Chemistry I Honors

Solutions Lesson #4Colligative Properties

What are Colligative Properties?

• Colligative (kohl-lig’-a-tiv) properties are observed behaviors of solutions that are deviations from the expected behaviors of pure solvent.

• There are four such behaviors that we will examine;– Freezing point

depression– Boiling point elevation– Vapor pressure lowering– Osmotic pressure

First – Freezing Point Depression

• We know from Unit 1 that all pure substances have a nature-given temperature at which they will freeze.

• This would be called the normal freezing point.

• For water, that temperature is 0°C.

• It is observed that when we dissolve a solute in water, (like salt, sugar, or any other regular solute), the resulting solution will actually freeze at a temperature below 0°C. (lower than expected)

• This same observation will be seen in any other solute-solvent pair.

Naming This Observation

• This phenomenon is called “freezing point depression” or “freezing point lowering” (the first name is more common).

Additional Thoughts

• There are calculations connected to this concept (and the other colligative properties too), but Chem I’s are not responsible for them.

• Just know that a solution will freeze at a lower temperature than the pure solvent.

• Should probably also know that this property is why we drop salt on the roads before a snow storm.

• The snow lands on the pavement and melts, but because the salt is there, the resulting solution will require a colder temperature to freeze – delaying the road icing.

Be Able to Explain Why This Works

Next – Boiling Point Elevation

• This will be a similar concept, but the result is the opposite from the first property.

• From Unit 1, we also learned that pure substances have their own nature-given boiling point.

• For water, that is 100°C

• But, when we dissolve a solute in water, the resulting solution will boil at a temperature that is higher than the normal, expected temperature.

• This is called “boiling point elevation”.

Have to Keep the Changes Straight

Some Additional Notes

• There are factors that affect the magnitude of the changes in both the freezing point and the boiling point.

• The first is the concentration of the solution (last lesson).

• The second is a “nature factor”. It turns out that different solvents are more sensitive to being made into a solution than others – you get a greater change in behavior.

• The final factor is how the solute actually dissolves – does it yield ions or complete molecules.

Next – Vapor Pressure Lowering• Consider a beaker of water

just sitting on a counter.– Over time, some of the

water molecules gain enough energy from the environment to “escape” from the surface of the liquid.

– This is simply called evaporation – the “escaped” molecules are now in the vapor phase.

• If we have a lid on the top of that beaker, the vapor molecules are essentially trapped in the space above the surface of the liquid.

• Since the vapor molecules are actually gas phase and moving (like gases do), they exert pressure on the surface of the liquid.

• That pressure is called “vapor presure”.

Vapor Pressure

• The number of vapor molecules that form is dependent on temperature.

• Higher the temperature, higher the number of vapor molecules and the higher the vapor pressure.

So What About a Solution?

• When a solute is dissolved in a solvent (like salt or sugar in water), the dissolved solute particles “get in the way” of the solvent molecules that are trying to escape from the surface of the liquid.

• This means that there will be fewer solvent molecules reaching the gas phase region above the liquid.

• Fewer solvent molecules in the gas phase will mean less vapor pressure (fewer particles = fewer collisions)

Concluding…

• A solution will have a lower vapor pressure than that observed in pure solvent.

• This is called “vapor pressure lowering”.

Last One – Osmotic Pressure

• This one is a lot more complicated - there is a lot of understanding that has to be put in place first.

• Please be patient with the explanations….

First - Osmosis

• Osmosis is a nature -driven event.

• It occurs when two systems are separated by a semi-permeable membrane.

• Semi-permeable means that some things can pass through the membrane, but other things cannot.

• In this discussion, one of the systems is simply pure water and the other system is a solution of some solute (like salt or sugar) in water.

What Occurs?• The nature-driven event

described in the previous slide is water (from the pure water side) moving through the membrane into the side that has the solution.

• Nature does this to reduce the concentration of the solution – trying to make the two sides have the same concentration.

• Very important to realize that the solute particles are not able to move through the membrane.

• That is why we describe the membrane as a semi-permeable membrane.

Osmosis

The red dots represent the water and the gray dots are the solute particles. Nature has the water molecules pass through the membrane to reduce the concentration of the solution. Note how the solute particles cannot pass through the membrane.

solvent solution

Osmotic Pressure

• Remember that this osmosis is a nature-driven event.

• Nature wants this event to occur spontaneously.

• But, we can stop the event – prevent the osmosis from occurring.

• If we push hard enough on the surface of the solution side of the system (basically increase the pressure above that side), we can prevent the water from passing through the membrane.

Finishing the Thought

• The pressure that is necessary to stop the osmosis from occurring is called the Osmotic Pressure.

• Finally – there is a connection between the concentration of a solution and the osmotic pressure.– The greater the concentration of the solution, the

greater the osmotic pressure.

Summarizing

• There are four Colligative Properties associated with solutions– Freezing Point Depression – the solution will freeze at

a lower temperature than the pure solvent– Boiling Point Elevation – the same solution will boil at

a higher temperature than the pure solvent– Vapor Pressure Lowering – the same solution will

have a lower vapor pressure than the pure solvent– Osmotic Presure – any solution will have a higher

osmotic pressure than its pure solvent.