Chapter 6: Acid and Alkali

6.1 Properties of Acid and Alkali

Acids and alkalis are chemical compounds that exhibit distinct physical and chemical properties in the presence of water.

Properties of Acids

• Taste: Sour (e.g., vinegar, lemon, lime).
• pH Value: Less than 7 (pH < 7).
• Litmus Paper Effect: Turns moist blue litmus paper to red.
• Feel: Corrosive on skin.
• Reaction with Metals: Reacts with active metals (e.g., zinc, magnesium) to produce hydrogen gas.
• Presence of Water: Acids only show acidic properties when dissolved in water because water allows acids to ionize and produce hydrogen ions (H⁺). Dry acids (e.g., dry citric acid crystals) do not show acidic properties.

Properties of Alkalis

• Taste: Bitter and slippery to touch (e.g., soap, detergent).
• pH Value: Greater than 7 (pH > 7).
• Litmus Paper Effect: Turns moist red litmus paper to blue.
• Feel: Corrosive in concentrated forms.
• Solubility: An alkali is a base that is soluble in water (producing hydroxide ions, OH^-).
• Presence of Water: Alkalis only show alkaline properties in the presence of water.

Role of Water in Acidic and Alkaline Properties

Without water, acids and alkalis cannot dissociate into ions. Therefore, they cannot change the color of dry litmus paper or react with metals.

The pH Scale and Indicators

The pH scale measures the degree of acidity or alkalinity of a substance, ranging from 0 to 14:

• pH < 7: Acidic (Lower pH = Stronger acid)
• pH = 7: Neutral (e.g., pure water, salt solution)
• pH > 7: Alkaline (Higher pH = Stronger alkali)

Indicator	Color in Acid	Color in Neutral	Color in Alkali
Litmus Paper	Red	Unchanged	Blue
Universal Indicator	Red / Orange / Yellow	Green	Blue / Purple
Phenolphthalein	Colorless	Colorless	Pink
Methyl Orange	Red	Yellow	Yellow

6.2 Neutralisation

Neutralisation is a chemical reaction between an acid and an alkali (base) to produce a salt and water only.

General Equation:
Acid + Alkali → Salt + Water

Examples of Neutralisation Reactions

• Hydrochloric Acid + Sodium Hydroxide → Sodium Chloride (Table Salt) + Water
• Sulphuric Acid + Potassium Hydroxide → Potassium Sulphate + Water
• Nitric Acid + Calcium Hydroxide → Calcium Nitrate + Water

Method of Neutralisation: Titration

Titration is a laboratory technique used to determine the exact volume of acid needed to completely neutralise a known volume of alkali using an indicator (such as phenolphthalein).

• The point at which the indicator permanently changes color is called the end point.

Applications of Neutralisation in Daily Life

Field / Context	Problem	Application of Neutralisation
Agriculture	Soil is too acidic for crop growth.	Farmers add slaked lime (calcium hydroxide) or powdered limestone to neutralise acidic soil.
Medicine	Stomach pain due to excessive gastric acid (gastritis).	Patients take antacids containing weak alkalis (magnesium hydroxide or aluminium hydroxide) to neutralise stomach acid.
Dental Hygiene	Bacteria in the mouth produce acids that decay teeth.	Toothpaste is mildly alkaline to neutralise dental acids and protect tooth enamel.
Hair Care	Shampoo is slightly alkaline and leaves hair rough.	Hair conditioner is mildly acidic to neutralise residual shampoo and make hair smooth.
Industrial Waste Treatment	Acidic factory effluent contaminates rivers.	Alkalis like lime are added to neutralise acidic industrial wastewater before discharge.