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Acids, Bases & Salts – Smart Revision Notes

 

Acids, Bases & Salts – Smart Revision Notes

Acids release H+ ions in water while bases release OH–. Their behavior depends on the degree of ionization. Strong acids like HCl dissociate almost completely:

$$ \text{HCl(aq)} \rightarrow \text{H}^+ + \text{Cl}^- $$

Weak acids like acetic acid ionize partially:

$$ \text{CH}_3\text{COOH } \rightleftharpoons \text{H}^+ + \text{CH}_3\text{COO}^- $$

pH Scale Essentials

pH tells how many H+ ions a solution has. Lower pH → stronger acid. Every unit change means a tenfold change:

$$ \text{pH} = -\log[\text{H}^+] $$

A solution with pH 3 is 100 times more acidic than pH 5.

Typical Acid Reactions

1. Acid + Metal → Salt + Hydrogen

$$ \text{2HCl} + \text{Zn} \rightarrow \text{ZnCl}_2 + \text{H}_2 $$

2. Acid + Carbonate → Salt + Water + CO2

$$ \text{2HCl} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 $$

Neutralization

When an acid reacts with a base, salt + water form:

$$ \text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O} $$

Neutralization releases heat — this is why antacids feel “relieving”.

Important Industrial Salts

Baking Soda (NaHCO₃) → releases CO₂ when heated (basis of baking):

$$ 2\text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2 $$

Washing Soda (Na₂CO₃·10H₂O) → used in detergents and water softening.

Plaster of Paris (CaSO₄·½H₂O) sets into gypsum when mixed with water.

Indicators – Quick Memory

Litmus: Acid → Red, Base → Blue
Phenolphthalein: Acid → Colourless, Base → Pink
Turmeric: Base turns it red

Trick Points (Exam Traps)

• Dilution always involves **adding acid to water**, not the other way — otherwise violent splashing occurs.

• Strong ≠ concentrated. Strength = ionization. Concentration = amount of solute.

• Basic salts like baking soda can show slightly alkaline nature.

• POP sets because of a hydration reaction, not “drying”.

• pH < 7 is not always dangerous — stomach acid has pH ~2 but is normal.

Rapid Revision Tips

✓ Remember: $$ \text{Acid Strength} \propto [\text{H}^+] $$ ✓ Acids react with carbonates → look for CO₂ test (lime water turns milky). ✓ For salts, always check: parent acid + parent base. ✓ Metal + Acid → Hydrogen gas pop test. ✓ Baking soda on heating → CO₂ → extinguishes flame. ✓ Use pH paper colours instead of guessing acidity/basicity.
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