Changes of State

What Causes State Changes? Particles of a liquid have more thermal energy than the particles of a solid. Particles of a gas have more thermal energy than the particles of a liquid. Solid --> Liquid --> Gas -->plasma energy/particle speed -------------> When a substance’s thermal energy increases or decreases sufficiently, a substance will change state.

Solid -->Liquid: Melting Melting: change of state from a solid to a liquid. Most compounds and elements have a specific temperature at which they melt, called a melting point. Exothermic or Endothermic? As a solid gains energy and begins to melt, its particles vibrate faster and faster, raising the solid’s temperature. At its melting point, particles vibrate so fast that they are able to break free from their fixed positions, and turn into a liquid.

Liquid --> Solid: Freezing Freezing: a change of state from a liquid to a solid. The particles are moving so slowly that they begin to form a regular patterns. THE FREEZING POINT = THE MELTING POINT Exothermic or Endothermic? Example (in photo): freezing rain occurs when rain freezes on contact with a solid surface or the ground.

Liquid --> Gas: Vaporization Vaporization: a change of state from a liquid to a gas. It takes place when the particles of a liquid gain enough energy to form a gas. Exothermic or Endothermic? There are two types of vaporization: 1. Evaporation: vaporization that takes place only at the surface of a liquid. Ex: a shrinking puddle gains energy from the ground, air, and sun, which allows the water molecules to escape and change into a gas.

Liquid --> Gas: Boiling 2. Boiling: vaporization that takes place at the surface of a liquid and below. When water boils, why do the bubbles float up to the surface? water vapor (gas bubbles) are less dense than water Exothermic or Endothermic? A substance’s boiling point is dependent on air pressure. The lower the air pressure the less energy is needed for the molecules to escape into a gas, therefore the lower the boiling point. a) close to sea level, water boils at 100 degrees b) in Denver, 1600 meters above sea level, water boils at 95 degrees C because of the lower air pressure.

Conclusions from Changes of State for Water Graph Given a set volume/mass of water, it takes far more time and energy for water to boil than it does for water to melt . This is because the difference in energy and particle speed between the liquid state and the gas state is much greater than the energy/particle speed difference between the solid state and the liquid state. A = ice (solid) B = melting point C = liquid being heated D = boiling point E = gas

Condensation Condensation: A change of state from gas to liquid. It’s the opposite of vaporization. Occurs when particles in a gas lose enough thermal energy to become a liquid. Water vapor can’t be seen. When you see fog or a cloud, you are seeing liquid water droplets suspended in the air. Exothermic or Endothermic? Examples: breathing on a mirror or ”seeing your breath” on a cold day. clouds -- water vapor (gas) in the atmosphere condenses into liquid droplets. dew -- water vapor in the air condenses onto surfaces at ground level e.g. cars, grass

Sublimation Sublimation: when the surface particles of a solid gain enough energy to form a gas and skip the liquid state. Ex: dry ice (solid CO2). It cannot exist as a liquid at normal temperature and pressure so it changes directly into a gas. The fog that forms above dry ice is a result of the dry ice cooling the water vapor in the air near it. It then condenses into a liquid, forming a fog.

endothermic exothermic endothermic exothermic endothermic vibrating faster, breaking free of fixed positions endothermic moving slower, coming together in fixed pattern exothermic moving very fast, spreading to fill space endothermic moving slower, coming closer together exothermic gaining a lot of energy, skip liquid state 5. Sublimation endothermic