Gases & Just how much water exists on Earth?

After our cloud in a bottle demonstration and discussions of the water cycle, students seemed to want to dive deeper into understanding water vapor and gases in general. This is an awesome initiative for a mixed age group of K-2nd graders. In looking into possible hands-on experiments for them to try, I found an easy and fun demonstration that uses the some of the same inexpensive materials we still had from our previous lesson. We used three empty plastic soda bottles filled about half way with warm water and several tablespoons of yeast. To turn it into an experiment rather than a demonstration, we fed each bottle of yeast/water with different nutrients: sugar, flour, or nothing at all. We placed thin walled balloons over the opening and waited. We all made predictions about which bottle would start to inflate the balloon first. The students had just recently baked pretzels with their teachers so we talked about how long they had waited for the dough to rise and what each of the ingredients were in that case.

We were very excited to see the balloons expand ever so slightly (in two of the three bottles) as we swirled the bottles to mix the bubbling yeast, sugar, and warm water. Some of our discussion as we waited included answering:

1. What caused the balloon to inflate?

2. What was the purpose of the balloon?

3. Which nutrient resulted in the best yeast growth? The most gas?

4. Do yeast require a food source to grow?

5. How does this relate to water vapor?

The following week we began by talking about percentages. We "made" a pizza (colored drawing on the board) and divided it up into quadrants - based on specific toppings that each of the students requested: plain cheese, or veggies with cheese, or meat with cheese, or no-cheese at all. Together we figured out the percentage of each favorite topping type on our pie and accordingly divided it up among the students. 

This provided us with a relatively standard starting point from which all students could move into a discussion of the overall water (percentages of: salt, fresh, frozen, potable) on our planet. Estimated at 97% ocean, 3% fresh water. 

To demonstrate we first filled a large graduated cylinder with 1000mL of water, this to represent all the water (in all forms) on the planet. Then we poured off 30mL of water into another cylinder, or 3% of the total water to represent earths fresh water. To the remaining 970mL we added salt, to represent the ocean, 97% undrinkable water. 

Next, using our globe, we asked the students what they noticed about the earth's poles...they are covered in snow! Almost 80% of the earth's fresh water is frozen in ice caps and glaciers. So we poured 6mL, from our 30mL of all fresh water, into another glass to represent non-frozen water. 

Using an eyedropper the students then removed a single drop of fresh water (from our 6mL) into a drinking glass, finally to represent clean, fresh, non-polluted water, 0.003% of the total! 

Students concluded that a very small amount of water is available to humans, but we stressed that this single drop is just a representation of the larger volume of water available to us on a global scale, remember we started with a pretend volume, 1000mL -- scientists estimate that there are over 300 million trillion gallons of water (in all forms) on earth, an inconceivable amount to picture even for an adult!