O Level Acids, Bases and Salts Worked Examples for 2026/2027 (Singapore MOE Syllabus) — Step-by-Step Worked Examples

Quick answer

Ever felt your heart sink during an exam when you see a Chemistry question that looks unfamiliar? It's common in O Level Chemistry, especially with topics like Acids, Bases, and Salts. But don't worry! By breaking down questions into simple steps, you can tackle any problem, no matter how it's worded.

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What you need to know

Acids are substances that release hydrogen ions ($𝐻^+$) in water, making the solution acidic. Bases release hydroxide ions ($OH^-$), making the solution basic or alkaline. Salts are formed when acids react with bases. It's like how in Singapore, we mix kaya and butter to make a delicious toast — each ingredient plays a role to create something new.

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How to tackle O Level questions

Understanding Acids, Bases, and Salts

One mistake I repeatedly see among my Sec 4 students is memorizing definitions without understanding. Think of acids as the tangy taste in your favourite hawker centre dish. Bases, on the other hand, are like the cleaning agents that keep the MRT stations spotless. When they react, they form salts, just like how a pinch of seasoning can bring a dish together.

Quick check:

• What ions do acids release in water?
• What ions do bases release?
• What is formed when acids react with bases?

Reaction Equations

Writing equations for reactions between acids and bases can be tricky. Students usually panic when they see application questions like this. Let's break it down.

Step 1: Identify the acid and base in the question.
Why: Knowing the reactants helps you predict the products.

Step 2: Write the word equation for the reaction. For example, hydrochloric acid + sodium hydroxide → sodium chloride + water.
Why: This gives a clear picture of the reaction happening.

Step 3: Convert the word equation into a chemical equation: $HCl + NaOH \rightarrow NaCl + 𝐻_2𝑂$.
Why: Chemical equations show the exact substances and their amounts.

Step 4: Balance the equation. Make sure the number of each type of atom is the same on both sides.
Why: Balancing ensures the law of conservation of mass is followed.

Quick check

• Write the balanced chemical equation for the reaction between sulfuric acid and potassium hydroxide.
• What is the salt formed in this reaction?

Common mistakes students make

1. Memorizing without understanding: Memorizing keywords won't help when the question is phrased differently. Always aim to understand the process.
2. Writing general answers: Be precise. If the question asks for a specific reaction, give the exact chemical equation, not just the word equation.
3. Forgetting to balance equations: This is crucial for full marks. Double-check your equations.

Exam tip

Precision matters more than length. In O-Level Chemistry, it's not about writing long answers but about being specific and accurate. Always double-check your equations and make sure they're balanced.

Worked examples

Question 1

Predict the products and write the balanced equation for the reaction between nitric acid and calcium hydroxide.

Solution

Step 1: Identify the reactants: Nitric acid ($HNO_3$) and calcium hydroxide ($Ca(OH)_2$).
Why: Knowing the reactants helps in predicting the products.

Step 2: Predict the products: Calcium nitrate and water.
Why: Acids react with bases to form a salt and water.

Step 3: Write the chemical equation: $HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 𝐻_2𝑂$.
Why: This represents the reaction in symbols.

Step 4: Balance the equation: $2HNO_3 + Ca(OH)_2 \rightarrow Ca(NO_3)_2 + 2𝐻_2𝑂$.
Why: Balancing ensures the number of atoms is the same on both sides.

Question 2

What is the salt formed when hydrochloric acid reacts with magnesium oxide?

Solution

Step 1: Identify the reactants: Hydrochloric acid (HCl) and magnesium oxide (MgO).
Why: This helps to identify the products formed.

Step 2: Predict the products: Magnesium chloride and water.
Why: A metal oxide reacts with an acid to form a salt and water.

Step 3: Write the chemical equation: $2HCl + MgO \rightarrow MgCl_2 + 𝐻_2𝑂$.
Why: This shows the reaction in chemical form.

Step 4: Balance the equation, but here it is already balanced.
Why: Always check, even if it looks balanced at first glance.

Question 3

Write the balanced equation for the reaction between sulfuric acid and sodium hydroxide.

Solution

Step 1: Identify the reactants: Sulfuric acid ($𝐻_2SO_4$) and sodium hydroxide (NaOH).
Why: Recognizing the reactants is the first step in predicting products.

Step 2: Predict the products: Sodium sulfate and water.
Why: Acids react with bases to form salt and water.

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Step 3: Write the chemical equation: $𝐻_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2𝐻_2𝑂$.
Why: This equation represents the full reaction.

Step 4: Balance the equation, but here it is already balanced.
Why: Balancing is essential for accuracy.

Question 4

Calculate the number of moles of water produced when 0.5 moles of hydrochloric acid react with sodium hydroxide.

Solution

Step 1: Write the balanced chemical equation: $HCl + NaOH \rightarrow NaCl + 𝐻_2𝑂$.
Why: A balanced equation is needed to find the mole ratio.

Step 2: Identify the mole ratio from the equation: 1:1:1:1.
Why: This tells you how many moles of each reactant and product are involved.

Step 3: Use the mole ratio to find moles of water: 0.5 moles of HCl produces 0.5 moles of $𝐻_2𝑂$.
Why: The 1:1 ratio means equal moles of acid and water.

Quick summary

• Acids release $𝐻^+$ ions; bases release $OH^-$ ions.
• Salts are formed from acid-base reactions.
• Write and balance chemical equations accurately.
• Understand the reactions, don't just memorize them.
• Be precise in exams — check your equations twice.

FAQ

Q: What is a strong acid?
A: A strong acid completely ionizes in water, releasing $𝐻^+$ ions. An example is hydrochloric acid.

Q: How do I know if a reaction forms a salt?
A: If an acid reacts with a base or metal, it usually forms a salt and water or hydrogen gas.

Q: Why is balancing equations important?
A: Balancing ensures that the mass and charge are conserved in a reaction, following the law of conservation of mass.

Q: Can acids react with metals?
A: Yes, acids react with metals to produce a salt and hydrogen gas.

Q: Is water always a product in acid-base reactions?
A: Yes, water is usually formed when acids react with bases.

Practise with free question sets

Work through more exam-style questions with answers and step-by-step solutions:

• 35+ O Level Acids, bases and salts Practice Questions for 2026 (based on Singapore MOE Syllabus)
• Topic study hub

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