The Dance of Particles

Why it matters

Ice, water and steam look like three different things. They are the same molecules — dancing three different dances. Take control of the temperature and watch the dance change.

Predict first

When ice is heated until it becomes steam, what happens to the water molecules?

the molecules never change — H₂O in ice is the same H₂O in steam. Only the spacing and speed change. That's why it's a physical change.

• They break apart into hydrogen and oxygen
• They stay the same molecules but move faster and spread out — correct
• They get bigger as they absorb heat
• New steam molecules are created and the water ones are destroyed

What you do

Drag the temperature slider through all three states. Watch the particle box and the state symbols light up together.

Check yourself

In which state do particles only vibrate about fixed positions?

In a solid, strong interparticle forces lock particles into a lattice — they vibrate but cannot swap places. That's why solids keep their shape.

Melting of ice is a physical change because…

Physical vs chemical is decided by ONE question: is a new substance formed? Ice → water is still H₂O, so it's physical.

Why does a gas fill its whole container?

The particles don't grow — there is just enormous empty space between them and almost no force holding them together.

• Solid — correct
• Liquid
• Gas
• All three
• heat is absorbed
• no new substance is formed — the molecules are unchanged — correct
• water molecules split into atoms
• it can happen at room temperature
• Gas particles expand to fill the space
• Gas particles repel each other strongly
• Particles move fast with almost no attraction, so they spread everywhere — correct
• Air pushes the gas outward