Changes in States of Matter

Grade 4 · Science · Study Guide

Learn how matter changes between solid, liquid, and gas as temperature changes.

All matter exists in one of three common states: solid, liquid, or gas. The state of matter depends on temperature and pressure. Matter can change from one state to another when thermal energy (heat) is added or removed.

When a solid is heated, it can melt and become a liquid. The temperature at which a solid melts is called the melting point. For example, ice melts into water at 0 degrees Celsius (32 degrees Fahrenheit). The reverse process, when a liquid cools and becomes a solid, is called freezing.

When a liquid is heated enough, it can turn into a gas through a process called evaporation. Evaporation can happen slowly at the surface of a liquid even below boiling point. When a liquid is heated to its boiling point, it turns into gas quickly throughout. For water, the boiling point is 100 degrees Celsius (212 degrees Fahrenheit). When a gas cools and turns back into a liquid, it is called condensation.

In each of these changes, the type of substance stays the same. When water freezes, it is still water, just in solid form. The molecules themselves have not changed, only how fast they move and how close together they are. Faster-moving molecules spread apart (gas), while slower molecules clump together (solid).

Key terms

Solid: A state of matter with a fixed shape and volume; particles are packed tightly and vibrate in place
Liquid: A state of matter with a fixed volume but no fixed shape; particles slide past each other
Gas: A state of matter with no fixed shape or volume; particles move freely and spread apart
Melting: The change from solid to liquid when heat is added
Evaporation: The change from liquid to gas when heat energy is added
Condensation: The change from gas to liquid when thermal energy is removed

Flashcards

Solid: State of matter with fixed shape and volume; particles packed tightly
Liquid: State of matter with fixed volume but not fixed shape; particles slide past each other
Gas: State of matter with no fixed shape or volume; particles move freely
Melting: Solid to liquid; occurs when heat is added (e.g., ice melting)
Freezing: Liquid to solid; occurs when heat is removed (e.g., water freezing to ice)
Evaporation: Liquid to gas; occurs when heat is added (e.g., puddles drying up)
Condensation: Gas to liquid; occurs when heat is removed (e.g., water droplets on a cold glass)
Boiling point: The temperature at which a liquid rapidly turns to gas; water boils at 100 degrees Celsius

Questions

What happens to the particles of a substance when it melts from a solid to a liquid?
1. They move faster and spread farther apart
2. They stop moving completely
3. They change into different particles
4. They become heavier
Show answer
Answer: They move faster and spread farther apart
When a solid melts, heat energy makes the particles vibrate faster and move farther apart, allowing them to slide past each other as a liquid.
When water freezes into ice, it becomes a different substance.
Show answer
Answer: False
Freezing is a physical change, not a chemical change. Water molecules remain the same H2O whether in solid (ice), liquid (water), or gas (steam) form.
The change from a liquid to a gas is called ______.
Show answer
Answer: evaporation
Evaporation is the process in which a liquid gains enough energy to escape as a gas. It can happen slowly at any temperature or quickly at the boiling point.
When you see water droplets form on the outside of a cold glass on a warm day, which process is occurring?
1. Melting
2. Evaporation
3. Condensation
4. Freezing
Show answer
Answer: Condensation
Condensation occurs when water vapor in warm air contacts the cold surface of the glass, loses energy, and turns back into liquid water droplets.
Adding heat to a substance can cause it to change from a solid to a liquid.
Show answer
Answer: True
Adding heat increases the energy and speed of particles, causing them to move farther apart until the substance changes from solid to liquid at its melting point.

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