Phase Changes

Understanding states of matter and transitions between them

Topics

How does condensation work?

Condensation occurs when water vapor in the air loses energy and transitions from gas to liquid. This happens when air cools below its dew point or when vapor comes into contact with a cold surface, causing water molecules to slow down and form liquid droplets.

How does water boil?

Water boils when its temperature reaches 100°C (212°F) at standard pressure and heat energy is added. Water molecules gain enough energy to overcome liquid bonds and escape as vapor, forming bubbles throughout the liquid.

How does water freeze?

Water freezes when its temperature drops to 0°C (32°F) and heat energy is removed. Water molecules slow down and form a crystalline structure held together by hydrogen bonds, transitioning from liquid to solid ice.

What is evaporation?

Evaporation is the process where liquid molecules at the surface gain enough energy to escape into the gas phase. It occurs at any temperature below the boiling point and is how liquids like water turn into vapor.

What is sublimation?

Sublimation is the phase change where a substance transitions directly from solid to gas without passing through the liquid phase. This occurs when the substance's vapor pressure exceeds atmospheric pressure at temperatures below its melting point, allowing solid particles to escape directly into the gas phase.

What is the triple point?

The triple point is the unique temperature and pressure condition where a substance can exist simultaneously in solid, liquid, and gas phases in equilibrium. At the triple point, all three phases coexist, and changing conditions slightly causes one phase to disappear.

Why does dry ice sublimate?

Dry ice sublimes because carbon dioxide's triple point is above atmospheric pressure. At normal atmospheric pressure, solid CO₂ cannot melt to liquid—it can only transition directly from solid to gas (sublimation) or from gas to solid (deposition), skipping the liquid phase entirely.

Why does ice float?

Ice floats because it is less dense than liquid water. When water freezes, its molecules form a crystalline structure that takes up more space than liquid water, making ice less dense and causing it to float.

Why does water boil at different temperatures?

Water boils at different temperatures because boiling point depends on atmospheric pressure. Higher pressure requires higher temperature to reach the vapor pressure needed for boiling, so water boils at higher temperatures under high pressure and lower temperatures under low pressure (like at high altitude).

Why does water expand when it freezes?

Water expands when it freezes because water molecules form a hexagonal crystalline structure with more space between molecules than in liquid water. This open structure makes ice less dense, causing expansion.