What is a mixture in the context of IB MYP Science?

In IB MYP Science, a mixture is defined as a combination of two or more substances that are not chemically bonded. Each substance in a mixture retains its own chemical properties. Mixtures can be homogeneous, where the composition is uniform throughout, or heterogeneous, where the components are visibly distinct.

How do mixtures differ from pure substances?

Mixtures differ from pure substances in that pure substances consist of a single type of element or compound with a fixed composition and distinct chemical properties. In contrast, mixtures contain two or more different substances that are physically combined and can be separated by physical means. Pure substances have a consistent composition, while mixtures can vary in their composition.

What are some common methods for separating mixtures?

Common methods for separating mixtures include filtration, which separates solids from liquids; distillation, which separates substances based on boiling points; and chromatography, which separates components based on their movement through a medium. These methods rely on the physical properties of the components in the mixture.

Can you provide examples of homogeneous and heterogeneous mixtures?

Yes, a homogeneous mixture, also known as a solution, has a uniform composition throughout, such as saltwater or air. A heterogeneous mixture has visibly different components, like a salad or a mixture of sand and iron filings. These examples illustrate the differences in how substances are distributed within the mixture.

Why is understanding mixtures important in the IB MYP Science curriculum?

Understanding mixtures is crucial in the IB MYP Science curriculum because it helps students grasp the concept of pure and impure substances, which is fundamental to chemistry. It also provides insight into real-world applications, such as material separation and purification processes, which are essential in various scientific and industrial fields.

Why is "Calling tap water 'pure'." a common mistake in Mixtures?

Why it happens: Tap water looks clean. How to avoid it: Tap water has dissolved minerals, chlorine, fluoride. It's a MIXTURE. Only distilled or deionised water is pure.

Why is "Calling fog a suspension." a common mistake in Mixtures?

Why it happens: Looks like droplets in air. How to avoid it: If particles DON'T settle, it's a colloid (or solution). Fog droplets are tiny enough to stay airborne — that makes it a COLLOID.

Pure and impure substancesIB MYP Sciences (Years 1-5)

Mixtures

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Mixtures — IB MYP Sciences: pure substances vs mixtures and how to tell them apart

A pure substance contains only one type of particle; a mixture contains two or more substances together but NOT chemically bonded. This MYP Sciences note covers the difference between pure and impure, types of mixture, and how melting and boiling points can tell you if a sample is pure.

What you’ll learn

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

MYP Sciences A — Distinguish between pure substances, mixtures, solutions, suspensions and colloids.
MYP Sciences A — Use a melting-point graph to identify whether a sample is pure.
MYP Sciences C — Decide whether a sample is a mixture or a compound from observation.

Pure substances vs mixtures

Pure = one type of particle. Mixture = several together, not bonded.

Pure substance — made entirely of ONE element or compound. Examples:

Pure water (H₂O only).
Pure copper (Cu atoms only).
Pure sugar (C₁₂H₂₂O₁₁ molecules only).

A pure substance has FIXED properties — fixed melting point, fixed boiling point, fixed composition.

Mixture — two or more pure substances physically combined but NOT chemically bonded. Examples:

Air (N₂ + O₂ + Ar + CO₂ + ...).
Sea water (H₂O + Na⁺ + Cl⁻ + many other ions).
Brass (Cu + Zn — an alloy).
A bag of mixed sweets.

Each substance keeps its own properties in a mixture. The mixture can be split apart by physical means.

How to tell them apart in the lab:

Pure substances melt and boil at a SHARP, FIXED temperature.
Mixtures melt over a RANGE of temperatures (sometimes 5-20 °C wide).

For example, pure water melts at exactly 0 °C and boils at 100 °C. Salt water freezes over a range below 0 °C and boils above 100 °C.

Pure: one element or compound; sharp MP/BP.
Mixture: two+ substances physically combined; MP/BP range.
Mixture components keep their own properties.

Types of mixture

Solutions dissolve, suspensions settle, colloids stay cloudy.

Solution — one substance (the SOLUTE, often solid) dissolves in another (the SOLVENT, often liquid). The particles spread out at the MOLECULAR or IONIC level — too small to see, even with a powerful microscope. Examples:

Salt water (Na⁺ + Cl⁻ dispersed through water).
Sugar in tea.
Alloys can be considered solid solutions (atoms of one metal in another).

Solutions are transparent (light passes through) and components DON'T settle.

Suspension — relatively large particles of solid mixed into a liquid but NOT dissolved. They eventually settle to the bottom if left alone. Examples:

Muddy water (sand suspended).
Calamine lotion (zinc oxide suspended).
Orange juice with pulp.

Stir or shake to suspend; let stand → particles settle.

Colloid — particles bigger than in a solution but small enough that they don't settle. They scatter light (Tyndall effect — a light beam through milk or fog is visible). Examples:

Milk (fat globules in water).
Fog (water droplets in air).
Whipped cream (air bubbles in cream).
Jelly (water in protein network).

Mixture type	Particle size	Settles?	Light?
Solution	Atom/ion/molecule	No	Transparent
Colloid	Mid-sized	No (suspended)	Scatters light (Tyndall)
Suspension	Large particles	Yes, with time	Cloudy

Solution: dissolves at molecular/ionic level. Clear.
Suspension: visible particles that settle out.
Colloid: in between — doesn't settle but scatters light.

Spotting a mixture vs a compound

If you can separate it physically, it's a mixture.

Key clues that distinguish a mixture from a compound:

Feature	Mixture	Compound
Composition	Varies (any ratio)	Fixed by formula
Properties	Components keep theirs	New properties
Energy on forming	Little or none	Usually a noticeable energy change
Separation	Physical methods (filter, evaporate)	Needs a chemical reaction
Example	Iron filings + sulfur (powder)	Iron sulfide FeS (after heating)

The iron/sulfur experiment is a perfect lab demonstration:

Mix iron filings with yellow sulfur powder → grey-yellow MIXTURE. A magnet picks the iron out. Properties are unchanged.
Heat strongly → glows red → forms iron(II) sulfide FeS. Now: dark grey, no magnetic attraction, new properties. It's a COMPOUND. To get the iron back you'd need a chemical reaction, not just a magnet.

Strong MYP criterion C question: 'A grey solid is shown. Design an investigation to decide if it is a mixture of iron and sulfur, or the compound iron sulfide.' Strong answer: test with a magnet (mixture: magnet picks iron out; compound: not magnetic), test with dilute acid (mixture: bubbles of H₂; compound: gives H₂S — smelly).

Mixture: physical separation works.
Compound: needs chemistry to break apart.
Iron + sulfur (mix) → heat → FeS (compound) is the classic demo.

How it’s examined

Criterion A tests definitions and classification. Criterion C asks you to deduce mixture/compound from given properties or an experiment plan.

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

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Step-by-step worked examples — Mixtures

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

1Pure or mixture?
Getting started• classification
Question
Classify each as a pure substance or a mixture: (a) tap water, (b) distilled water, (c) air, (d) gold ring 24-carat (pure).
Step-by-step solution
1. Step 1
  (a) Tap water has dissolved minerals → MIXTURE.
2. Step 2
  (b) Distilled water is just H₂O → PURE.
3. Step 3
  (c) Air has many gases mixed → MIXTURE.
4. Step 4
  (d) 24-carat gold is pure Au → PURE.
Answer
(a) mixture, (b) pure, (c) mixture, (d) pure.
2Sharp MP tells you it's pure
Building confidence• melting point
Question
A sample melts at exactly 80 °C with no temperature range. A second sample melts between 75 °C and 90 °C. Which is pure?
Step-by-step solution
1. Step 1
  Pure substances have a sharp, fixed melting point.
2. Step 2
  Range melting is characteristic of a mixture.
Answer
The first sample (sharp MP at 80 °C) is pure.
3Types of mixture
Building confidence• solution, suspension, colloid
Question
Classify each as solution, suspension or colloid: (a) salt water, (b) muddy water, (c) milk.
Step-by-step solution
1. Step 1
  (a) Salt dissolves at ionic level → SOLUTION.
2. Step 2
  (b) Solid sand settles out with time → SUSPENSION.
3. Step 3
  (c) Fat globules stay dispersed and scatter light → COLLOID.
Answer
(a) solution, (b) suspension, (c) colloid.
4Mixture or compound?
Stretch• experiment
Question
A dark grey solid contains iron and sulfur. Describe a test to decide whether it's a mixture or the compound iron sulfide.
Step-by-step solution
1. Step 1
  Test with a magnet. If iron is picked out — it's a mixture (iron + sulfur). If nothing is attracted — it's iron sulfide (Fe is no longer free).
2. Step 2
  Test with dilute HCl. A mixture gives H₂ (from Fe + HCl) — odourless, pops with a splint. A compound (FeS) gives H₂S — smells like bad eggs.
Answer
Use a magnet (picks iron from a mixture; nothing from FeS) or dilute HCl (mixture → H₂; FeS → H₂S, bad-egg smell).

Key Definitions and Keywords — Mixtures

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

Pure substance
Examiner keyword
Made entirely of one element or compound. Fixed composition; sharp MP and BP.
Mixture
Examiner keyword
Two or more substances physically combined but NOT chemically bonded.
Solution
Examiner keyword
A mixture where one substance is dissolved at molecular/ionic level in another. Transparent.
Suspension
A mixture with relatively large particles that settle out over time.
Colloid
A mixture with particles bigger than solute molecules but small enough not to settle. Scatters light (Tyndall effect).

Common Mistakes and Misconceptions — Mixtures

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

✕Calling tap water 'pure'.
Why it happens
Tap water looks clean.
How to avoid it
Tap water has dissolved minerals, chlorine, fluoride. It's a MIXTURE. Only distilled or deionised water is pure.
✕Calling fog a suspension.
Why it happens
Looks like droplets in air.
How to avoid it
If particles DON'T settle, it's a colloid (or solution). Fog droplets are tiny enough to stay airborne — that makes it a COLLOID.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Mixtures — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Mixtures

Short Study Notes in the form of Slides

Short Study Notes — Mixtures

Detailed Notes

What you’ll learn

How it’s examined

1Pure or mixture?

2Sharp MP tells you it's pure

3Types of mixture

4Mixture or compound?

Pure substance

Mixture

Solution

Suspension

Colloid

✕Calling tap water 'pure'.

✕Calling fog a suspension.

Practice questions

Past paper style quiz

Assess your understanding

Mixtures — frequently asked questions