Tuesday, March 26, 2013

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!

Friday, March 8, 2013

The water cycle

Last week we began talking about the water cycle, particularly as it relates to cloud formation. The students brought up many excellent questions for us to explore as we begin our spring semester on the topic of water. Here are just a few:

·      How are clouds made?
·      How is water made?
·      How does water get clean?
·      How does water evaporate? What is evaporation?
·      What is water vapor?
·      How does water get in the pond?
·      How does water stay in a pond?
·      How and when does rain happen?
·      How can animals live in water?
·      Why do we need water?

I was also thrilled to hear they were thinking about experiments that they want to try:

·      Will snow melt faster in the sun or shade?
·      Where is the best spot to collect the most water?
·      How long does it take water to evaporate?
·      Will snow melt faster than ice?
·      Which melts faster when you add water to it, ice or snow?
·      What melts faster, packed or unpacked snow? 

In order to introduce the water cycle we drew a circular diagram with arrows depicting land with a pool of water (rivers, ocean, lakes), the sun and clouds, and from the clouds, rain drops. The students were already aware of the terms: collection,  precipitation, condensation, and evaporation but not precisely what they meant. So we decided to go through each of the cycle terms together. 

For visualizing evaporation we each put a small squeeze of hand sanitizer onto our hands and talked about how it felt: "wet, squishy, sticky, cold". Then some of us waved our hands through the air to simulate wind and students described how our hands felt: "dry!". They decided that the hand sanitizer had evaporated from their hands. And we talked about how the sun does a similar process, as it warms the water that is collected on the land (in our rivers, oceans, lakes) it causes some portion of the water to evaporate and that means that water vapor - gas that we can't see - is able to rise up into the sky.  So what happens to water vapor as it rises into the atmosphere, I asked them. At a certain level in the sky the air temperature changes (many students were familiar with this thanks to being well-seasoned travelers; having seen water condensation on the windows of an airplane in flight) and at that high altitude, those vapors turn into tiny water droplets (and with dust) collect to form clouds. 

This came back to one of their original questions - How and when does rain happen? We talked about how on some days it is cloudy but also sunny and the shape and color of those clouds may appear fluffy and white, while on other days, clouds may appear quite grey even black.  Intuitively is seemed, they decided that on the grey-cloud days it was more likely to rain. I explained that as the clouds get more and more full of tiny droplets of water, they may appear grey at the bottom, at some point the cloud can't hold anymore water droplets, and it rains (thank you gravity). 

In order to visualize condensation we ventured to carry out by now universally famous, Make a Cloud in a Bottle Experiment. For those who are not familiar, we poured a small amount of warm water into a 2 liter plastic bottle and, in order to produce dust debris, we lit a match and let the particles/smoke fall into the bottle. We quickly closed the bottle off with the cap and began squeezing the bottle in the middle. By adding particles such as smoke and by squeezing the bottle we caused the air pressure to drop, and created a cloud!

Finally we made our way out to a nearby field with our clipboards, paper, and pencils, to look at the sky. We were lucky to find, on that chilly but beautiful winter morning, the sky was absolutely full of clouds. The color and shape of the clouds varied widely across the sky. 

One second grader knew the names of all the different types of clouds and he wrote down every name and definition. Many students actually sketched the clouds they saw, some chose those in the distance while others picked clouds right overhead. 
Here is a Kindergartner working on her sketch.

They made special care of the shading - for grey clouds that looked to be just about to empty on us, they used darkened pencil marks. 

Next week we plan to talk more in depth about gases that we cannot see and we have a very cool experiment that will help us to visualize water vapor.