Which is Sugar and Which is Salt?
Objectives:
Students use the senses to observe and distinguish differences. Students see and describe how evaporation leaves crystals that are different for sugar and salt.
If a teacher asks you which is sugar and which is salt you would want to taste it to find out. I want to see if you can tell with your eyes instead.
Show the students two clear glasses with some sugar in one and salt in the other. Ask for guesses. "How do you know?" They feel and look the same. If you crumble a few grains of either you can tell the difference by the smell. But even if the students discover which is which while handling the crystals, mix up your glasses again behind a blind so that no one knows - even you! At that point label one of them ‘A’ and the other ‘B’. Start a page in the journal with the date and a picture of the glasses.
Materials:
Two clear glasses
Spoon or mixing stick
Sugar
Salt
Water for the two glasses
Enough petri dishes or clear, flat plastic plates for each pair of students
Wrapping clear plastic if using plates
Procedure and Monitoring:
Dissolve the sugar and salt in the glasses. Make the solution a little thick, to the point that almost no more can be dissolved. Ask the students if they can tell now which is which. Have the students write down a guess as to which glass has sugar and which has salt. Have your materials managers pass out all the petri dishes. Divide the class into equal groups and designate one side A and the other B. Pour just a teaspoon of the salt solution (in alphabetical order) into all the petri dishes on side A and pour just a teaspoon of the sugar solution for everyone on side B. (It doesn't matter which is A or B at this point as long as all A's are one solution and all B's are the other)
Tell the class you are going to put them all together where they will dry out in a few days.
When the water in the solution evaporates a crystal pattern will randomly occur in the dishes. Kids love the sight of it. Look for the differences by using a magnifying glass or microscope.
Talk to Write
The sugar and salt are still there in the dishes but the water is not. Water in the air is called water vapor. When water is a gas, it’s invisible.
Summarize what we've seen and answer our guesses from 3 to 5 days ago in the journals. How do we use our senses to learn? Now can you tell which is sugar and which is salt just by looking at the crystals.