Matter can change from one physical state to another when conditions change.
Ice can melt into liquid water, liquid water can evaporate into water vapour and water vapour can condense back into liquid droplets.
In this lesson you will investigate several of these changes using safe everyday observations.
Goal: observe changes of state, identify the starting and ending states and connect what you see with a simple particle explanation.
Before you start
Review Matter and Its Common States.
You should already know that solids normally have a definite shape and volume, liquids have a definite volume but change shape with their container and gases spread through the available space.
Materials
You will need:
- two or three ice cubes
- two plastic cups
- cold water
- room temperature water
- a shallow plate or tray
- a teaspoon
- paper towels
- a timer or clock
- a notebook or worksheet
An adult or teacher should help prepare the cold water if necessary.
Safety first
This investigation does not require boiling water, flames or dangerous chemicals.
Keep water away from electrical equipment.
Wipe up spills quickly so nobody slips.
Do not taste investigation materials.
Use plastic containers if there is a risk that glass could be dropped.
What is a change of state?
A change of state happens when matter changes from one physical state to another.
Common examples include:
- solid to liquid
- liquid to solid
- liquid to gas
- gas to liquid
These changes have special names.
Starting state Ending state Change Solid Liquid Melting Liquid Solid Freezing Liquid Gas Vaporization Gas Liquid CondensationEvaporation and boiling are both forms of vaporization
Vaporization means a liquid changes into a gas.
Evaporation happens at the surface of a liquid and can occur without the liquid boiling.
This is why a wet surface can gradually dry at ordinary temperatures.
Boiling also changes liquid into gas, but boiling happens throughout the liquid under suitable temperature and pressure conditions.
You do not need to boil anything for this lesson.
Investigation 1: Watch ice melt
Place an ice cube on a shallow plate.
Observe it immediately.
Record:
- its state
- its shape
- whether liquid water is already visible
Start the timer.
Observe the ice again after several minutes.
What should you notice?
The solid ice gradually becomes liquid water.
This change is called melting.
The starting state is solid.
The ending state is liquid.
Record your melting observation
Observation time Solid ice present? Liquid water present? What changed? Beginning After a few minutes LaterWhere does the energy for melting come from?
If the surroundings are warmer than the ice, energy can transfer from the surroundings to the ice.
This energy helps the solid change into liquid water.
You may notice that the plate and nearby air feel much warmer than the ice.
Particle explanation for melting
In solid ice, water particles are held in an organized structure.
As energy is transferred to the ice, particle motion increases and the solid structure can break down.
The particles can then move around one another more freely as liquid water.
Does melting create a new substance?
No.
Ice and liquid water are both forms of the same substance, water.
The physical state changed, but the substance remained water.
Investigation 2: Observe condensation
Fill one plastic cup with cold water and add ice.
Dry the outside of the cup carefully with a paper towel.
Place the cup on the table.
Wait several minutes and watch the outside surface.
What appears on the outside?
You may see tiny droplets of liquid water forming on the outside of the cold cup.
Ask yourself:
Did this water pass through the wall of the cup?
In an undamaged waterproof cup, the answer is no.
The droplets come mainly from water vapour already present in the surrounding air.
What is condensation?
Air contains water vapour.
Water vapour is water in the gas state and is normally invisible.
Air close to the cold cup cools.
Under suitable conditions, some water vapour changes into liquid water.
This gas to liquid change is called condensation.
Record your condensation observation
Time Outside of cup What do you observe? Beginning Dry After a few minutes LaterCompare with a room temperature cup
For a useful comparison, put room temperature water in the second cup.
Dry its outside.
Place it beside the cold cup.
Observe both cups at the same times.
Ask:
Which cup develops more visible droplets on the outside?
Why make this comparison?
The two cups help you examine whether the cold surface is important to the condensation you observe.
Try to keep other conditions similar.
For example, use the same type of cup and place both cups in the same location.
Particle explanation for condensation
Gas particles move freely through the available space.
When water vapour cools enough under suitable conditions, water particles can come together to form liquid water.
The particles have not disappeared.
The water has changed from the gas state to the liquid state.
Investigation 3: Observe evaporation
Place one teaspoon of room temperature water on a shallow plate or tray.
Mark or remember the area covered by the water.
Observe it during the lesson.
If some water remains at the end, leave the plate in a safe place and observe it again later.
What happens during evaporation?
Some particles at the surface of liquid water have enough energy to escape into the surrounding air as water vapour.
This process is called evaporation.
The starting state is liquid.
The ending state is gas.
Evaporation does not require boiling
This is an important idea.
A puddle can disappear on an ordinary day.
Wet clothes can dry.
Water left in an open container can slowly decrease.
These are familiar examples of evaporation.
Where does evaporated water go?
The water does not become nothing.
Water particles move into the air as invisible water vapour.
They may later take part in another change of state.
Can water vapour become liquid again?
Yes.
The condensation investigation shows the reverse direction.
Liquid water can evaporate into water vapour.
Water vapour can later condense into liquid water.
Investigation 4: Think about freezing
You began the lesson with ice that was already frozen.
Before it became ice, that water was liquid.
Freezing is the change from liquid to solid.
It is the reverse of melting.
Why are we not freezing water during the lesson?
A classroom lesson does not need to demonstrate every change in real time.
Freezing ordinary water usually requires a freezer and more time than this lesson provides.
Instead, you can use the ice cube as evidence that liquid water can become solid when suitable cooling occurs.
Particle explanation for freezing
As liquid water loses enough energy under suitable conditions, particle motion decreases and the particles become organized into the solid structure of ice.
The substance is still water.
Its physical state has changed.
Changes can happen in opposite directions
Change Direction Reverse change Melting Solid to liquid Freezing Freezing Liquid to solid Melting Evaporation Liquid to gas Condensation Condensation Gas to liquid EvaporationEnergy and changes of state
Changes of state involve energy transfer.
Melting and vaporization require energy to be transferred into the substance.
Freezing and condensation involve energy being transferred away from the substance to its surroundings.
The exact energy behaviour can be studied in more detail in later science courses.
Temperature and state are connected
Temperature affects how particles move and can influence which state is stable.
However, temperature is not the only factor that can matter.
Pressure can also affect state changes.
For this beginner investigation, we focus mainly on familiar changes caused by heating or cooling under ordinary conditions.
Do all substances melt and boil at the same temperatures?
No.
Different substances have different physical properties.
A temperature that melts one substance may leave another substance solid.
This is one reason scientists identify substances partly by measuring properties such as melting and boiling temperatures.
State change or chemical change?
A change of state is a physical change.
The chemical identity of the substance does not have to change.
For example:
ice to liquid water
is a physical change.
Both are water.
A different kind of change
Some changes produce new substances.
Those are chemical changes.
For example, burning a material is not simply a change from one state to another.
This lesson focuses only on physical changes of state.
Common mistake 1: Thinking melting means disappearing
When ice melts, the matter does not disappear.
The solid becomes liquid water.
Common mistake 2: Thinking evaporation means the water is destroyed
Evaporated water enters the surrounding air as water vapour.
The water particles still exist.
Common mistake 3: Thinking visible mist is water vapour
Water vapour itself is normally invisible.
Visible mist contains tiny liquid water droplets.
Those droplets can form when water vapour condenses.
Common mistake 4: Thinking condensation water comes from inside the cold cup
When droplets appear on the outside of a waterproof cold cup, they generally formed from water vapour in the surrounding air.
The water did not need to pass through the cup wall.
Common mistake 5: Thinking evaporation only happens on hot days
Evaporation can occur at many temperatures.
Its rate can change with conditions such as temperature, air movement, humidity and exposed surface area.
Practice 1
An ice cube becomes a puddle.
What is the change?
Melting.
Starting state: solid.
Ending state: liquid.
Practice 2
Liquid water becomes ice in a freezer.
What is the change?
Freezing.
Starting state: liquid.
Ending state: solid.
Practice 3
A small puddle slowly disappears from a plate.
What change is likely responsible?
Evaporation.
Liquid water enters the gas state as water vapour.
Practice 4
Droplets form on the outside of a cold cup.
Which change is involved?
Condensation.
Water changes from gas in the surrounding air to liquid droplets.
Practice 5
Which process is the reverse of melting?
Freezing.
Practice 6
Which process is the reverse of condensation?
Vaporization.
Evaporation is one form of vaporization.
Practice 7
Is melting a chemical change?
No.
Melting is a physical change of state.
Practice 8
A student says that a wet floor can only dry if the water reaches its boiling temperature.
Is that correct?
No.
Water can evaporate from its surface without boiling.
Use evidence from your observations
Complete a table like this.
Observation Starting state Ending state Name of change Ice becomes water Solid Liquid Melting Droplets form from water vapour Gas Liquid Condensation Water gradually leaves an open surface Liquid Gas Evaporation Liquid water becomes ice Liquid Solid FreezingThink like a scientist
For every observation, separate what you actually saw from your explanation.
For example:
Observation:
Tiny water droplets appeared on the outside of the cold cup.
Explanation:
Water vapour in the surrounding air cooled and condensed into liquid water on the cold surface.
This distinction helps you reason carefully from evidence.
Could you turn one observation into a fair test?
Yes.
For example, you could ask:
How does exposed surface area affect the rate at which water evaporates?
You would then need to identify:
- the independent variable
- the dependent variable
- controlled variables
- the measurement method
This connects changes of state with the scientific investigation skills you have already learned.
Final challenge
Choose one of these changes:
- melting
- freezing
- evaporation
- condensation
Explain it using four sentences.
Your explanation should identify:
- the starting state
- the ending state
- the name of the change
- what happens to particle arrangement or movement
Final review checklist
Can you:
- identify melting as solid to liquid?
- identify freezing as liquid to solid?
- identify vaporization as liquid to gas?
- identify condensation as gas to liquid?
- explain that evaporation can happen without boiling?
- explain where droplets on a cold cup come from?
- distinguish a physical state change from a chemical change?
- use a simple particle model to explain each state change?
The main idea
Matter can move between physical states when conditions change.
Melting changes a solid into a liquid.
Freezing changes a liquid into a solid.
Vaporization changes a liquid into a gas.
Condensation changes a gas into a liquid.
These are physical changes, so a substance can remain chemically the same even though its state changes.
By observing ice, evaporation and condensation, you can see evidence of processes that happen constantly in everyday life and throughout nature.
Continue learning
Review Matter and Its Common States whenever you need to compare the properties of solids, liquids and gases.