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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The core idea
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Learning objectives
- describe the characteristic properties of acids as in reactions with metals, bases and carbonates to form salts
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 pH < 7.
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
- Acid + metal → salt + hydrogen gas (H₂) (reactive metals above hydrogen; dilute acids).
- Acid + metal oxide/metal hydroxide → salt + water (H₂O) (neutralisation).
- Acid + carbonate → salt + water (H₂O) + carbon dioxide (CO₂).
- H₂ test: lighted splint → squeaky pop. CO₂ test: limewater turns 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; taste and smell are never chemical tests.
Acids are corrosive. 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
Metals above hydrogen in the reactivity series usually react with dilute hydrochloric acid or dilute sulfuric acid to form hydrogen gas. Metals below hydrogen (e.g. copper, silver) do not. This school-level displacement rule should not be extended to oxidising acids such as nitric acid, which can produce different reduction products. This classroom rule is for common dilute, non-oxidising acids, especially hydrochloric acid and dilute sulfuric acid. Oxidising acids such as nitric acid can give different products, so do not apply the hydrogen rule to them automatically. See: Reactivity Series.
C. Acid + metal oxide or hydroxide → salt + water
Metal oxides and metal hydroxides neutralise acids to form salt and water. 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)
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)
4. Common Mistakes
- Writing “acid + metal gives hydrogen” without stating dilute acid and a reactive metal.
- 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: “pop” must be “squeaky pop”; “limewater turns milky”.
- 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
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
Match reaction and gas-test evidence
Hints
Hint 1: reaction family
Hint 2: confirmatory observation
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
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
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
Check whether the reaction condition is met
Hints
Hint 1: qualify the metal rule
Hint 2: name the salt
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
“Reactive metals above hydrogen in the reactivity series react with dilute acids to form a salt and hydrogen gas.”
Mind stretcher 2: Which test is wrong?Extension
Question: A student bubbles the gas from acid + metal into limewater. Why is this a bad test?
Show Answer
Acid + metal produces hydrogen, not carbon dioxide. Limewater is a test for CO₂. For hydrogen, you use a lighted splint and listen for a squeaky pop.
8. Quiz
Test yourself on indicators, the three acid reactions, and gas-test keywords.
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