9th Class Chemistry Chapter 7 Notes PDF 2026 – Acids, Bases and Salts

Definition

Arrhenius: an acid produces $H^{+}$ ions in water; a base produces $O H^{-}$ ions in water.

Brønsted–Lowry (broader): an acid is a proton ( $H^{+}$ ) donor; a base is a proton acceptor.

Conjugate base: what's left after an acid donates a proton. Conjugate acid: what forms after a base accepts one.

Remember

The Brønsted–Lowry idea explains substances Arrhenius's definition misses — e.g. $N H_{3}$ has no $O H^{-}$ to release, but is still a base because it accepts a proton from water.

Example 1 — identifying a conjugate base

In $C H_{3} C O O H + H_{2} O ⇌ C H_{3} C O O^{-} + H_{3} O^{+}$ , which species is the conjugate base?

$C H_{3} C O O H$ donates a proton to become $C H_{3} C O O^{-}$ — that's the acid's conjugate base.
( $H_{2} O$ accepted a proton to become $H_{3} O^{+}$ , its conjugate acid.)

Answer: $C H_{3} C O O^{-}$

Remember

Strength describes how completely an acid ionizes in water (a fixed chemical property); concentration describes how much acid is dissolved in a given volume — a dilute strong acid and a concentrated weak acid are entirely different things, so the two words are never interchangeable.

Rule

Acid $+$ base $\to$ salt $+$ water (neutralization).

Acid $+$ metal $\to$ salt $+$ hydrogen gas.

Acid $+$ carbonate $\to$ salt $+$ water $+$ carbon dioxide.

Example 2 — testing for carbon dioxide with lime water

What happens when $C O_{2}$ passes through lime water for (i) a short time, (ii) a long time?

Short time: $C O_{2}$ reacts with calcium hydroxide to form insoluble calcium carbonate, turning the lime water milky/cloudy.
Long time: excess $C O_{2}$ converts the insoluble calcium carbonate into soluble calcium bicarbonate, and the cloudiness clears again.

Answer: Milky at first, then clears with excess $C O_{2}$

Remember

The gas that makes bread rise while baking is carbon dioxide, released when baking soda ( $N a H C O_{3}$ ) reacts and decomposes — trapped bubbles of this gas make the dough soft and light.

Quick Recap

●Arrhenius: acid gives $H^{+}$ , base gives $O H^{-}$ in water. Brønsted–Lowry: acid donates a proton, base accepts one
●Conjugate base/acid: what's left/formed after donating/accepting a proton
●Strength: how completely it ionizes. Concentration: how much is dissolved — not the same thing
●Acid+base $\to$ salt+water. Acid+metal $\to$ salt+H $_{2}$ . Acid+carbonate $\to$ salt+water+CO $_{2}$

9th Class Chemistry Chapter 7 Notes

Defining Acids and Bases

Strength vs Concentration

Reactions of Acids and Bases

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