BACKGROUND:
Water is an universal solvent which
means it can dissolve many other substances within the molecular structure
of water. Water becomes salty because many different components that
erode from the land will dissolve and become part of the water. Over
eons of time the water cycle evaporates only fresh water, leaving the “salts”
behind.
The term solution means a system in which
one or more substances are uniformly mixed or dissolved in another substance.
A solution has two components, a solute and a solvent. The solute
is the substance that is dissolved. The solvent is the substance
doing the dissolving. A solute plus a solvent equals a solution.
Water is considered a universal
solvent, in other words, many other substances can be dissolved into water.
Seawater is an example, it contains many ions of dissolved elements like
sodium, chlorine, bromine, calcium, carbon, and many more. Seawater
starts as fresh water but as water falls on the land causing the erosion
of rocks, minerals become a part of the water, and then become part of
seawater. Salt water is neither dirty nor polluted, it is a
solution that is clean, unless polluted by humans or nature.
PROCEDURE:
- Go over the terms solute,
solvent, and solution. Make sure students know that
solute + solvent = solution.
- You may want to put boiling water with salt in a
jar and then put a lid on it. Make sure the jar can withstand
boiling water. Watch what happens. The steam will make fresh
water droplets. If it is a warm day you might want to put an ice
cube in a jar and put it in a warm place. Students can watch
as the water turns from solid, to liquid, to gas. Water droplets
should form on the lid, illustrating condensation. Afterwards,
take the top off and release the steam. Where did it go?
Although it seems that it has disappeared, it's really in a gaseous
state in the atmosphere. You have proven to the students that
water vapor is part of the atmosphere and the salt is left behind.
- The worksheet has the students determining
solutes, solvents, or solutions. The answers are:
- salt + water = seawater
- kool aid + water = punch
- orange
+ water = orange juice
- apples + water = apple juice
- dirt + water = dirty water
