The Dazzling Doohickey of Density
A) Objective: To demonstrate the density and the relative densities of liquids, and how this can be used to determine the density of other objects.B) Time: 20 minutes
C) Materials:
- graduated cylinder or tall drinking glass (anything to show the layers)
- syrup, glycerin, water, vegetable oil, alcohol
- food colouring for water and alcohol
- solids to check densities [small screw (anything metal), bean, cork]
D) Theory: All materials have characteristic densities. As long as the materials do not react or mix, a less dense substance will float on a more dense layer. By adding solids to the column you can compare their densities to those of the liquids a nd each other.
E) Procedure:
- "Has anyone seen a rainbow in a puddle before? Why does this happen?"
- Explain how all different liquids are different. The small particles that make up the liquid can be positioned really close together or really far apart. Put 1 person on a desk and 5 people on another desk (for this age group, it's better if the 5 people are of the same sex since the have to get close to each other.) Which one is heavier?
- Density is the mass of something divided by the amount of space that it takes up -- its volume. So the 1 person and the 5 people are on desks of the same size, but the 5 people are much heavier, so that box of people would be denser. This is the same idea as the density column.
- "If I pour the same amount of 2 different liquids on top of each other and one has 20 particles in it and the other 200, which is more dense? Which one do you think will float on top of the other? Let's try this out with some things that you all h
ave at home."
- Slowly pour about 15 mL of each liquid into the cylinder -- the most dense first (don't let the syrup drip on the sides of the cylinder.)
- Ask what would happen if you covered the column and shook it up.
- The layers would not all form again because the mixing allows more than just the surface of the layers to touch and so the particles can slip in between each other and not ever come apart unless they repel each other like oil and water.
- Ask the class if they can think of any examples of how densities can be a problem in our lives -- like oil spills!
- Ask if the class thinks we can find the density of some solids using our column. Get volunteers to choose an object to drop in and guess which layer it will stop in.
F) Safety:
- Make sure students don't fall off desks.
- Instructors should be careful with the graduated cylinder, it's easy to knock over and expensive to replace!
G) Real Life Examples:
- oil spills
- compare tin foil and a lead ball of the same volume
- when cooking pasta, putting oil in water. . . does the oil float?
