On a rainy day, we may remember the appearance of patches of oily slicks on the road, as cars and other combustion vehicles occasionally leave their calling in the wake of start-stop-traffic.
As you might recall, certain oils from cars will not dissolve in water – this leaves the rainbow-hued oil patch you see on the road.
Is there a teachable moment here? The obvious statement that oil and water will not mix may lead you and your kids to speculate on gradients of oil & water mixing.
Unlike Solutions Will Not Mix
Place three glasses of water side-by-side, each with a drop or two of food coloring added so that they’re all different colors.
Dissolve salt into the water of one glass. Try to add the briny solution (a drop at a time) into one of the other glasses of colored water. The brine will sit atop of the water colored solution (it will not dissolve).
This instance, similar to the original gasoline and water phenomenon, further illustrates how one substance can be forced to be unlike another.
How can you two such different substances mix?
Chemistry in the Moment: A Kitchen Table Experiment
There’s a proven concept in chemistry that states: Like substances will dissolve similar substances. If you’re trying to dissolve something, look to the water. The concept seems strange, it’s not a leap to make our experiences adapt to a table-top experiment.
A set of three glasses contain three solutions—each a different color (or substance). The first contains black tea, the second contains green tea and the last vessel contains honey. The black and green teas dissolve together easily.
Pouring green tea into the glass containing honey, however, produces a watery mixture that will stand atop of the honey.
Adding sugar to the green tea first, however, will force the mixture to dissolve readily when you add it to the honey.
How Does ‘Like Dissolve Like?’
The chemist’s adage of ‘like dissolving like’ is where the teachable moment comes into play. Although the oil and water will never naturally mix, the contrary investigation allows for further exploration of the concept. Scientists term water the ‘universal solvent,’ it may be manipulated to suit a variety of different situations.
Cold water dissolves salt, but not sugar. Heating the water allows the sugar to dissolve and (more) rapidly dissolves the salt. Sugar is a molecule that is part carbon and part oxygen/hydrogen. The molecule half resembles grease. Water is a tightly packed group of H2O molecules—it is packed tightly because of its size, and contains no other atoms (there are no carbon atoms).
The molecule is symmetrical—it is simply shaped. It will readily dissolve molecules similar to it. Sugar, in this case, will dissolve in hot water for several reasons: (1) Molecules of hot water are not tightly packed (as in the colder alternative). (2) The warming sugar molecules also lose cohesion with one another. (3) The molecules can make room for one another—and allowing for solvation.
Perhaps the best illustration would be to attempt to dissolve sugar into solid water (ice?)—it doesn’t work that way, does it? Thus pure gasoline, termed octane, is all carbon and hydrogen (with no oxygen atoms) group of molecules. Water, as illustrated, is dissimilar to octane and will not dissolve it.
Where Does That Leave Us?
Bringing experimentation to a familiar setting–the kitchen table allows a more relaxed conversation to occur. Avoid chemistry ‘jargon’ at first, then add in the correct terms as your child becomes interested.
All too often, we grope in the dark to find the appropriate response to questions that seem beyond our reach, and base our answers on an intuitive gut instinct. However, as is the case with all hard science, chemistry is an experimental endeavor.