SPI Chemistry · Acids, Bases and Salts

Salts and Their Uses

A salt is what is left when an acid's hydrogen is replaced by a metal or ammonium ion; its parent acid and base decide whether its solution is acidic, neutral or basic.

All Acids, Bases and Salts notes

Why this matters

Five questions in the bank, two EASY and three MODERATE: picking the acid salt, the salt whose solution turns blue litmus red, the formula of baking soda, what common salt is used to make, and the ion the brown ring test finds. Most are a name or formula to recall, and one needs the strong-or-weak rule.

Concept 1 of 3: Normal salts, acid salts and the nature of a salt solution

Every salt has a parent acid and a parent base. If both are strong, they cancel and the solution is neutral. If one is stronger, the solution leans its way: a strong acid makes it acidic, a strong base makes it basic.

Definition

A normal salt has all the acid's replaceable hydrogen replaced: $\mathrm{NaCl}$ , $\mathrm{Na_2SO_4}$ .
An acid salt still has some replaceable hydrogen: $\mathrm{NaHCO_3}$ , $\mathrm{NaHSO_4}$ . It comes from partly neutralising an acid that has two hydrogens to give.
Strong acid + strong base gives a neutral solution (pH 7).
Strong acid + weak base gives an acidic solution (pH below 7): it turns blue litmus red.
Weak acid + strong base gives a basic solution (pH above 7).

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.

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Example 1 · Acids, Bases and Salts · Salts and Their Uses

An aqueous solution of a salt turns blue litmus to red. The salt could be the one obtained by the reaction of:

Expecting an acid salt to give an acidic solution

An acid salt is named for the replaceable hydrogen it keeps. Sodium hydrogencarbonate is an acid salt, yet its solution is mildly basic, because its parent acid (carbonic acid) is weak.

Calling every salt neutral

Only a salt of a strong acid and a strong base is neutral. A salt of a strong acid and a weak base is acidic, and a salt of a weak acid and a strong base is basic.

Drill 1 more on normal salts, acid salts and the nature of a salt solution

Concept 2 of 3: Salts made from common salt

Common salt, sodium chloride, is the starting point for many everyday chemicals. Electrolysis of brine gives sodium hydroxide, chlorine and hydrogen; brine with ammonia and carbon dioxide gives baking soda, and baking soda gives washing soda.

Definition

Chlor-alkali process: electricity through brine gives sodium hydroxide, chlorine gas and hydrogen gas: $\mathrm{2NaCl + 2H_2O \rightarrow 2NaOH + Cl_2 + H_2}$ .
Baking soda, sodium hydrogencarbonate $\mathrm{NaHCO_3}$ : from brine, ammonia and carbon dioxide. On heating it gives off carbon dioxide.
Washing soda, $\mathrm{Na_2CO_3 \cdot 10H_2O}$ : baking soda is heated to sodium carbonate, which is crystallised from water.
Bleaching powder, $\mathrm{CaOCl_2}$ : chlorine (from brine) passed over dry slaked lime.
Plaster of Paris, $\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$ , is made by heating gypsum, not from common salt.

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.

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Example 2 · Acids, Bases and Salts · Salts and Their Uses

The chemical formula of baking soda is:

Swapping baking soda and washing soda

Baking soda is sodium hydrogencarbonate,

\mathrm{NaHCO_3}

. Washing soda is sodium carbonate with ten waters,

\mathrm{Na_2CO_3 \cdot 10H_2O}

Thinking slaked lime comes from common salt

Slaked lime,

\mathrm{Ca(OH)_2}

, is made from limestone (quicklime and water). Bleaching powder uses slaked lime, but only its chlorine comes from common salt.

Drill 1 more on salts made from common salt

Concept 3 of 3: Tests for ions and gases

Each common ion or gas has one test with a result you can see: a coloured ring, a white precipitate, milky lime water or a pop. Learn the test and its result as a pair.

Definition

Nitrate: the brown ring test. Add fresh iron(II) sulphate solution, then pour concentrated sulphuric acid down the side: a brown ring forms where the layers meet.
Sulphate: barium chloride solution gives a white precipitate that does not dissolve in dilute hydrochloric acid.
Chloride: silver nitrate solution gives a white precipitate.
Carbonate: a dilute acid gives a gas that turns lime water milky (carbon dioxide).
Hydrogen burns with a pop when a burning splinter is brought near.

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.

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The same idea in a practice question:

Practice questionModerate

Example 3 · Acids, Bases and Salts · Salts and Their Uses

Brown ring test is used for the detection of ____

Swapping the sulphate and chloride tests

Barium chloride finds sulphate. Silver nitrate finds chloride. Both give a white precipitate, so the reagent is what tells them apart.

Reading the brown ring as a sulphate test

The brown ring test uses iron(II) sulphate and sulphuric acid as reagents, but the ion it detects is nitrate.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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

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