Melting and solidifying
This Topic is About...
- I will describe melting and solidifying.
- I will use the particle model to describe changes of state.
- I will learn that a change of state is a physical process that does not change the type of substance.
- I will make a prediction for an investigation and identify factors in a fair test.
You’re going to be a science explorer!
Key Words
- change of state
- melting
- physical process
- solidifying
Tap to Learn the Meanings!
- change of state: When a material changes from one form (solid, liquid, gas) to another.
- melting: When a solid turns into a liquid because of heat.
- physical process: A change that does not make a new substance, like melting or freezing.
- solidifying: When a liquid turns into a solid as it cools down.
Great job learning about how materials change their state!
Change of state
Most materials and substances can change from one form to another when they are heated or cooled. This is called change of state. Solids, liquids and gases are different states of matter.
Sabera’s ice-cream started as a solid. When the Sun heated the ice-cream it changed to a liquid. This change of state from solid to liquid is called melting.
We can show melting in this way:
solid heating causes melting → liquid
Solidifying is the opposite of melting. Solidifying is when a substance or material changes from a liquid to a solid. Liquid substances solidify when they lose heat.
We can show solidifying in this way:
liquid cooling causes solidifying → solid
Let’s Investigate!
Question: What happens to ice when it is heated?
Equipment: Ice cubes, a saucer
Method:
- Put an ice cube on the saucer. Is the ice liquid or solid? How do you know?
- Touch the ice. Does it feel hot or cold?
- Hold the ice cube in your hand over the saucer.
- Predict what you think will happen to the ice.
- Observe what happens to the ice and check if your prediction was correct.
The change of state that happens when you heat ice is:
solid state (ice) — heat → liquid state (water)
Follow-up Questions:
Tap to See Answers
- 1: The heat made the ice absorb energy, causing it to melt.
- 2: No, the ice did not change into a new substance. It remained water, only changing its state from solid to liquid.
- 3a: You can change the liquid water back into ice by cooling or freezing it.
- 3b: This process is called freezing or solidification.
Excellent work! You’ve discovered how heat causes a change of state.
Substances change state when they melt or solidify. Change of state is called a physical process because the substance only changes its form. The substance does not change into a different substance.
Change of state and the particle model
We know that the particles in solids and liquids are always moving. The particles in solids cannot move very much because they are tightly packed together.
The particles in a substance gain energy when we heat the substance. Some of the heat energy changes to movement energy. This makes the particles move faster. With enough energy, particles move so fast that they move away from each other. When this happens in a solid, the solid melts and becomes a liquid.
When a liquid loses heat energy and cools, its particles slow down and move closer together. If the particles lose enough energy, the liquid changes state and becomes a solid.
Let’s Investigate!
Question: Which solids melt first when heated?
Equipment: An ice cube, a square of chocolate, a cube of butter, three saucers, a watch or timer
Method:
- Place an ice cube on a saucer. Do the same with the chocolate and the butter.
- Put the saucers in a warm place, such as a sunny windowsill.
- Predict which solid will melt first.
- Observe the saucers every five minutes for 15 minutes.
Follow-up Questions:
Tap to See Answers
- 1: The warmth provides heat energy that makes the solids melt faster.
- 2: Chocolate usually melts first because it has a lower melting point. Predictions may vary.
- 3: Ice often takes the longest to melt since it needs more heat energy to reach its melting point.
- 4: When heated, the particles in each solid move faster and spread apart, changing from a solid to a liquid state.
- 5: It is a fair test if all solids are placed in the same warm place and observed for the same time.
- 6: The conclusion should describe which solid melted first and explain why, based on temperature and material properties.
- 7: If you continue heating the melted ice (water), it will eventually boil and change into water vapour (gas).
Fantastic work! You’ve explored how heat energy causes different solids to melt at different rates.
Metals like gold and silver will also melt if they are heated enough. Metals must be heated to very high temperatures to make them melt. When the melted metal cools, it solidifies and forms a solid again.
