BACKGROUND:
Water is an excellent solvent because
it forms very strong bonds (covalent). Substances with weak bonds
(mainly ionic) usually dissolve into the stronger substances or solvents.
Dissolution is not a chemical change.
However, not all substances will dissolve
in water. Salts will dissolve, the covalent bond of water "rips"
the ionic bonds of the salts. Sand will not dissolve in water because
the "bond" of water is not strong enough to dissolve the sand.
However, some strong acids can dissolve sand.
Dissolution will proceed faster in warmer
water, because it has more room for the molecules of the salt to “fit”
between the molecules of water.
PROCEDURE:
- Ask students if a body of
salt water were to evaporate, would anything be left. If the students
debate over this you might want to set out a dish of salt water and a dish
of fresh water and have them observe what happens when the water evaporates.
- In this lab, the students will measure
all the correct amounts and follow the lab sheet. Have them describe
what happens when they mix the materials. Once the students record
their findings on one item, have them clean out the dish and proceed to
the next item. Do not dispose of the sand and mud in the sink.
You may want to save the sand and dry it out, to be used again.
- Students may ask you why the oceans
aren't sweet. The oceans are not sweet because sugar does not dissolve
as fast as salt and does not stay dissolved. Sugar has strong bonds
(covalent) whereby salts have weak bonds(ionic). Also, salts are
much more abundant in the rocks. Have you ever tasted a sweet rock?
