Acids, Bases
& Salts

Substances that change colour to indicate acidic/basic nature.

• Litmus: Acid turns Blue to Red. Base turns Red to Blue.
• Phenolphthalein: Acid remains colourless. Base turns Pink.
• Methyl Orange: Acid turns Red. Base turns Yellow.

Smell the Change

Substances whose odour changes in acidic/basic media. e.g. Onion, Vanilla, Clove Oil.

In a Basic medium, the characteristic smells are destroyed.

Acid + Metal

When an acid reacts with a metal, it forms a salt and Hydrogen gas.

Hydrogen Pop Test

Bring a burning candle near the gas. Hydrogen gas burns with a characteristic 'pop' sound.

Acids react with metal carbonates to release Carbon Dioxide gas.

Lime Water Test

Passing $CO_2$ through lime water turns it milky due to the formation of insoluble Calcium Carbonate.

Excess $CO_2$ makes milkiness disappear due to soluble Calcium Bicarbonate!

2.6 Neutralization Reaction

Acid + Base $\rightarrow$ Salt + Water

2.7 Metallic Oxides + Acid

Metallic oxides (like $CuO$) react with acids to give salt and water. This proves that metallic oxides are basic in nature!

Concept: Ionization

Compounds like Glucose and Alcohol contain Hydrogen, but their solutions do not conduct electricity.

This is because they do not dissociate to produce $H^+$ ions in water. Hence, they are not categorized as acids.

Dry HCl Gas vs Litmus

Dry HCl gas does not change the colour of dry blue litmus paper.

It only changes the colour of a wet (moist) blue litmus paper.

Conclusion: Acids produce $H^+$ ions only in the presence of water! In the absence of water, dissociation of $HCl$ does not occur.

How to Dilute Acids ⚠️

The process is highly exothermic.

Always add Acid to Water slowly with constant stirring.

If water is added to a concentrated acid, the sudden heat generated may cause the mixture to splash out and cause burns, or the glass container may break.

The pH Scale

A scale for measuring $H^+$ ion concentration (0 to 14).

• pH < 7: Acidic (Higher $H^+$)
• pH = 7: Neutral (Pure water)
• pH > 7: Basic (Higher $OH^-$)

Plants require a specific pH range for healthy growth (usually close to 7).

Adjusting Soil pH

If soil is too acidic, farmers treat it with materials like quick lime ($CaO$) or slaked lime ($Ca(OH)_2$) to neutralize the acidity.

pH of Salts

• Strong Acid + Strong Base: Neutral Salt (pH = 7)
• Strong Acid + Weak Base: Acidic Salt (pH < 7)
• Weak Acid + Strong Base: Basic Salt (pH > 7)

Electrolysis of aqueous sodium chloride (Brine).

Products: Sodium Hydroxide (Cathode), Chlorine (Anode), Hydrogen (Cathode).

Formula: $CaOCl_2$

Formed by the action of Chlorine gas ($Cl_2$) on dry slaked lime $Ca(OH)_2$.

Uses:

• Bleaching cotton and linen in the textile industry.
• Disinfecting drinking water to make it free of germs.
• As an oxidizing agent in many chemical industries.

Formula: $NaHCO_3$

Sodium Hydrogen Carbonate. It is a mild, non-corrosive basic salt.

Baking Powder

Mixture of Baking Soda + a mild edible acid (like Tartaric acid).

Role of Tartaric Acid: It neutralizes the bitter taste of Sodium Carbonate formed during heating, making cakes soft and spongy!

Formula: $Na_2CO_3 \cdot 10H_2O$

Sodium Carbonate Decahydrate. Formed by recrystallization of sodium carbonate from water.

Uses:

• Removing permanent hardness of water.
• Used in glass, soap, and paper industries.
• Domestic cleaning agent.

Formula: $CaSO_4 \cdot \frac{1}{2}H_2O$

Calcium Sulphate Hemihydrate. Obtained by heating Gypsum carefully at 373 K.

Setting into Gypsum

On mixing with water, it sets into a hard solid mass called Gypsum:

(Hence, POP must always be stored in a moisture-proof container!)

Are salt crystals really dry?

Heating Blue Copper Sulphate crystals ($CuSO_4 \cdot 5H_2O$) makes them turn white as water of crystallization is lost.

Adding a few drops of water back restores the blue colour immediately!