What are the main differences between acids and bases in terms of their properties?

In the context of the IB MYP Science curriculum, acids and bases are distinguished by their chemical properties. Acids typically have a sour taste, can turn blue litmus paper red, and react with metals to produce hydrogen gas. They have a pH less than 7. Bases, on the other hand, have a bitter taste, feel slippery, can turn red litmus paper blue, and have a pH greater than 7. Both acids and bases can conduct electricity when dissolved in water due to the presence of ions.

How do acids and bases react to form salts?

In chemical reactions, acids and bases can react with each other in a process called neutralization. During this reaction, the hydrogen ions (H⁺) from the acid combine with the hydroxide ions (OH⁻) from the base to form water (H₂O). The remaining ions from the acid and base form a salt. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the products are water and sodium chloride (NaCl), which is a common table salt.

What is the role of pH in determining the strength of an acid or base?

The pH scale, which ranges from 0 to 14, is used to measure the acidity or alkalinity of a solution. In the IB MYP Science curriculum, understanding pH is crucial for determining the strength of acids and bases. A pH less than 7 indicates an acidic solution, with lower values representing stronger acids. A pH greater than 7 indicates a basic solution, with higher values representing stronger bases. A pH of 7 is considered neutral, which is the pH of pure water.

Why are indicators used in experiments involving acids and bases?

Indicators are substances used in science experiments to determine the acidity or basicity of a solution. They change color at specific pH levels, providing a visual representation of the solution's pH. Common indicators include litmus paper, phenolphthalein, and bromothymol blue. In the IB MYP curriculum, using indicators helps students understand the concept of pH and the properties of acids and bases through hands-on experiments.

Can you explain the concept of a buffer solution in relation to acids and bases?

A buffer solution is a special type of solution that resists changes in pH when small amounts of an acid or base are added. It is typically made from a weak acid and its conjugate base or a weak base and its conjugate acid. In the IB MYP Science curriculum, buffer solutions are important for maintaining stable pH levels in various chemical and biological processes. For example, the human body uses buffer systems to maintain the pH of blood, which is crucial for proper physiological function.

Why is "Treating 'base' and 'alkali' as identical." a common mistake in Acids, Bases and Salt?

Why it happens: Many alkalis are bases, and vice versa. How to avoid it: All alkalis are bases (they accept H⁺). NOT all bases are alkalis — copper oxide is a base but doesn't dissolve in water.

Why is "Thinking pH 4 is only twice as acidic as pH 8." a common mistake in Acids, Bases and Salt?

Why it happens: It looks linear. How to avoid it: Each pH unit is a 10× change. pH 4 is 10 000× more acidic than pH 8.

Why is "Forgetting that acid + metal produces hydrogen." a common mistake in Acids, Bases and Salt?

Why it happens: It's easy to confuse with acid + carbonate (CO₂). How to avoid it: Memorise the three products: metal → H₂, base → water only, carbonate → water + CO₂.

Chemical reactionsIB MYP Sciences (Years 1-5)

Acids, Bases and Salt

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Acids, Bases and Salts — IB MYP Sciences: pH, neutralisation and how salts form

Acids in your stomach, bases in cleaning products, salts in your food — chemistry shapes everyday life. This MYP Sciences note covers the pH scale, what makes an acid or a base, common neutralisation reactions, and how salts are made.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Define acids, bases and alkalis using ion language.
MYP Sciences A — Use the pH scale to classify a solution.
MYP Sciences A — Write word equations for acid + metal, acid + base, acid + carbonate.
MYP Sciences B — Plan a titration to find the concentration of an acid.
MYP Sciences D — Evaluate everyday neutralisation (indigestion remedies, acidic soil).

Acids, bases and the pH scale

Acids release H⁺; alkalis release OH⁻; pH measures how acidic or alkaline a solution is.

An acid is a substance that, when dissolved in water, releases H⁺ ions (protons). A base is a substance that accepts H⁺ ions. A soluble base is called an alkali — it releases OH⁻ ions in water.

The pH scale is a numerical measure of acidity:

pH	Class	Examples
0-1	Strongly acidic	Concentrated HCl, battery acid
2-3	Acidic	Lemon juice, vinegar
5-6	Weakly acidic	Coffee, milk
7	Neutral	Pure water
8-9	Weakly alkaline	Sea water, baking soda solution
10-13	Alkaline	Ammonia, soap
14	Strongly alkaline	Concentrated NaOH

Universal indicator colours across the pH scale.

A pH change of 1 unit = 10× change in H⁺ concentration. pH 3 is 100× more acidic than pH 5.

Acid → H⁺ in water. Alkali → OH⁻ in water.
pH scale: 0 (most acidic) — 7 (neutral) — 14 (most alkaline).
Universal indicator: red (acid) → green (neutral) → purple (alkali).
1 pH unit = 10× change in H⁺ concentration.

Three key reactions of acids

Acid + metal, acid + base, acid + carbonate — all produce a salt.

1. Acid + metal → salt + hydrogen

$\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2$

Hydrogen gas pops with a lighted splint — used as the gas test. Works with reactive metals (above hydrogen in reactivity); copper and below don't react.

2. Acid + base (or alkali) → salt + water (neutralisation)

$\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$

The active step is H⁺ + OH⁻ → H₂O. This is how indigestion remedies work (stomach HCl + base → neutralised). Also used to treat acidic soil with lime (calcium oxide).

3. Acid + carbonate → salt + water + CO₂

$2\text{HCl} + \text{CaCO}_3 \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$

Effervescence (bubbles of CO₂) is the visible sign. Limestone fizzes in acid; chalk in vinegar does the same. CO₂ test: turns limewater milky.

Acid + metal → salt + H₂ (gas, pops with splint).
Acid + base → salt + water (neutralisation).
Acid + carbonate → salt + water + CO₂ (gas, fizzes; turns limewater milky).

How salts form and how they're named

Replace the H of an acid with a metal: that's a salt.

A salt is what you get when the hydrogen in an acid is replaced by a metal (or by ammonium, NH₄⁺). Each common acid gives a particular family of salts:

Acid	Salt family	Example
HCl (hydrochloric)	chlorides	NaCl, MgCl₂
H₂SO₄ (sulfuric)	sulfates	Na₂SO₄, CuSO₄
HNO₃ (nitric)	nitrates	KNO₃, NaNO₃
CH₃COOH (ethanoic)	ethanoates	Na ethanoate

Worked example. Predict the salt formed when sulfuric acid reacts with sodium hydroxide.

H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O. The salt is sodium sulfate (Na₂SO₄).

Many salts are useful in everyday life: NaCl (food), CaSO₄ (plaster of Paris), KNO₃ (fertiliser), NH₄NO₃ (fertiliser, explosive precursor).

Salt = H of acid replaced by metal.
HCl → chlorides; H₂SO₄ → sulfates; HNO₃ → nitrates.
Salts are everywhere: food, agriculture, construction.

How it’s examined

Criterion A tests pH classification, word equations and reaction products. Criterion B/C: titrations and salt preparation IAs. Criterion D may ask about indigestion, soil treatment or acid rain.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Step-by-step worked examples — Acids, Bases and Salt

Step-by-step solutions to past-paper-style questions on acids, bases and salt, written exactly the way a tutor would explain them at the board.

1Classifying solutions
Getting started• pH
Question
Classify each as strongly acidic, weakly acidic, neutral, weakly alkaline or strongly alkaline: (a) battery acid pH 1, (b) coffee pH 5, (c) pure water pH 7, (d) ammonia pH 11.
Step-by-step solution
1. Step 1
  (a) pH 1 — strongly acidic.
2. Step 2
  (b) pH 5 — weakly acidic.
3. Step 3
  (c) pH 7 — neutral.
4. Step 4
  (d) pH 11 — alkaline (close to strongly alkaline).
Answer
(a) strongly acidic, (b) weakly acidic, (c) neutral, (d) alkaline.
2Acid + metal
Building confidence• reactions
Question
Write a word equation for the reaction between magnesium and dilute hydrochloric acid, and name the gas produced.
Step-by-step solution
1. Step 1
  Acid + metal → salt + hydrogen.
2. Step 2
  Magnesium + hydrochloric acid → magnesium chloride + hydrogen.
3. Step 3
  Hydrogen pops with a lighted splint.
Answer
Magnesium + HCl → magnesium chloride + hydrogen. Gas = hydrogen (pops with lighted splint).
3Naming salts
Building confidence• salts
Question
Name the salt formed when (a) HNO₃ + KOH, (b) H₂SO₄ + Mg(OH)₂.
Step-by-step solution
1. Step 1
  (a) HNO₃ + KOH → KNO₃ + H₂O. Salt = potassium nitrate.
2. Step 2
  (b) H₂SO₄ + Mg(OH)₂ → MgSO₄ + 2H₂O. Salt = magnesium sulfate.
Answer
(a) potassium nitrate (KNO₃), (b) magnesium sulfate (MgSO₄).
4Identifying a carbonate
Stretch• tests
Question
A solid X fizzes vigorously when dilute HCl is added. The gas produced turns limewater milky. What is X likely to contain?
Step-by-step solution
1. Step 1
  Fizz + limewater turns milky → the gas is CO₂.
2. Step 2
  Acid + something → CO₂ usually means the substance is a CARBONATE (CO₃²⁻).
Answer
X contains a carbonate (CO₃²⁻). The gas test (limewater milky) confirms CO₂.

Key Formulae — Acids, Bases and Salt

The formulae you need to memorise for acids, bases and salt on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Neutralisation
$H⁺ + OH⁻ → H₂O$
When to use
The ion-level summary of any acid–alkali neutralisation.

Key Definitions and Keywords — Acids, Bases and Salt

Definitions to memorise and the exact keywords mark schemes credit for acids, bases and salt answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Acid
Examiner keyword
A substance that releases H⁺ ions when dissolved in water.
Base
Examiner keyword
A substance that accepts H⁺ ions (neutralises an acid).
Alkali
Examiner keyword
A soluble base that releases OH⁻ ions in water.
pH
Examiner keyword
A measure of acidity from 0 to 14. <7 acid, 7 neutral, >7 alkaline.
Neutralisation
Examiner keyword
Acid + base → salt + water. The active step: H⁺ + OH⁻ → H₂O.
Salt
Examiner keyword
A compound formed when the hydrogen of an acid is replaced by a metal (or ammonium).
Universal indicator
A mix of dyes that shows different colours at different pH values.

Common Mistakes and Misconceptions — Acids, Bases and Salt

The traps other students keep falling into on acids, bases and salt questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Treating 'base' and 'alkali' as identical.
Why it happens
Many alkalis are bases, and vice versa.
How to avoid it
All alkalis are bases (they accept H⁺). NOT all bases are alkalis — copper oxide is a base but doesn't dissolve in water.
✕Thinking pH 4 is only twice as acidic as pH 8.
Why it happens
It looks linear.
How to avoid it
Each pH unit is a 10× change. pH 4 is 10 000× more acidic than pH 8.
✕Forgetting that acid + metal produces hydrogen.
Why it happens
It's easy to confuse with acid + carbonate (CO₂).
How to avoid it
Memorise the three products: metal → H₂, base → water only, carbonate → water + CO₂.

Practice questions

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Acids, Bases and Salt — frequently asked questions

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Acids, Bases and Salt

Short Study Notes in the form of Slides

Short Study Notes — Acids, Bases and Salt

Detailed Notes

What you’ll learn

How it’s examined

1Classifying solutions

2Acid + metal

3Naming salts

4Identifying a carbonate

Neutralisation

Acid

Base

Alkali

pH

Neutralisation

Salt

Universal indicator

✕Treating 'base' and 'alkali' as identical.

✕Thinking pH 4 is only twice as acidic as pH 8.

✕Forgetting that acid + metal produces hydrogen.

Practice questions

Past paper style quiz

Assess your understanding

Acids, Bases and Salt — frequently asked questions