To name the salt, combine the metal name with the ending from the acid: HCl ⟶ chloride, H₂SO₄ ⟶ sulfate. For example, magnesium + hydrochloric acid ⟶ magnesium chloride.
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Neutralisation & Salt Production
🍹 Neutralisation Reactions
Acids can be neutralised by alkalis, bases, and metal carbonates. The three key word equations are:
acid + alkali ⟶ salt + water
acid + base ⟶ salt + water
acid + metal carbonate ⟶ salt + water + carbon dioxide
🦩 Which Salt Do You Get?
The salt produced depends on two things:
The acid used (determines the second part of the salt name).
The metal in the base, alkali, or carbonate (determines the first part).
Acid
Formula
Salt Type Produced
🌺 Hydrochloric acid
HCl
Chlorides
🌺 Nitric acid
HNO₃
Nitrates
🌺 Sulfuric acid
H₂SO₄
Sulfates
Reaction
Example
Products
Acid + Alkali
HCl + NaOH
NaCl + H₂O
Acid + Metal oxide
H₂SO₄ + CuO
CuSO₄ + H₂O
Acid + Carbonate
2HCl + CaCO₃
CaCl₂ + H₂O + CO₂
🌴 Exam Tip
If the question asks you to name the salt, just combine the metal name with the acid ending. For example: copper oxide + sulfuric acid ⟶ copper sulfate + water.
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Making Soluble Salts
🦜 The Method for Making Soluble Salts
Soluble salts can be made by reacting an acid with an excess of an insoluble solid (a metal, metal oxide, hydroxide, or carbonate). Here is the step-by-step method:
Step 1: Warm the acid gently in a beaker.
Step 2: Add the insoluble solid (e.g. copper oxide) a little at a time, stirring after each addition.
Step 3: Keep adding the solid until it is in excess — i.e. some solid remains undissolved at the bottom. This ensures all the acid has reacted.
Step 4:Filter the mixture to remove the excess solid. The filtrate (liquid) contains the dissolved salt.
Step 5:Evaporate the water from the filtrate (gently heat or leave to crystallise slowly) to obtain pure, dry salt crystals.
🌴 Why Excess Solid?
Adding excess solid ensures that all the acid has been used up. If there is leftover acid, your salt would be contaminated. The excess solid is simply removed by filtration — easy!
🌺 Required Practical Alert
You must be able to describe how to make a pure, dry sample of a named soluble salt. For example: to make copper sulfate, react sulfuric acid with excess copper oxide, filter, then evaporate/crystallise.
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The pH Scale & Neutralisation
🍹 Acids and Alkalis in Solution
Acids produce H⁺ ions (hydrogen ions) when dissolved in water.
Alkalis produce OH⁻ ions (hydroxide ions) when dissolved in water.
🦩 The pH Scale
The pH scale runs from 0 to 14.
pH 7 = neutral (e.g. pure water).
Below pH 7 = acidic — the lower the number, the stronger the acid.
Above pH 7 = alkaline — the higher the number, the stronger the alkali.
pH Range
Type
Examples
0 – 3
Strong acid
Stomach acid, hydrochloric acid
4 – 6
Weak acid
Lemon juice, vinegar
7
Neutral
Pure water
8 – 10
Weak alkali
Baking soda, toothpaste
11 – 14
Strong alkali
Bleach, oven cleaner
🌴 The Neutralisation Ionic Equation
When an acid reacts with an alkali, the key reaction is:
H⁺(aq) + OH⁻(aq) ⟶ H₂O(l)
Hydrogen ions from the acid combine with hydroxide ions from the alkali to form water. This is why the pH moves towards 7.
🦜 Measuring pH
Universal indicator — changes colour depending on the pH. Compare to a colour chart.
pH probe (meter) — gives a precise numerical pH reading. More accurate than indicator.
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Strong & Weak Acids 🚨 Higher
🍹 Strong Acids
Strong acids are completely ionised in aqueous solution — every molecule splits into ions.
The ionisation is a reversible reaction — an equilibrium exists.
CH₃COOH ⇌ CH₃COO⁻ + H⁺ (partially ionised)
🌴 pH and H⁺ Ion Concentration
For the same concentration, a strong acid has a lower pH than a weak acid (because more H⁺ ions are released).
Each decrease of 1 pH unit means the concentration of H⁺ ions increases by a factor of 10.
So pH 3 has 10 times more H⁺ ions than pH 4, and 100 times more than pH 5.
🚨 Don't Confuse These!
Dilute / Concentrated = refers to the amount of substance dissolved in a given volume of water. You can have a concentrated weak acid or a dilute strong acid.
Weak / Strong = refers to the degree of ionisation. A strong acid fully ionises; a weak acid only partially ionises.
These are two completely different concepts. A common exam trap!
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Question 01 [2 marks]
Write the word equation for the reaction of an acid with a metal. State the two products formed.
🌺 Answer
acid + metal ⟶ salt + hydrogen. The two products are a salt and hydrogen gas.
Question 02 [2 marks]
Write a balanced symbol equation for the reaction of magnesium with hydrochloric acid.
🌺 Answer
Mg + 2HCl ⟶ MgCl₂ + H₂
Question 03 [2 marks]
Explain why the reaction between magnesium and hydrochloric acid is described as a redox reaction.
🌺 Answer
It is a redox reaction because the magnesium is oxidised (it loses electrons) and the hydrogen ions are reduced (they gain electrons to form hydrogen gas).
Question 04 [3 marks]
Write word equations for the reactions of acids with: (a) an alkali, (b) a base, (c) a metal carbonate.
🌺 Answer
(a) acid + alkali ⟶ salt + water (b) acid + base ⟶ salt + water (c) acid + metal carbonate ⟶ salt + water + carbon dioxide
Question 05 [2 marks]
Name the salt produced when sulfuric acid reacts with copper oxide. Write the word equation for this reaction.
🌺 Answer
The salt is copper sulfate. Word equation: sulfuric acid + copper oxide ⟶ copper sulfate + water.
Question 06 [4 marks]
Describe how you would make a pure, dry sample of copper sulfate crystals from copper oxide and sulfuric acid.
🌺 Answer
1. Warm the sulfuric acid gently in a beaker. 2. Add copper oxide a little at a time, stirring, until it is in excess (some remains undissolved at the bottom). 3.Filter the mixture to remove the excess copper oxide. 4. Gently heat or leave the filtrate to evaporate/crystallise, forming pure, dry copper sulfate crystals.
Question 07 [2 marks]
What ions do acids and alkalis produce in aqueous solution?
🌺 Answer
Acids produce H⁺ ions (hydrogen ions) in aqueous solution. Alkalis produce OH⁻ ions (hydroxide ions) in aqueous solution.
Question 08 [2 marks]
Write the ionic equation for neutralisation. State what happens to the pH during a neutralisation reaction.
🌺 Answer
H⁺(aq) + OH⁻(aq) ⟶ H₂O(l). During neutralisation, the pH moves towards 7 (neutral).
Question 09 [2 marks]
Name two methods of measuring pH and state which is more accurate.
🌺 Answer
Universal indicator (gives an approximate pH from the colour) and a pH probe/meter (gives a precise numerical reading). The pH probe is more accurate.
Question 10 [2 marks]Higher
Explain the difference between a strong acid and a weak acid. Give one example of each.
🌺 Answer
A strong acid is completely ionised in aqueous solution (e.g. hydrochloric acid, HCl). A weak acid is only partially ionised in aqueous solution (e.g. ethanoic acid, CH₃COOH).
Question 11 [3 marks]Higher
A strong acid has a pH of 2 and a weak acid has a pH of 4. Both are the same concentration. How many times more H⁺ ions does the strong acid have compared to the weak acid? Explain your answer.
🌺 Answer
The strong acid has 100 times more H⁺ ions than the weak acid. Each pH unit represents a tenfold change in H⁺ concentration. The difference is 2 pH units, so 10 × 10 = 100 times more H⁺ ions.
Question 12 [3 marks]Higher
Explain the difference between the terms 'dilute' and 'weak' when describing acids. A student says "concentrated sulfuric acid is a strong acid, so dilute sulfuric acid must be a weak acid." Explain why the student is wrong.
🌺 Answer
Dilute/concentrated refers to the amount of acid dissolved in a given volume of water. Weak/strong refers to the degree of ionisation. The student is wrong because sulfuric acid is a strong acid regardless of concentration — it is always fully ionised in solution. Diluting it reduces the number of H⁺ ions per volume, but each molecule still fully ionises. Strength and concentration are independent properties.