7-2: Concentration and Solubility

7-2: Concentration and Solubility

8th Grade Physical Sciences


Concentration • Concentration is a

measure of how much of a solute is in a solvent – i.e.: how much salt is in the water.

• Maple syrup starts as a dilute solution, when the water is evaporated, it becomes a concentrated solution – syrup!


Concentration • A concentrated solution has

a lot of solute dissolved in a certain amount of solvent.

• Concentrated fruit juices remove the solvent (water), leaving a lot of solute (sugars) in a small amount of solvent (water).

• “Just add water”


Concentration • A dilute solution only has a

little solute dissolved in a certain amount of solvent.

• If you add too much solvent (water) to the concentrated fruit juice, it becomes diluted and tastes watery.

• “Don’t add too much water”


Measuring Concentration

• To measure concentration, you compare the amount of solute to the total amount of the solution.



• Example: 100 kg of maple tree sap has about 1 kg of sugar in it.

• The concentration is therefore: 1 kg/100 kg X 100% = 1%



• After boiling the water away, maple syrup still has about 1 kg of sugar in it, but there is only 2 kg of the solution remaining.

• The concentration is therefore: 1 kg/2 kg X 100% = 50%


Solubility

• Solubility is a measure of how much of a solute can dissolve in a solvent – at a given temperature.


Solubility

• If you add solute and it dissolves, you have an unsaturated solution.

• When it no longer dissolves and just sinks to the bottom, it is called saturated.


Solubility

• When air is saturated with water, fog or clouds form.


Solubility • Too much sugar in

ice tea will cause sugar to build up at the bottom.

• The ice tea would be “saturated” with sugar.


How Do We Measure Solubility?

• To measure solubility, we just add a solute until it no longer dissolves.

• The amount added is the solubility.

• For sugar, 180g can be dissolved in 100g of water at 0˚C.

• That is more sugar than water!!!


How Do We Measure Solubility?

• Using a table of solubility, you could do an experiment to find out whether you have salt or sugar.

• Just measure the solubility and you will know which one you

have.


What Affects Solubility?

• Three things affect solubility:

1. Pressure

2. Temperature

3. Type of solvent



• Increasing pressure increases the solubility of a gases.

• When soda water is made, they put it under high pressure with CO2 gas.

• When you open the soda, the pressure drops and the gas comes out as bubbles.

Question? How does this affect scuba divers?



• Increasing temperature increases the solubility of solids in liquids.

• Warm water dissolves more sugar than cold water.

• BUT increasing temperature decreases the solubility of gases in liquids.

• Warm soda is flat – less fizzy than cold soda.



• Solvents affect solubility.

• Some solutes will not dissolve in water – like oil.

• The rule is like dissolves like, meaning…

1. Polar solutes will be dissolved by polar solvents.

2. Nonpolar solutes will be dissolved by nonpolar solvents.



• So oil based paints (nonpolar) can’t be cleaned with water (polar).

• You must use turpentine (nonpolar).

• Latex paint (polar) must be cleaned with water (polar).


Supersaturated Solutions

• Sometimes we can create a solution where more solute is dissolved than should be allowed – this is called a supersaturated solution.

• Adding even a small amount of undissolved solute will cause a chain reaction – and the extra solute will come out of the solution.

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7-2: Concentration and Solubility Summary

• Concentrated solutions have a lot of solute dissolved.

• Dilute solutions have a little solute dissolved. • Temperature, pressure, and solvents affect how

much of a solute can dissolve. • If no more solute can dissolve, it is called a

saturated solution. • Supersaturated solutions have more than they

should have in them – and the smallest particle added can cause the extra solute to come out of the solution.

7-2: Concentration and Solubility

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