Matter and its changes (grade 6) · lesson

Atoms and molecules

💡 Atoms join to make molecules, and a formula tells you which atoms and how many.

An atom is the smallest piece of an element. Gold, oxygen, and carbon are elements. Pure gold is made of only gold atoms.

Atoms can join to make a molecule. A water molecule is two hydrogen atoms joined to one oxygen atom. Its formula is H₂O.

In a formula, each letter is a kind of atom. The small number after a letter tells how many of that atom. No number means one.

🧠 Let's work it out

Read CO₂.
C has no number, so there is one carbon atom.
O has a 2, so there are two oxygen atoms.
That is 3 atoms in all.
If a formula has only one kind of letter, like O₂, it has only one kind of atom.

What do you think comes next?

atom: the smallest piece of an element
element: a substance made of only one kind of atom
molecule: two or more atoms joined together

🔗 PhET simulation: Build a Molecule · PhET Interactive Simulations ↗

Solids, liquids, and gases

💡 Solids, liquids, and gases are made of the same kinds of particles, just packed and moving in different ways.

In a solid, particles are packed tight. They shake in place but do not move around. That is why a solid keeps its shape.

In a liquid, particles are still close, but they can slide past each other. That is why a liquid flows.

In a gas, particles are far apart and zoom around. There is lots of empty space, so a gas can be squeezed into less room.

🧠 Let's work it out

Ask two things: how close are the particles, and can they move around?
If ice melts, the particles stay close but break free from their spots. So the water can flow.
The particles do not grow, shrink, or break apart.

What do you think comes next?

particle: a tiny bit of matter, like an atom or molecule
state of matter: solid, liquid, or gas

🔗 PhET simulation: States of Matter: Basics · PhET Interactive Simulations ↗

Temperature and thermal energy

💡 Temperature measures how fast particles move on average, while thermal energy is the total for all the particles.

Particles never stop moving. When something warms up, its particles move faster. When it cools, they slow down.

Temperature tells you the average speed of the particles. Thermal energy is the energy of all the particles added up.

So a big pot and a small cup can be the same temperature. The big pot has more particles, so it has more thermal energy.

🧠 Let's work it out

A sealed balloon goes into a freezer and shrinks.

Claim: the air cooled.
Evidence: it got smaller.
Reasoning: cold particles move slower, so they hit the balloon wall less hard.
The wall is pushed out less, so the balloon shrinks. No air left the balloon.

What do you think comes next?

temperature: a measure of how fast particles move on average
thermal energy: the total energy of all the moving particles in something

🔗 Khan Academy: Physical properties of matter · Khan Academy ↗

Changing state

💡 Adding energy melts and boils matter, and taking energy away condenses and freezes it.

Melting and boiling need energy. The energy frees the particles from each other. Freezing and condensing happen when matter loses energy.

Water melts and freezes at 0 °C. It boils and condenses at 100 °C.

While ice melts, its temperature stays at 0 °C. The heat goes into freeing the particles, not into speeding them up. Once all the ice melts, the temperature rises again.

🧠 Let's work it out

Read a heating table: −10, −5, 0, 0, 0, 5 °C.
The flat part at 0 °C is where the ice is melting.
Energy is still going in. It is just being used to change the state.

What do you think comes next?

melt: change from solid to liquid
evaporate: change from liquid to gas
condense: change from gas to liquid
freeze: change from liquid to solid