon dioxide, and ammonia.
  • Uses: Used in baking (to make cakes fluffy), as an antacid, and in fire extinguishers.
• Washing Soda (Sodium Carbonate – Na₂CO₃ · 10H₂O):
  • Preparation: Made by heating baking soda.
  • Uses: Used in glass, soap, and paper industries. Used for washing clothes (cleaning agent).
• Plaster of Paris (CaSO₄ · 1/2H₂O):
  • Preparation: Produced by heating gypsum (calcium sulfate dihydrate).
  • Uses: Used for making casts for broken bones, in construction, and for making decorative items.

**Neutralization**

Neutralization is the reaction between an acid and a base, resulting in salt and water.

Examples & Applications:

• Antacids: Used to relieve heartburn by neutralizing excess stomach acid (HCl). Antacids contain bases like magnesium hydroxide ($Mg(OH)_2$) or aluminum hydroxide ($Al(OH)_3$).
• Agriculture: Lime (calcium oxide, CaO) or slaked lime (calcium hydroxide, Ca(OH)₂) is used to treat acidic soil. This raises the soil’s pH, making it more suitable for plant growth.