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Acids, Bases and Salts: SPI Chemistry

Acids, Bases and Salts has 8 questions in the SPI bank, five EASY and three MODERATE, and none is from the 2022 paper. Almost all of them test a word or a formula: what makes a compound an acid, what a strong acid is, which salt is baking soda, which test finds a nitrate. Only one needs a short chain of reasoning, from a salt's parent acid and base to the colour it gives litmus. Learn the pairs that look alike and these are safe marks.

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Subtopic notes

Formula & revision sheet

1 formulas · 5 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet

Acids, Bases and Indicators

Formulas (1)

What acids and bases are · Reactions of an acid $\text{acid} + \text{base} \rightarrow \text{salt} + \text{water} \qquad \text{acid} + \text{metal} \rightarrow \text{salt} + \mathrm{H_2}$

Reference tables (2)

Strong and weak acids and bases, and the pH scale7 rows

Substance	Strong or weak	Kind
Hydrochloric acid, $\mathrm{HCl}$	Strong acid	Hydracid (no oxygen)
Sulphuric acid, $\mathrm{H_2SO_4}$	Strong acid	Oxyacid
Nitric acid, $\mathrm{HNO_3}$	Strong acid	Oxyacid
Ethanoic acid, $\mathrm{CH_3COOH}$	Weak acid	Organic acid
Carbonic acid, $\mathrm{H_2CO_3}$	Weak acid	Oxyacid
Sodium hydroxide, $\mathrm{NaOH}$	Strong base	Alkali
Ammonium hydroxide, $\mathrm{NH_4OH}$	Weak base	Alkali The one weak base to remember.

Strong or weak is about splitting into ions, not about how much acid is in the water.

Indicators6 rows

Indicator	In an acid	In a base
Blue litmus	Turns red	Stays blue
Red litmus	Stays red	Turns blue
Phenolphthalein	Colourless	Pink Colourless with phenolphthalein can mean acidic or neutral.
Methyl orange	Red	Yellow
Turmeric	Stays yellow	Reddish brown
Universal indicator	Red to orange (pH below 7)	Blue to violet (pH above 7)

Universal indicator is green at pH 7.

Watch out for (6)

Calling every base an alkali
→ What acids and bases are
Forgetting that acids need water
→ What acids and bases are
Confusing concentrated with strong
→ Strong and weak acids and bases, and the pH scale
Reading oxyacid or hydracid as a strength
→ Strong and weak acids and bases, and the pH scale
Testing an acid with red litmus
→ Indicators
Taking 'no colour' with phenolphthalein as neutral
→ Indicators

Salts and Their Uses

Reference tables (3)

Normal salts, acid salts and the nature of a salt solution3 rows

Parent acid	Parent base	Solution	Example
Strong	Strong	Neutral, pH 7	Sodium chloride, potassium nitrate
Strong	Weak	Acidic, pH below 7	Ammonium sulphate, $\mathrm{(NH_4)_2SO_4}$ Ammonium salts of strong acids are acidic: ammonium hydroxide is a weak base.
Weak	Strong	Basic, pH above 7	Sodium carbonate, sodium ethanoate

The stronger parent decides which way the solution leans.

Salts made from common salt5 rows

Common name	Formula	Made from	Used for
Caustic soda	$\mathrm{NaOH}$	Electrolysis of brine	Soaps, paper
Baking soda	$\mathrm{NaHCO_3}$	Brine, ammonia and carbon dioxide	Baking powder, antacid
Washing soda	$\mathrm{Na_2CO_3 \cdot 10H_2O}$	Heating baking soda, then crystallising	Cleaning, removing hardness of water Washing soda is a carbonate; baking soda is a hydrogencarbonate.
Bleaching powder	$\mathrm{CaOCl_2}$	Chlorine and dry slaked lime	Bleaching cloth, disinfecting water
Plaster of Paris	$\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$	Heating gypsum at 373 K	Plaster for broken bones

The first four start from common salt; plaster of Paris does not.

Tests for ions and gases5 rows

Looking for	Add or do	You see
Nitrate	Iron(II) sulphate, then concentrated sulphuric acid	A brown ring The sulphate here is part of the reagent: the test finds nitrate.
Sulphate	Barium chloride solution	White precipitate
Chloride	Silver nitrate solution	White precipitate
Carbonate	Dilute acid, gas through lime water	Lime water turns milky
Hydrogen gas	A burning splinter	A pop sound

Barium for sulphate, silver for chloride, iron(II) sulphate for nitrate.

Watch out for (6)

Expecting an acid salt to give an acidic solution
→ Normal salts, acid salts and the nature of a salt solution
Calling every salt neutral
→ Normal salts, acid salts and the nature of a salt solution
Swapping baking soda and washing soda
→ Salts made from common salt
Thinking slaked lime comes from common salt
→ Salts made from common salt
Swapping the sulphate and chloride tests
→ Tests for ions and gases
Reading the brown ring as a sulphate test
→ Tests for ions and gases

Question weightage by concept

6 concepts · 8 questions — where the marks actually sit, so you know what to drill first

Acids, Bases and Indicators3 questions · 38%

Concept	Qs	Share
What acids and bases are	1	13%
Strong and weak acids and bases, and the pH scale	1	13%
Indicators	1	13%

Salts and Their Uses5 questions · 63%

Concept	Qs	Share
Normal salts, acid salts and the nature of a salt solution	2	25%
Salts made from common salt	2	25%
Tests for ions and gases	1	13%

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