1. Matter and Its States

Matter is anything that has mass and takes up space (volume). Everything you can touch — air, water, your phone — is matter. Matter comes in three everyday states, and the difference is all about how its tiny particles are arranged and how fast they move.

• Solid — particles are packed tightly in fixed positions, so a solid keeps its shape and volume (an ice cube).
• Liquid — particles are close but can slide past each other, so a liquid keeps its volume but takes the shape of its container (liquid water).
• Gas — particles are far apart and move freely, so a gas spreads to fill any container (water vapor / steam).

Adding or removing heat drives the changes of state: melting (solid→liquid), freezing (liquid→solid), boiling/evaporating (liquid→gas), and condensing (gas→liquid). These are physical changes — the substance is still water, just in a different form.

A physical change (melting, cutting, dissolving) changes form but not the substance. A chemical change (burning, rusting) makes a new substance.

⚠️ Common misconception: melting ice is NOT a chemical change. It is still water — only the state changed.

💡 Tip: heat usually moves matter 'up' the ladder solid → liquid → gas; removing heat moves it back down.