Saturation & Solubility

Is there a limit to how much sugar you can dissolve in a cup of tea?

Saturation

• saturated – a solution that contains as much solute as can possibly be dissolved under the existing conditions of temperature and pressure

• unsaturated - a solution with less than the maximum amount of solute (it could hold more)

• supersaturated – a solution that contains a greater amount of solute than that needed to form a saturated solution (this is an unstable situation; such a solution will not remain supersaturated for long)

Supersaturation Demo

(this might happen another day…)• If I dissolved 33g of NaCH3COO in 30g of

water, calculate how many grams that would be in 100g of water. (think proportions)

Sketch this graph and locate the point you just calculated (x amount of NaCH3COO in 100g of water) at 75°C.

• What adjective would you use to describe this solution at 75°C?

• If the solution were cooled to 25°C (and nothing crystallized), how would you describe the solution?

Supersaturation Demo

1. What does the flask look like to start with?2. What happened over time as it sat still?3. What did I add?4. Then what happened? What does it look like

now?5. How would you describe the remaining

solution after this change?

Solubility

• Solubility is the amount of a solute that will dissolve in a specific solvent under given conditions.– determined experimentally– expressed in grams of solute per 100 grams of

solvent (at a specific T and P)

Solubility

• On a graph showing solubility versus temperature, (see Table D in your Reference Tables)

• The line represents saturation for a particular substance at different temperatures.

• A point above the line represents supersaturation (AS LONG AS ALL OF THE SOLUTE IS DISSOLVED),

• while, a point below the line indicates an unsaturated solution.

Find the potassium nitrate (KNO3) curve on Table D

How many grams of KNO3 will dissolve in 100g of water at 50°C?

Approximately 83 grams of KNO3 will dissolve in 100 g of water at 50˚C

If you only put 70g of KNO3 in the water at 50°C, how would you describe the solution?

70g of KNO3 at 50 ˚C is below the line…therefore the solution is unsaturated

If about 83 grams can dissolve at 50 degrees, with only 70 dissolved, you should be able to dissolve about 13 more grams.

It is HARD to make a supersaturated solution. You would be lucky to get 70g of KNO3 to

dissolve at 40°C.

How much would you expect to dissolve at 40˚C?

Probably about 63g.

If you put 70g potassium nitrate in 100g of water at 40˚C, how much solid would you expect to have on the bottom?

Probably about 7g.

Solubility is affected by:

temperature•how temperature affects solubility of a solid depends on if the dissolving process was endothermic or exothermic:– endothermic when dissolving – higher temperature results

in increased solubility– exothermic when dissolving – higher temperature results

in decreased solubility•gaseous solutes are less soluble at higher temperatures– increased kinetic energy of the solute (gas) increases the

tendency for the gas to escape from the solvent

Solubility is affected by:

the nature of the solute and solvent•“like dissolves like” – – polar and ionic solutes dissolve in polar solvents

but not in nonpolar solvents; – nonpolar solutes dissolve in nonpolar solvents but

not in polar solvents (ex.: nonpolar oil molecules stay separated from polar water molecules in salad dressing)

Solubility is affected by:

pressure•solutions of solids and liquids not affected much by pressure•gas solubility is strongly affected by pressure•increased pressure increases the rate at which gas molecules strike the surface of solute •Henry’s Law: the solubility of a gas is proportional to the partial pressure of the gas above the liquid

Dissolving rate is affected by:

• surface area – because solvent particles pull solute particles from the surface of the solute

• stirring – because dissolved solute builds up next to the solid solute causing the dissolving process to slow down

• temperature – solvent particles move faster at higher temperatures, so more solvent particles come into contact with the solid solute in a shorter period of time