November 2 What is the difference between evaporation and boiling? Objectives :* Liquids * Vapor pressure * Identify the factors that affect vapor presure * Learn to use table H * Intermolecular attractions * Evaporation * Boiling

LIQUIDS

LIQUID A form of matter that has definite volume but no definite shape. A liquid takes the shape of the container is in. Particles are hold together by forces of attractions that are called intermolecular forces.

VAPOR The gas phase of a substance that is ordinarily a solid of liquid at that temperature. The vapor above the surface of a liquid exerts a characteristic pressure called vapor pressure.

EVAPORATION It happens at ALL temperatures and only at the surface of the liquid. The molecules at the surface that can escape the liquid inside a closed container produce the vapor pressure.

VAPOR PRESSURE OF A LIQUID Pressure exerted by the gas above the liquid in a closed system. Measured with a manometer. It increases with temperature.

TABLE H – vapor pressure of four liquids

Evaporation is an endothermic process *Evaporation is an endothermic process. When a liquid evaporates absorbs heat from the surroundings. If the liquid is evaporating over our skin we feel cold. Transpiration is a cooling process. Explain why.

STRONG INTERMOLECULAR ATTRACTIONS If a liquid has STRONG intermolecular attractions the molecules tend to stay together as liquid. A lot of energy is needed to separate the molecules. The liquid will have * LOW VAPOR PRESSURE and * HIGH BOILING POINT

Liquids with weak forces of attraction will have *high vapor pressure and *low boiling points.

Boiling A liquid boils when the vapor pressure of the liquid equals the external pressure. The temperature at which the liquid boils is the boiling point.

NORMAL BOILING POINT The temperature at which the vapor pressure of the liquid is equal to 1atm or 101.3 kPa. For water is 1000C. Its vapor pressure at that temperature is 101.3 kPa. (Table H)

CONDENSATION the phase change from liquid to gas. It is an exothermic process.

DISTILLATION A method of separating homogeneous mixtures of solids in liquids or different liquids. By boiling and condensing the vapors, the mixtures can be separated. When a mixture of different liquids is heated up the liquid with weaker intermolecular attractions will boil first. The vapors can be condensed to separate the liquids.

Pressure Cooker it works by increasing the pressure on top of the water. The BP at higher T will be higher and the food cooks faster.

HEAT OF VAPORIZATION The heat needed to completely vaporize 1 gram of liquid at is BP.

SOLIDS * Have definite shape and volume. * Particles are close together (packed). * Particles have 2 types of movement rotation and vibration in their places * Particles are arranged in a crystalline structure, that is a geometrical pattern that repeats itself.  

AMORPHOUS SOLIDS Lack an ordered internal structure, don’t have crystalline structure. Do not have a definite melting point. Example glass, asphalt, rubber

SUBLIMATION Change of state from solid to gas without going through the liquid state. Endothermic Examples: Dry Ice Iodine Moth balls

DEPOSITION Phase change from gas directly to solid.