Unit 4 - Solubility8.2 Factors the Affect Solubility

Context – Review from Friday• Things dissolve in water if they can be

attracted to water• Molecular compounds dissolve by

being surrounded by water – the molecules themselves are not broken up• Ionic compounds dissociate into ions

Ionic Compounds in Water• Not all ionic compounds are soluble in water. It

depends on the ionic bonds in the crystal lattice

• Compounds with strong ionic bonds are hard to pull apart. ▫ The water molecules will only be weakly attracted. The

compound is insoluble.• Compounds with weak ionic bonds are easy to pull apart.

▫ The water molecules are strongly attracted. The compound is very soluble.

• Use the solubility chart

• Dissolved ions are also called electrolytes – solution can conduct electricity

Solubility Categories•Highly soluble: when more than 1.0g of

the substance will dissolve in 100mL of solvent

•Slightly soluble: if between 0.1g – 1.0 g of substance will dissolve per 100mL

•Insoluble: If less than 0.1g of solute will dissolve in 100mL

• If something is soluble, there are certain factors that can change how soluble something is…

Factors that affect the RATE of dissolving

1. Temperature▫ Higher temperatures means greater

kinetic energy, more collisions between solute and solvent

2. Agitation▫ More collisions with solute and fresh

solvent3. Particle Size

▫ Increase the surface area for dissolution by grinding or crushing the solute into powder

Factors that Affect Solubility1. Molecule Size

▫ Small molecules are more soluble than large molecules

2. Temperature▫ Most solids have higher solubility at

higher temperature▫ Doesn’t really affect liquid-liquid

solubility▫ Gas solubility decreases at higher

temperature

3. Intermolecular Forces (molecules only!)▫“Like dissolves Like”

4. Pressure (gases only!)▫Changes in pressure do not affect liquid

and solids▫The solubility of gases is directly

proportional to the pressure of the gas above the liquid

How much solute??•There is a limit to the amount of solute

which can dissolve in a solvent•A solution which contains the maximum

amount of solute is saturated•If the solution contains less than the

maximum amount of solute, it is unsaturated▫The crystals of solute added will continue

to disappear.

•If there is extra solute which cannot be dissolved, the solution is supersaturated▫solid may and usually does

come out of solution.▫A change in temperature

(cooling a solid dissolved in liquid) can cause a saturated solution to become supersaturated

•As we know, the maximum amount of solute which a solution can dissolve depends on the temperature of the solvent

•** Different trends for gases

•We use a graph called a solubility curve to decide on masses

• A solubility curve is a plot of solubility versus temperature.

• The line represents a saturated solution.

• Points below the line represent unsaturated solutions.

• Points above the line represent a supersaturated solution.

Solubility Curve for Ionic Compounds

• What is the solubility of potassium nitrate at 80ºC ?

At what temperature will sodium nitrate have a solubility of 95 g/100 cm3 ?

•What is the solubility of sodium chloride at 25ºC in 100mL of water ? • From the

solubility graph we see that sodium chlorides solubility is 36 g.

Solubility Curve for Gases

Solubility of gases in water shows an opposite trend!

Practice!•Worksheet•P. 289 # 9, 10•P. 301 # 1-4