Lesson 2.2 Changes of State of Matter Vocabulary: Melting – Melting Point – Freezing – Vaporization – Evaporation

Lesson 2.2 Changes of State of Matter

Vocabulary:

Melting –

Melting Point –

Freezing –

Vaporization –

Evaporation –

Boiling –

Boiling point –

Condensation –

Sublimination –

What happens to the Particles of a Solid as It Melts?

Particles of a liquid have more thermal energy than the same substance in solid form. As a gas, the particles have even more thermal energy.

A change from a solid to a liquid involves an INCREASE in thermal energy. A change from a liquid to a solid involves a DECREASE in thermal energy.

The change in state from a solid to a liquid is called melting. In pure, crystalline solids, melting occurs at a specific temperature called the melting point. Chemists compare melting points when trying to identify an unknown material. The melting point of pure water is 0°C.

The energy needed to melt an ice cube comes mostly from the air in the room. The added thermal energy makes the water molecules vibrate faster, raising their temperature.

At a solid’s melting point, its particles vibrate so fast that they break free from their fixed positions. At 0°C, the temperature of ice stops increasing. Any added energy continues to change the arrangement of the water molecules from ice crystals into liquid water, and the ice melts.

Freezing: The change of state from a liquid to a solid is called freezing. It is the reverse of melting. At a liquid’s freezing point, its particles are moving so slowly that they begin to take on a fixed position. When water is put in a freezer, the water loses energy to the cold air in the freezer. The water molecules move more and more slowly as they lose energy, then become solid ice. The freezing point of water 0°C is the same as its melting point.

What happens to the Particles of a Liquid as it Vaporizes?

The change in state from a liquid to a gas is called vaporization. Vaporization occurs when the particles in a liquid gain enough energy to move independently. This is how clouds form and why a puddle dries up.

Two types of vaporization:

Evaporation – vaporization that takes place only on the surface of a liquid (like a shrinking puddle). Water in the puddle gains energy from the ground, the air, or the sun. The added energy helps some of the water molecules escape into the air.

Boiling: vaporization that takes place both below and at the surface of a liquid. When water boils, vaporized water molecules form bubbles below the surface. The bubbles rise and break the surface of the liquid. The temperature at which a liquid boils is called its boiling point.

Condensation: Change in state from a gas to a liquid (reverse of vaporization) Example: when breathing on a mirror, the warm water vapor in your breath reaches the cooler surface of the mirror, and the water vapor condenses into liquid droplets. Condensation occurs when particles in a gas lose enough thermal energy to form a liquid.

Clouds typically form when water vapor in the atmosphere condenses into tiny liquid droplets. When the water droplets get heavy enough, they fall as rain. Water vapor is a colorless gas – steam, clouds, and fog are not – they are tiny water droplets suspended in air. If you can see it, it is condensation, not water vapor.

What Happens to the Particles of a Solid as it Sublimes?

In places where winters are cold, the snow may disappear even when the temperatures are below freezing.

Sublimation occurs when the surface particles of a solid gain enough energy that they form a gas.

During sublimation, particles of a solid do not pass through the liquid state as they form a gas.

Dry Ice

Dry ice is solid carbon dioxide. At ordinary atmospheric pressure, carbon dioxide cannot exist as a liquid. So instead of melting, it changes directly into a gas. When dry ice becomes a gas, it cools water vapor in the nearby air. The water vapor then condenses into a liquid, forming fog near the dry ice.