1. Evaporation
Why liquid evaporate?
Liquid to gas below boiling point – called evaporation
Evaporation occurs when some particles have enough energy to escape as a gas from
surface of liquid.
Liquids that evaporate quickly at r.t.p. called volatile liquids.
 They usually have boiling points just above room temp.
 Eg. Petrol and perfumes
Why is evaporation different from boiling?
 both involve a liquid changing to gas
 difference :
Boiling Evaporation
Occur only at boiling point Occur at temperatures below boiling
point
Occur throughout liquid Occur only at the surface of the liquid
Occur rapidly Occurs slowly
- Condensation
When gas cooled efficiently, changes into liquid, process called condensation.
When water vapour touches cold surface , condensation occurs and liquid water is
obtained
What happen to particles of gas when it condenses?
 Reverse of boiling.
 Heat energy given out.
 As temp. Drops, gas particle lose energy and move more slowly until the movement
is slow enough for gas to change to liquid.
- Sublimation
When dry ice (dry carbon dioxide) exposed to temp. Higher than -78 degrees, it directly
turns to carbon dioxide gas without melting.
Like dry ice, some solids change directly into a gas without going thru the liquid state.
Occurs because particles at surface of solid have enough energy to break away from
solid and escape as gas
 Eg. Iodine and ammonium chloride – solids that sublime
Substances that sublime can also from a gas into a solid without liquid state (Process
also called condensation.)
Usefulness of sublimation
 Dry ice
 industrial refrigeration (refrigerator)
 transporting frozen goods
 especially for refrigerating food like ice cream & meat as it keeps them very cold and
changes directly into a gas without leaving any liquid behind

2. Diffusion
- Smell spread when millions of tiny gas particles escape from liquid surface and move around
freely until they reach your nose and is detected as aromas. This observation of spreading of
aroma proves that particles move freely and fill up any available space.
- Diffusion is process by which particles move freely and fill up any available space.
- Example : pg. 16
- Particles of gas when in confined area, is evenly spread out.
 different particles diffuse into spaces between each other
 homogenous mixture – looks same throughout as different type of particles spread
out evenly
- Different mass / size of particles diffuse at different rates.
 different gas diffuse at different rate
 Eg. Hydrogen particle lighter and smaller than air particle.
 Balloon with hydrogen gas shrink faster than balloon filled with air.
Rubber used to make balloon have millions of tiny holes and smaller
particles diffuse more quickly through holes than large air particles (consist
of oxygen, nitrogen, carbon dioxide)
Effect of Molecular Mass on Rate of Diffusion
- Mass of gas particle is molecular mass
- The smaller the molecular mass of the gas, the lighter it is
 Hydrogen is lightest of all gases , 20 times lighter than air.
- Example on pg. 17
 Particles with lower molecular mass diffuse faster than particles with higher
molecular mass
Diffusion in liquids / gases
- Example pg. 18
 Small crystal of potassium manganite (VII) introduced to beaker of distilled water,
dissolve to form deep purple solution and settle at bottom of beaker. Diffusion takes
place till solution becomes uniformly purple.
- Diffusion is the movement of particles from a region of higher concentration to a region of
lower concentration. (apply to gas also)
Effects of temperature on rate of diffusion
- If earlier example uses boiling water, solution will become uniformly purple within much
shorter time.
- Rate of diffusion increases as temperature of solution increases
- Higher the temperature, faster the rate of diffusion
 Example : inflated balloon shrink faster on hot day than on cold day, a lump of sugar
dissolve faster in hot water than in cold water
- Particles gain more heat as temperature increases and move faster , thus increasing the rate
of diffusion.