CONCEPTS: solutions, solubility, concentration
BACKGROUND: How fast something dissolves depends on the surface area, and stirring
SUBSTANCES NEEDED: hard candy, salt, sugar, sugar cubes
EQUIPMENT NEEDED: spoons, cups
PREPARATION: glue 27 sugar cubes together to make one big cube
1 - Drop a piece of hard candy (mostly sugar) into a cup of water.
Observe as it slowly dissolves. It may take several hours.
Where did the candy go? What could make it dissolve faster?
Allow children to try experiments: stirring, more water, break into pieces, etc.
2 - Which will dissolve faster, a sugar cube or an equal amount of granulated sugar?
Try and see.
Display the large (glued together) sugar cube.
Build a similar one without glue and take it apart to illustrate the increased surface area exposed
when a solid is broken up into pieces.
Part 2: Solubility
BACKGROUND: When one substance dissolves in another the solution may
seem different in some ways but no new substances
have been created (no chemical reaction). There is a maximum amount of a
substance that will dissolve in another.
When the solution is "saturated", no more will dissolve.
Solutions can be diluted but the dissolved substance is still there.
SUBSTANCES NEEDED: salt, sugar, others. . . (note: salt often has a small amount
of a moisture absorbing agent added and this will not dissolve,
making the solution cloudy--try pure salt (canning/pickling or Kosher salt)
EQUIPMENT NEEDED: cups or jars, spoons, shallow dishes
1 - How much salt (or sugar) will dissolve in water? Add some salt to a cup
or small jar of water one teaspoon at a time.
Each time stir or shake until it all dissolves before adding the next teaspoon.
(A lot will dissolve! Especially sugar--100 ml of water will dissolve over 50 packets of sugar,
and the volume will more than double!) When no more will dissolve, the solution is saturated.
The saturated solution can be poured off once the excess solid has settled.
You can't tell by looking that its saturated, but no more will dissolve.
Present students with a saturated solution and a dilute one--how can they tell which is which?
2 - Spoon a puddle of the saturated solution onto a shallow dish or piece of plastic wrap. As the water evaporates, the dissolved substance reappears.
Pour some soda-pop into a solid dish and let evaporate over several days--note appearance of dissolved solids.
3 - Dilution. Pour some of the saturated solution into a large container and
add some more water. Add more and more water.
Pour some of the diluted solution into a different container with water.
Is all the salt or sugar still there? How is the solution different?
No matter how much you dilute the solution, some salt is always there.
Spoon some out and let is evaporate to show some is still there.
CONNECTIONS: Even though diluted, pollutants are still there.
Some pollutants will be accumulated in microorganisms and fish making them toxic even though
the concentration in the water is low. Is dilution the solution to pollution?
Part 3: Substances that won't dissolve
BACKGROUND: some substances don't dissolve much at all
SUBSTANCES NEEDED: sand, cornstarch
EQUIPMENT NEEDED: cooking oil bottle (1/2 full)
1 - Try to dissolve some sand or cornstarch in water--none will dissolve!
2 - Insoluble liquids. Add water to a half-full bottle of cooking oil to make a
fascinating bottle of water and oil.
Add a drop of food color (dissolves in the water only) for interesting effect.
Shake up the bottle, the oil and water always separate.
CONNECTIONS: Your body has a hard time excreting things that don't dissolve in
water--cholesterol clogs your arteries the way grease clogs water pipes in your house.
Part 4: Effect of temperature
CONCEPTS: solubility, temperature
BACKGROUND: How much of a substance dissolves in a liquid may depend
on temperature. Gases dissolve in water.
SUBSTANCES NEEDED: sugar, bottle of pop
EQUIPMENT NEEDED: hot plate, pot, cups, spoons
1 - Effect of temperature. Some substances such as sugar or saltpetre
(Potassium Nitrate) are much more soluble in hot water than in cold.
Make a saturated solution with room temperature water.
Use less than a cup of water or it will require a lot of the solid.
Then, heat the water--eg. sit a small jar of solution in a pot of water and
heat on a hot plate. More will now dissolve. A lot more!
When solution cools, the solid will reappear because there is too much
dissolved for the lower temperature.
Spoon some of the hot solution onto a shallow dish and not reappearance
of solid as it cools.
2 - Gases in water. Heat water--notice bubbles forming.
This is air coming out of solution--unlike most solids,
gases are more soluble in colder water (opposite of the sugar).
Open a can of cold pop vs. a can of warm pop.
The carbon dioxide gas is more soluble when its cold so there's less to escape.
CONNECTIONS: Since gases are more soluble in colder water, there is more
oxygen in colder water and thus fish are plentiful in the cold North Atlantic.