IB MYP Sciences – States and properties of matter Study Guide

This study guide teaches states and properties of matter for IB MYP Sciences. It is aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014, which lists “states and properties of matter” among the topics explored in the MYP sciences on-screen examinations. MYP has no SL/HL split, and the page suits MYP years 4 and 5, when students entered for eAssessment prepare for it.

MYP has no prescribed content list – schools design their own units – so your school’s unit may be ordered differently. This page covers the core ideas behind a topic the IB’s brief names. Your teacher will share the task-specific clarifications for any assessed task.

Use it with the revision notes for final-week recall and the practice questions to test yourself. For the course overview, see the IB MYP Sciences hub and the printable checklist.

What this unit covers

Area	What you should be able to do	Criterion it mostly trains
Particle model	Describe arrangement, movement and energy of particles in solids, liquids and gases; explain diffusion	A
Changes of state	Name each change; explain energy transfer and why temperature stays constant	A
Heating and cooling curves	Read and sketch curves; find melting and boiling points; compare energy on flat sections	A, C
Density	Use ρ = m / V; measure volume by displacement; predict floating and sinking	A, B, C
Gas behaviour	Explain pressure, and the effect of temperature and volume, in words	A
Mixtures and separation	Tell pure substances from mixtures; choose and explain a separation method; calculate Rf	A, B, C

How this links to MYP concepts and assessment

The brief gives energy, movement, transformation and models as examples of related concepts in MYP sciences. This topic uses all four. The particle model is a model. Changes of state are transformations. Temperature is linked to the movement of particles, and heating transfers energy.

The brief also sets out three on-screen examination tasks: Knowing and understanding (25 marks, criterion A), Investigation skills (50 marks, criteria B and C) and Applying science (25 marks, criterion D). Matter questions can appear in any of them. The existing guides on criteria in practice and investigation skills explain how each criterion works in general.

The particle model

All matter is made of tiny particles (atoms, molecules or ions). The particle model explains the properties of the three states.

	Solid	Liquid	Gas
Arrangement	Close together, regular pattern	Close together, random	Far apart, random
Movement	Vibrate about fixed positions	Move around and slide past each other	Move quickly in all directions
Forces between particles	Strong	Weaker than in a solid	Very weak
Shape	Fixed	Takes the shape of the container	Fills the container
Volume	Fixed	Fixed	Fills the container
Can it be compressed?	Almost not	Almost not	Yes, easily

Two ideas carry most marks:

• Temperature is linked to the average kinetic energy of the particles. Heat a substance and its particles move (or vibrate) faster.
• Gases compress because there is empty space between the particles. In solids and liquids the particles are already touching.

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It is fast in gases, slower in liquids and extremely slow in solids. It is faster at higher temperature (particles move faster) and faster for lighter particles.

In a classic demonstration, ammonia and hydrogen chloride diffuse towards each other along a glass tube. A white ring of ammonium chloride forms nearer the hydrogen chloride end, because ammonia particles are lighter and move faster.

Changes of state

Change	From → to	Energy
Melting	solid → liquid	absorbed
Freezing	liquid → solid	released
Boiling / evaporation	liquid → gas	absorbed
Condensation	gas → liquid	released
Sublimation	solid → gas	absorbed
Deposition	gas → solid	released

When a substance melts or boils, the energy supplied is used to overcome the forces of attraction between particles, not to make the particles move faster. That is why the temperature stays constant during a change of state even though heating continues.

Evaporation and boiling

Evaporation	Boiling
Happens at any temperature below the boiling point	Happens only at the boiling point
Only at the surface	Throughout the liquid (bubbles form inside)
Slow	Fast
Cools the liquid left behind	Temperature stays at the boiling point

Evaporation cools because the particles with the most energy escape from the surface. The average energy of the particles left behind falls, so the temperature falls. This is why sweating cools you.

Purity and melting point

A pure substance melts and boils at fixed, sharp temperatures. Pure water boils at 100 °C at standard atmospheric pressure and melts at 0 °C. An impurity usually lowers the melting point, makes it melt over a range of temperatures, and raises the boiling point. Salty water boils above 100 °C. So a sharp melting point is evidence of purity.

Heating and cooling curves

A heating curve plots temperature (y-axis) against time (x-axis) while a substance is heated steadily.

Temperature
   |                         ______ boiling (liquid + gas)
   |                        /
   |                       /  liquid warming
   |          ___________ /
   |         / melting (solid + liquid)
   |        /
   |       / solid warming
   |______/______________________________ Time

• Sloping sections: one state is warming up. Particles gain kinetic energy.
• Flat sections: a change of state. Energy goes into overcoming forces between particles. The flat section at the lower temperature is the melting point; the higher one is the boiling point.
• A cooling curve is the mirror image: flat sections at the condensation and freezing points, where energy is released.

The boiling plateau is usually much longer than the melting plateau. Boiling separates the particles completely, so far more energy is needed than for melting, where the particles stay close together.

Worked example: comparing energy on a heating curve

A 100 W heater supplies energy at a steady rate to a solid. The melting plateau lasts 120 s and the boiling plateau lasts 600 s. Assume all the heater’s energy goes into the substance.

1. Energy supplied = power × time.
2. Melting: 100 W × 120 s = 12 000 J.
3. Boiling: 100 W × 600 s = 60 000 J.
4. Boiling needs 60 000 / 12 000 = 5 times as much energy as melting.

In practice some energy heats the container and air, so the substance absorbs less than this.

Density

Density is the mass per unit volume of a substance.

density = mass / volume        ρ = m / V
mass = density × volume        m = ρ × V
volume = mass / density        V = m / ρ

Common units are g/cm³ and kg/m³. To convert g/cm³ to kg/m³, multiply by 1000. Water has a density of about 1.00 g/cm³ (1000 kg/m³).

An object floats in a liquid if its density is less than the liquid’s and sinks if its density is greater. Ice (about 0.92 g/cm³) is less dense than liquid water, which is unusual – most substances are densest as solids. That is why ice forms on top of ponds.

Worked example 1: a regular block

A metal block measures 6.0 cm × 3.0 cm × 2.0 cm and has a mass of 97.2 g.

1. Volume = 6.0 × 3.0 × 2.0 = 36 cm³.
2. ρ = m / V = 97.2 / 36 = 2.7 g/cm³.
3. In kg/m³: 2.7 × 1000 = 2700 kg/m³. It sinks in water, because 2.7 > 1.00.

Worked example 2: an irregular object by displacement

A measuring cylinder holds water to the 50.0 cm³ mark. A small metal ornament of mass 112.5 g is lowered in, and the level rises to 62.5 cm³.

1. Volume of ornament = 62.5 − 50.0 = 12.5 cm³.
2. ρ = 112.5 / 12.5 = 9.0 g/cm³.

Read the scale at the bottom of the meniscus, with your eye level with the surface. Lower the object gently on a thread so no water splashes out.

Gas behaviour (qualitative)

A gas exerts pressure because its particles collide with the walls of the container. Each collision pushes on the wall. More frequent or harder collisions mean higher pressure.

Change (fixed amount of gas)	What happens to the particles	Result
Heat it, volume fixed (sealed can)	Move faster; hit walls more often and harder	Pressure rises
Squash it into a smaller volume, temperature fixed (syringe)	Same speed, but less space, so more collisions per second with each area of wall	Pressure rises
Heat it, pressure allowed to stay the same (balloon)	Move faster and push the walls out	Volume increases
Cool it (balloon in a freezer)	Move slower, fewer and softer collisions	Balloon shrinks

Always explain in steps: particle speed → frequency or force of collisions → pressure. Don’t say the particles themselves get bigger when a gas expands. The particles stay the same size; the space between them grows.

Mixtures and separation

• An element contains only one type of atom.
• A compound contains two or more elements chemically joined, in fixed proportions.
• A mixture contains two or more substances not chemically joined. It can be separated by physical methods.
• A solution is a mixture of a solute dissolved in a solvent. A substance that dissolves is soluble; one that doesn’t is insoluble.

In chemistry, “pure” means a single substance with nothing else mixed in. It is not the everyday meaning of “natural” or “clean”.

To separate	Method	Why it works
Insoluble solid from a liquid (sand and water)	Filtration	Solid particles are too large to pass through the filter paper
Dissolved solid from a solution (salt from salt water)	Evaporation or crystallisation	Solvent evaporates; solute is left. Heat gently and let crystals form as it cools
Solvent from a solution (pure water from seawater)	Simple distillation	Water boils, the vapour is condensed and collected; salt stays behind
Two miscible liquids with different boiling points (ethanol, 78 °C, and water, 100 °C)	Fractional distillation	The liquid with the lower boiling point reaches the top of the column first and is condensed
Coloured substances in a mixture (inks, dyes)	Paper chromatography	Substances move up the paper at different rates, depending on solubility in the solvent and attraction to the paper
Magnetic from non-magnetic solid (iron filings and sulfur)	Magnet	Only iron is attracted

Chromatography and Rf values

Rf = distance moved by the spot / distance moved by the solvent front

Both distances are measured from the baseline. Rf is always between 0 and 1. The same substance, with the same solvent and paper, always gives the same Rf, so you can identify it by comparing with known references. A pure substance gives a single spot.

Worked example. The solvent front moves 9.0 cm. An ink gives spots at 3.6 cm and 6.3 cm.

1. Rf = 3.6 / 9.0 = 0.40.
2. Rf = 6.3 / 9.0 = 0.70.
3. Two spots, so the ink is a mixture of at least two dyes.

Draw the baseline in pencil (graphite doesn’t dissolve and run), and keep it above the solvent level so the spots don’t wash into the solvent.

Common errors

• Saying temperature rises during melting. It stays constant; the energy overcomes forces between particles.
• Saying “the particles expand” when a gas is heated. The spaces between them increase.
• Confusing evaporation and boiling, or saying evaporation only happens at 100 °C.
• Forgetting to subtract the starting reading in a displacement measurement.
• Mixing units: g with m³, or giving a density with no unit.
• Measuring Rf distances from the bottom of the paper instead of from the baseline.
• Choosing filtration to separate salt from salt water. Dissolved salt passes through filter paper.
• Describing a gas as having “no forces” between particles. The forces are very weak, not zero.

Next steps

Go to the revision notes for condensed tables and a self-test, then try the practice questions, which include a design task and a data-analysis task. Why substances have different melting points is explained by bonding in atomic structure and bonding. The course models guide explains how your eAssessment option affects which topics you meet.

Official syllabus

International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. Published by the International Baccalaureate Organization.