Solutions – Unit 4

We have covered solutions in grade 9 chemistry. Recall that any substance on Earth can have one of two possible appearances:

1)homogeneous (same appearance) – a substance that is uniform in its appearance throughout, you can not observe a difference anywhere in the substance since it is pure

Examples – elements with identical atoms or compounds with identical molecules

-solutions with dissolved particles

2)heterogeneous – (different appearance) – a substance where 2 distinct phases can be seen within, the substance is not considered to be pure

Examples – suspensions that will settle to the bottom

-mechanical mixtures that may be physically separated

-colloids that will refract light

From this we can discuss homogenous solutions. Solutions contain 3 important components:

1)Solution – a homogeneous mixture of a solute and a solvent. The solute and solvent may be of the same phase, however often they are not. The most common and important solution is a solute dissolved in water.

2)Solute - the component that is being dissolved

-may be a gas, liquid or solid (most commonly solid)

3)Solvent – the component that performs the dissolving

-may be gas, liquid or solid (most commonly liquid)

-water is known as the ‘universal solvent’ since it has specific properties that will allow it to dissolve nearly any solute

Examples of solutions by phase:

Gas / Gas -Air (78% N2, 21 % O2, 1 % Ar and CO2)

-Oxygen / Acetylene for torches

Gas / Liquid -Carbonated drinks – CO2 dissolved in water and released slowly

Liquid / Liquid -ethanol dissolved in water to make a variety of alcohols

Solid / Liquid -salt dissolved in seawater

-sugar cubes dissolved in coffee

-NOTE : ionic solids will form ions when dissolved in a solution : NaCl (s) + H2O (l)  Na+(aq) + Cl-(aq)

-recall that aqueous solutions that contain ions are called electrolytes

Solid / Solid -will form a special type of solution called an alloy

-alloys include brass (tin and zinc) or steel (iron and zinc)

Unsaturated, Saturated and Supersaturated Solutions

Unsaturated Solution – is able to take more of a solute at a specific temperature

-in a graph, this would be found under the line indicating the saturation curve

example : adding a small amount of salt to water so that you can observe that it has completely dissolved

Saturated Solution - has the MAXIMUM amount of a solute that can dissolve at a specific temperature

-in a graph, this would be found at the point on the line corresponding to that temperature

example : you have added an amount of salt to water just to the point where adding any more would cause that amount not to dissolve in the water

Supersaturated Solution - contains more solute that could normally dissolve at a specific temperature

NOTE : this will create an unstable situation, agitation will lead to crystallizing the solid from the solution if it is cooled

-in a graph, this would be located above the point on a line indicating the saturation curve

how we get there :

1)get to the point of saturation where no more solute can be added without

it sitting undissolved in the solution

2)heat the solution

3)add more of the solute to create the potentially unstable situation

4)cool the solution to create the instability

5)you now have a supersaturated solution since there is more solute than expected at this temperature

Solubility

Solubility is generally reported in mass of solute that will dissolve in 1 kg of solvent at a given temperature (i.e. solubility of KNO3 in water at RT = 400 g/kg)

Since the solubility of different solutes in the same solvent will be different, we can use solubility to separate solutes apart. For example, if we have a solution of NaCl and Pb(NO3)2 in water at 90  C both are soluble. If we cool the solution to 10  C, the lead nitrate precipitates out of the solution, and we can pour off the water with the salt still dissolved. We can now boil the water to leave just the salt behind.

Homework :Q 1-8 p. 269