Changes of Phase

Phase “Phase” refers to the state of matter a material is in: solid, liquid or gas. – (we will ignore other states like plasma for now since they make life a bit complicated) A change in phase means that the material is breaking or forming bonds (so there is a change in energy. It also means that the physical and sometimes chemical properties have changed.

A Touch of Background Matter is held together by bonds These bonds occur due to electrons from one atom or molecule being attracted to the nuclei of another atom or atoms There is a competing force which tends to break things up, that is the motion of molecules from their temperature. The higher the temperature, the more things break up.

Three States of Matter Solids – All molecules are bonded – Shape is rigid, with a defined surface – Volume is fixed – Each molecule stays in one place Gas – All bonds between molecules are broken – Shape is defined by container, if any – Volume is defined by container, if any – Molecular movement is rapid and random Liquid – SOME bonds between molecules are broken – Shape is defined by container, if any, and with a surface – Volume is fixed – Molecular movement is fast, but short distance

Change of State Solid  Liquid  Gas It takes energy (heat, warmth) to break the bonds The temperature ( = kinetic energy) that the bond breaking occurs reflects the energy of the bond The melting temperature = freezing temperature 

Evaporation and Boiling Point Evaporation of a liquid occurs at any temperature How quickly a liquid evaporates depends on the temperature It occurs ONLY at the surface of the liquid Boiling is different: It occurs at one temperature It occurs throughout the liquid It occurs when the pressure of the evaporating liquid equals or is greater than the air pressure pushing down on the surface of the liquid (Vapor Pressure > Atmospheric Pressure)

Vapor Pressure Start with a liquid in a container Some of the liquid will evaporate In an open container, they can escape In a closed container, liquids will continue to evaporate But they will not escape So the number of gas molecules will increase, and so will the pressure

Vapor Pressure, continued As more and more gas molecules evaporate, the pressure increases. But as more and more molecules are in the gas phase, some will begin to condense. Eventually, the rate of molecules evaporating and condensing will be equal to each other The pressure inside the container will get to a new constant pressure This is the vapor pressure of this liquid. It will be constant for this temperature time pressure

Vapor Pressure and Temperature At any one temperature, a liquid will have a constant vapor pressure Molecules will constantly be moving in and out of the gas phase But as temperature increases, more molecules will start evaporating, and the vapor pressure will increase Temperature pressure HEAT

Vapor Pressure and Temperature, continued Vapor Pressure depends on the temperature AND the liquid being heated. Some liquids, like water, evaporate fairly easily and have a moderate vapor pressure Others, like acetone, evaporate very easily, and have a very high vapor pressure Others, like olive oil, have a low vapor pressure, because it is quite difficult for the molecules to evaporate Compare the vapor pressure at any one temperature (say room temperature) Or compare the temperatures needed to reach atmospheric pressure Pressure Temperature Room Temp. acetone water oil 1.0 atm