Physical & Chemical Properties of Acids
Acid properties and reactions: indicator behaviour, and the 3 core reactions with metals, bases and carbonates—plus key observations and gas tests.
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This lesson connects each acid reaction to what you would observe and how you would identify any gas produced.
1. Definition
An acidic solution contains hydrogen ions, H + (aq), and has a pH below the neutral value (pH < 7 at room temperature).
2. Key Ideas
- Indicators:
- acids turn blue litmus red
- acids have pH < 7 (see pH Scale)
- Three core acid reactions:
- acid + metal → salt + H₂
- acid + metal oxide/metal hydroxide → salt + H₂O
- acid + carbonate → salt + H₂O + CO₂
- Core gas tests:
- H₂: lighted splint → squeaky pop
- CO₂: limewater turns milky/chalky
3. Detailed Explanations
| Reactant added to a suitable acid | Products | Useful observation |
|---|---|---|
| suitable metal above hydrogen, with dilute hydrochloric or sulfuric acid | salt + hydrogen | metal gets smaller; hydrogen gives a squeaky pop with a lighted splint |
| basic metal oxide or hydroxide | salt + water | solid may dissolve; no gas is required |
| carbonate | salt + water + carbon dioxide | effervescence; carbon dioxide turns limewater milky |
A. Indicator evidence and safe handling
An acidic solution turns blue litmus paper red and gives a pH below 7 with Universal Indicator. These observations are safe ways to identify acidic behaviour; never use taste or direct smelling to test laboratory chemicals.
Handle laboratory acids as directed: hazards depend on the acid and its concentration. Never taste or directly smell chemicals. If acid spills on skin, rinse immediately with plenty of water.
B. Acid + metal → salt + hydrogen (usually)
Many reactive metals react with dilute acids to produce hydrogen gas and a salt.
Example (dilute hydrochloric acid + magnesium): Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Expected observations:
- effervescence (bubbles)
- magnesium dissolves / gets smaller
- H₂ gives a squeaky pop with a lighted splint
Use the reactivity series: suitable metals above hydrogen react with dilute hydrochloric acid or dilute sulfuric acid to release hydrogen. Copper and silver are below hydrogen and do not. A coating can slow or stop a reaction, as with lead(II) chloride on lead.
Do not apply this rule automatically to nitric acid or concentrated sulfuric acid: these can give different products. The acid’s identity and conditions matter.
C. Acid + metal oxide or hydroxide → salt + water
Basic metal oxides and hydroxides, and amphoteric ones when reacting as bases, neutralise acids to form salt and water. Do not assume that every oxide is basic; oxide classification is developed in the Oxides lesson. Other bases do not always form water; for example, ammonia reacts with an acid to form an ammonium salt.
Example (copper(II) oxide + nitric acid): CuO(s) + 2HNO₃(aq) → Cu(NO₃)₂(aq) + H₂O(l)
The black solid dissolves to form a blue copper(II) nitrate solution. Unlike copper metal, copper(II) oxide reacts with acid without producing hydrogen. The reacting oxide ions combine with hydrogen ions to form water:
CuO(s) + 2H + (aq) → Cu²⁺(aq) + H₂O(l)
A base is a substance that neutralises an acid. In O-Level, this includes metal oxides and metal hydroxides. See:
Introduction to Bases
.
D. Acid + carbonate → salt + water + carbon dioxide
Acids react with carbonates (or hydrogencarbonates) to give carbon dioxide gas.
Example (calcium carbonate + dilute hydrochloric acid): CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Test for CO₂:
- bubble the gas through limewater
- limewater turns milky/chalky (white precipitate of CaCO₃ forms)
CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)
4. Common Mistakes
- Writing “acid + metal gives hydrogen” without naming a suitable metal and dilute hydrochloric acid or dilute sulfuric acid.
- Saying “blue litmus becomes blue” (wrong direction): acids turn blue → red.
- Writing “gas produced is carbon dioxide” for acid + metal (wrong gas).
- Forgetting gas-test keywords: hydrogen gives a pop with a lighted splint; carbon dioxide turns limewater milky. Name the test and observation together.
- Assuming every metal reacts with a dilute acid; first check whether the metal is above hydrogen in the reactivity series.
5. Exam Tips
Reagent + observation + conclusion. Example: “Add dilute HCl(aq): effervescence; gas gives squeaky pop → hydrogen produced.”
- Use state symbols: acids in water are (aq).
- For gas tests, write the observation and the inference (don’t stop at “a gas is produced”).
6. Worked Examples
Modelled example 1
Identify the gas (metal + acid)
Problem
Magnesium ribbon is added to dilute hydrochloric acid and a colourless gas is produced. State the gas and describe a confirmatory test.
Study the worked solution
Use the reaction pattern
Method
Identify hydrogen as the gaseous product.Reason
A reactive metal with dilute acid forms a salt and hydrogen.Working
Mg + 2HCl → MgCl₂ + H₂.Choose the specific test
Method
Bring a lighted splint to the collected gas.Reason
Hydrogen ignites characteristically under this test.Working
Use a lighted splint.State observation and conclusion
Method
Report a squeaky pop and identify hydrogen.Reason
The precise observation distinguishes the confirmatory test from vague “burning”.Working
Squeaky pop confirms H₂.
Guided practice 2
Identify the gas (carbonate + acid)
Problem
Marble chips are added to dilute nitric acid. Effervescence occurs and the gas turns limewater milky. Identify the gas and explain the observation.
Match reaction and gas-test evidence
Hints
Hint 1: reaction family
Acid + carbonate gives salt, water and one gaseous oxide.
Hint 2: confirmatory observation
The gas that turns limewater milky is CO₂.
View solution step by step
Infer from the reactants
Method
Predict carbon dioxide from acid plus carbonate.Reason
Marble chips are calcium carbonate.Working
Acid + carbonate → salt + water + CO₂.
Use the confirmatory evidence
Method
Identify the gas as CO₂ and explain the milky appearance.
Reason
Carbon dioxide forms a white CaCO₃ precipitate in limewater.
Working
Limewater turns milky/chalky; gas = CO₂.
Common misconception 3
Check the metal before applying the rule
Learner claim
A student says, “Every metal reacts with dilute hydrochloric acid to make hydrogen.” Use copper to explain why this is too broad.
Test the metal against hydrogen
View solution step by step
Check the reactivity condition
Method
Locate copper relative to hydrogen.Reason
Only metals above hydrogen displace hydrogen ions from dilute hydrochloric or sulfuric acid.
Working
Copper is below hydrogen.Correct the claim
Method
State the condition and the copper result.Reason
Copper cannot displace hydrogen ions under these conditions.
Working
Metals above hydrogen react with these dilute acids; copper gives no observable reaction with dilute HCl.
Examiner practice 4
Complete the word equation
Examination question
Complete: acid + basic metal oxide → ______ + ______. [2 marks]
Name both product classes
View solution step by step
Identify the salt product
1 markMethod
Write salt as the first product.Reason
The acid anion combines with the metal cation.Working
First blank: salt.Identify the neutralisation product
1 markMethod
Write water as the second product.Reason
Metal oxide neutralises the acid without producing a gas.
Working
acid + metal oxide → salt + water.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Self-mark one mark for each product class.
Challenge 5
Predict reaction/no reaction
Boundary transfer
Zinc is added to dilute sulfuric acid. Predict the products, write the balanced equation and state how you would identify the gas.
Check whether the reaction condition is met
Hints
Hint 1: qualify the metal rule
Zinc is above hydrogen in the reactivity series.
Hint 2: name the salt
The acid supplies sulfate ions.
View solution step by step
Check the reactivity condition
Method
Locate zinc above hydrogen in the reactivity series.Reason
A metal above hydrogen can displace hydrogen ions from a dilute acid.Working
Zn is above H, so a reaction occurs.Write and test the products
Method
Form zinc sulfate and hydrogen, then name the gas test.Reason
Sulfuric acid supplies sulfate; hydrogen is confirmed with a lighted splint.Working
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g); the gas gives a squeaky pop.
7. Mind Stretchers
Mind stretcher 1: Improve the answerExtension
Question: A student writes: “Acids react with metals.” Rewrite this as a precise, fully qualified statement.
Show answer
“Suitable metals above hydrogen in the reactivity series react with dilute hydrochloric acid or dilute sulfuric acid to form a salt and hydrogen gas.”
Mind stretcher 2: Which test is wrong?Extension
Question: A student bubbles the gas from magnesium + dilute hydrochloric acid into limewater. Why does this not confirm the expected gas?
Show answer
Magnesium + dilute hydrochloric acid produces hydrogen, not carbon dioxide. Limewater is a test for CO₂. For hydrogen, you use a lighted splint and listen for a squeaky pop.
Try independently: Copper metal and black copper(II) oxide are added to separate samples of dilute hydrochloric acid. Predict the observations and explain why they differ.
Show answer and reasoning
Copper metal gives no observable reaction: it cannot displace hydrogen ions. Copper(II) oxide dissolves to form a copper(II) chloride solution, with no hydrogen produced. The oxide acts as a base and reacts with hydrogen ions to form water. In dilute solution the copper(II) chloride is blue or blue-green, depending on concentration.
8. Quiz
Practise the ideas in the Acid–Base Chemistry topic check.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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