This page needs the IB Demystified site-wide snippets. Ask the site administrator to install them.
IB DemystifiedMYP Sciences
States of matter
Keep heating ice and its temperature does something surprising: it stops rising, twice. Explaining those pauses with particles is the key to melting, boiling, evaporation and everything in between.
Recommended for MYP 1 · About 3 lessons · Criteria A, B, C and D
Figure 1. Heating ice at a steady rate. The temperature stops rising at B–C and D–E.
describe the properties of solids, liquids and gases
explain those properties using the arrangement, movement and forces of particles
name and describe the six changes of state
use melting and boiling points to predict the state of a substance
interpret heating and cooling curves and explain why the temperature stays constant during a change of state
explain evaporation and diffusion, and what affects how fast they happen
Before you start
You will use these skills. If any feel shaky, review them first.
everyday examples of solids, liquids and gases
reading a thermometer, including temperatures below 0 °C
reading values from a line graph
Key vocabulary
Particle model
The idea that all matter is made of tiny particles that are always moving.
Melting point
The temperature at which a solid melts. A pure substance melts at one fixed temperature.
Boiling point
The temperature at which a liquid boils, forming bubbles of gas throughout.
Evaporation
A liquid turning into a gas at its surface, at any temperature.
Sublimation
A solid turning directly into a gas without melting.
Diffusion
The spreading of particles from where they are concentrated to where they are less concentrated.
Understanding states of matter
What is it?
Matter exists as solids, liquids and gases. A solid has a fixed shape and volume; a liquid has a fixed volume but flows; a gas fills any container and can be compressed.
Why does it happen?
In a solid, strong forces hold particles in fixed positions where they vibrate. In a liquid the particles are still touching but can move past each other. In a gas the particles are far apart and move quickly in all directions. Heating gives particles more energy; at the melting and boiling points that energy is used to overcome the forces between them, which is why the temperature pauses in Figure 1.
How do we know?
Particles are far too small to see, but the model explains what we observe: gases can be squashed, smells spread through still air, and a heating curve has flat sections. In 1827 the botanist Robert Brown saw tiny particles released from pollen grains jiggling randomly in water. Decades later, scientists explained this as the particles being struck by moving water particles that are far too small to see.
Why does it matter?
Changes of state keep food cold in fridges, cool our bodies when we sweat, drive the water cycle and let us purify water. Melting points are used to check whether a substance is pure.
What does it connect to?
The particle model underpins density, dissolving, gas pressure and chemical reactions. It connects to the water cycle and weather in Earth science, and to temperature control in biology.
Changes of state
Figure 2. The six changes of state.
Melting (solid to liquid) and freezing (liquid to solid) happen at the same temperature for a pure substance.
Evaporation happens at the surface at any temperature; boiling happens throughout the liquid at the boiling point. Condensation is gas turning back into liquid.
Sublimation (solid to gas) and deposition (gas to solid) skip the liquid state, as with dry ice and frost.
Melting, evaporating, boiling and sublimation take in energy. Freezing, condensing and deposition release energy. The particles themselves do not change; only their arrangement and movement do.
States of matter in the real world
Gritters spread salt on icy roads because salty water freezes below 0 °C. Chemists check the purity of a new medicine by measuring its melting point: impurities lower it and make it melt over a range. Clouds form when water vapour in rising air cools and condenses onto tiny specks of dust.
Worked examples
Example 1: predicting a state
Bromine melts at −7 °C and boils at 59 °C. What state is bromine in on a winter day at −10 °C?
Compare the temperature with the melting point: −10 °C is below −7 °C.
Below its melting point, a substance is a solid.
So bromine is a solid at −10 °C.
Example 2: reading a heating curve
Use Figure 1. What is happening between C and D, and why does the temperature rise?
C–D is a sloping section between 0 °C and 100 °C, so the water is all liquid.
No change of state is happening, so the energy supplied makes the particles move faster.
Faster particles mean a higher temperature, so the line slopes upwards.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: why do puddles dry faster in some places?
Guided investigation · about 2 hours in total (mostly waiting) · pairs
Research question
How does the surface area of water (20, 40, 60, 80 and 100 cm²) affect the mass of water that evaporates in 2 hours?
Scientific background
Evaporation happens only at the surface, where the fastest-moving particles can escape into the air. A larger surface gives more particles the chance to escape at the same time.
Hypothesis
If the surface area increases, then more water will evaporate in 2 hours, because more particles are at the surface where they can escape.
Independent variable
Surface area of the water: 20, 40, 60, 80 and 100 cm².
Dependent variable
Mass of water lost in 2 hours, measured with a balance.
Control variables
Same starting volume, 50 cm³, in every dish.
Same place in the room, away from heaters, windows and draughts, because moving or warmer air speeds up evaporation.
Same time, 2 hours, and same water temperature at the start.
Apparatus
Five dishes of different sizes, ruler, 50 cm³ measuring cylinder, electronic balance, water, timer.
Method
Measure each dish and calculate the area of its base.
Pour 50 cm³ of water into each dish and record the mass of each dish with its water.
Leave all five dishes side by side for 2 hours.
Record each mass again and calculate the mass of water lost.
Repeat on another day if possible and calculate means.
Safety. Wipe up spills at once so nobody slips, and keep water away from the balance's electrical supply.
Results from one student are used in the Criterion C questions below. Then evaluate: one of her dishes gave a result that does not fit. What could have caused it, and how would you prevent it?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
