CHANGES OF STATE

Evaporation: A change of phase from liquid to gas that takes place at the surface of a liquid. Boiling: a change of phase from liquid to gas that takes place beneath the surface of a liquid. Condensation: The change of phase from gas to liquid. Freezing: the change of phase from liquid to solid. Sublimation: the process in which a substance goes from solid to gas, bypassing the liquid state. Precipitation: rain, sleet, snow, hail that falls to Earth when cloud particles become too heavy

Water Cycle

Changes of State –solid to liquid OR liquid to solid melting point: the temperature at which a substance changes from solid to liquid OR liquid to solid –Liquid to vapor OR vapor to liquid Boiling point: the temperature at which a substance changes from liquid to vapor OR vapor to liquid.

There is NO change in temperature during a change of state. The heat required when the temperature changes is given by Q = mc  T The heat required during a change of state is used to break apart those vibrating bonds, which does not affect the temperature!

There is NO change in temperature during a change of state. The heat required during a change of state is given by Q = mL where L is the latent heat of either vaporization or fusion

Melting Ice If a cup of ice is left out in a room, it will eventually begin to melt and the water will eventually come to thermal equilibrium with the room temperature. While the ice is melting, the temperature remains at 0 degrees Celsius until no water remains. Once the complete change of state has taken place, the temperature of the water will start increasing.

Phase Change Graph TT TT TT Ice/water Q = mL f water/steam Q = mL v

Examples How much heat must be added to 3 kg ice at -7º C till it just begins to melt? C ice = 2100 J/kgK Q = mc  T Q = 3 (2100) (7) Q = J

How much heat must be added to 3 kg of ice at 0º C to melt all of it? L f ice = 3.33 x 10 5 J/kg Q = mL f Q = 3 (3.33 x 10 5 ) Q = J

How much heat must be added to 3 kg of water at 30º C so that all of it becomes steam at 100 º C ? L v water = 22.6 x 10 5 J/kg First, the water must reach its boiling temperature, then it must all be boiled. Q = mc  T + mL v Q = 3 (4186)(70) + 3(22.6 x 10 5 ) Q = J

How much heat must be added to 4 kg of water at 100º C so that all of it becomes steam at 120 º C ? L v water = 22.6 x 10 5 J/kg c steam = 2010 J/kgK First, the water must all be boiled, then the steam must heat up. Q = mL v + mc  T Q = 4(22.6 x 10 5 ) + 4(2010)(20) Q = J

Water Plasma???? For water vapor to change into “water plasma” would require a temperature of approximately 12,000 degrees Celsius!

The boiling point of water Is the boiling point of water ALWAYS 100 degrees Celsius? Where is there less atmospheric pressure?

The boiling point of water In the mountains, there is less atmospheric pressure pushing down on the surface of water. Therefore, when heat is added, it is easier for liquid water molecules to “escape”- in other words, to “boil”. Boiling water at high altitudes is NOT 100˚C! On top of Pike’s Peak, in Colorado, water will boil at only 86˚C. You have to boil your potatoes a lot longer if you’re in the mountains! Boiling water there is just not as hot as boiling water closer to sea level.

The effect of increased pressure on boiling temperature of water If pressure above the surface of water increases, it is more difficult for liquid to change into gas- so the temperature of liquid water will climb higher than 100 degrees. Pressure cookers cook food more quickly because the temperature of the water inside is HOTTER than 100˚C!

–When two substances at different temperatures are in contact, one will gain the heat, one will lose the heat and the final temperature of both will be the same. heat gained = heat lost m 1 c  T = - m 2 c  T m 1 c(T f – T o ) = - m 2 c(T f – T o ) and T f = T f

calorimeter: an insulated container used in experiments to study heat transfer. When placing a hot object inside a calorimeter filled with water, the heat lost by the object is gained by both the water and the calorimeter. - heat lost = + heat gained - Q object = Q water + Q calorimeter

By FAR, the most common form of matter in the universe both by mass and volume is PLASMA!