Weight: What? Are you dense?

WEIGHT and DENSITY

Current topics: density
Prior topic: Chemistry, mass, weight, gravity
Time of lesson: 20-30 minutes
Things you should have before starting this lesson:

A scale
Your lego hydrogen and oxygen "atoms"
2 plastic bags you get for produce at the grocery store, one filled with water and tied closed. The other filled with air and tied off.
I also use four drinking glasses, one nearly full of water, one with a tablespoon of baking soda, one with two table spoons of vinegar, and one empty one.
A tea candle
A lighter
The weights you are going to use in your Pinewood Derby car.  If it is lead, put it in a plastic bag.
A few things that are roughly the same size as your weights, but of lighter density.

Density

We already learned about mass, which is how much stuff is in something.  But they take up different amount of volume. What weighs more, a one-pound hammer or a one-pound bag of feathers?

It is a trick question.  They have the same weight.

Weight is a measure of mass. So both the hammer and the bag of feathers weigh the same.  The mass can come in different sizes. The one pound bag of feathers would take up a lot more space (volume) compared to the hammer.

The measure of how much something weighs based upon a unit of volume is called "density".

(Take out the glass of water and the empty glass).  Here is a glass of water.  Here is another glass.  It is not empty.  It is full of air.  Air has mass and weighs something. Which weighs more? Water or air?

Water weighs more in the same sized glass.  So water has a higher density.  So using sizes, the water weighs 829 times more than air*.  That is why air bubbles rise in water. The earth's gravity is pulling down on the heavier water more than the lighter air.

(Now is the time when you take out the tea candle, light it and put it at the bottom of one glass.)
This candle is burning. It needs air* to keep burning.

(Then pour the vinegar into the glass with the baking soda.)

These fizzing bubbles is carbon dioxide gas. Carbon dioxide is clear just like air.  It is also one and a half times as dense as air.

(Carefully and slowly pour the carbon dioxide gas out of the glass directly over the lit candle. This is kind of overkill on this topic, but but kids LOVE this trick and this will keep this lesson fun.)

The carbon dioxide is invisible but it is heavier than air and so it sinks to the bottom of this glass. The fire needs air to burn, so it goes out when the carbon dioxide covers it.  Carbon dioxide doesn't burn.

(Clear that stuff to the side. Now give the lightest thing that is roughly the same size as your weights.)

What weighs more, this (give the next heaviest thing) or this?  (Take away the lightest object and repeat until you get to your weights). Watch his amazement as he feels the density of Lead or Tungsten.

We are going to use these weights in our car. Using weights with the highest density is better than lower density.

Most kids will be using weights made out of zinc* (yes, the weights we are an alloy, but let's keep this simple).

(Show him this table:)
http://www.lenntech.com/periodic-chart-elements/density.htm

Zinc has a density of 7.13 grams per cubic centimeter.
(He will naturally want to know what is the most dense material.)
Osmium is very, very dense but it is not very much of it on earth so it is  hard to find, very expensive, and is dangerous. Actually, most of the most dense materials are really dangerous - Iridium, Uranium, Americium.

Platinum is not dangerous but Pinewood Derby weights made of platinum would cost nearly as much as a real car (U$7,000 - okay, a cheap, horrible car).   We could fill it with gold, but it costs about the same as platinum.  (Here is where if you decide if you skip over Tungsten depending on whether you are going to get it. Point to what you are using.)

Lead is about 50% heavier than Zinc.
(Tungsten is almost 3x heavier than Zinc.)

Parents' note: I did a science experiment to see what the actual effective density difference, which accounts for difference in molecules and alloys. I saw the claim on the website that sold me the tungsten weight claiming that it was 3.2x denser.  It's not true.  I weighed the zinc weights - the standard ones that almost all kids use and those hav a density of 7.43 grams/milliliter. The lead fishing weight I used was 11.4 grams/ml.  The tungsten weights were 20.4 grams/ml. So the tungsten is 2.7x denser (not 3.2x) and the lead is about 50% denser than zinc.  Using lead is a big advantage over zinc. My son has used lead in the past because it was cheap.  This year, we splurged and are using tungsten for the first time.

Next up Center of Gravity