What is a State of Matter?

Classify ice, liquid water, and steam by state and explain why each has its shape and volume behavior.

Ice: solid — particles locked in a rigid lattice, fixed shape and fixed volume.
Liquid water: liquid — particles close together but free to flow, fixed volume, takes container's shape.
Steam: gas — particles far apart and moving freely, no fixed shape or volume, expands to fill the container.

A block of solid CO₂ (dry ice) disappears at room temperature without becoming a puddle. What process is this and why?

Dry ice goes directly from solid to gas without a liquid stage — this process is called sublimation.
At normal atmospheric pressure, CO₂'s solid-liquid-gas triple point lies above 1 atm, so liquid CO₂ can't exist at everyday pressure.
Heat energy from the room is enough to break the solid lattice directly into gas particles.

Explain, at the particle level, why a gas can be compressed easily but a liquid almost cannot.

In a gas, particles are far apart with mostly empty space between them, so pushing them closer together is easy.
In a liquid, particles are already almost touching, so there is very little empty space left to remove.
This is why gases are compressible and liquids (like in hydraulic systems) are treated as nearly incompressible.