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Oxygen in Water: A Bubbly Science

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Author: Tricia Edgar

It doesn't take a degree in chemistry to know that water is made up of two elements: hydrogen and water. Sure, you're familiar with that famous formula, but did you know that some types of water can have more oxygen than others? Use a nifty test kit to discover the amounts of oxygen in water that is sitting and running. Where do you think you'll find the most oxygen?

Problem:

Does sitting or running water have higher levels of oxygen?

Materials:

  • Measuring cup
  • Dissolved oxygen test kit
  • Water
  • 2 dishpans
  • Notebook
  • Pencil

Procedure:

  1. Create a hypothesis, your best guess about what is going to happen. Do you think that still water, slow-moving water, or bubbling water will have the most oxygen? Why?
  2. Fill up two dishpans with water. Let them sit overnight.
  3. The next morning, collect your dissolved oxygen test kit and other supplies. Take a sample from the water that has been sitting in the dish pan overnight.
  4. Think about the test you're about to conduct and the problem question: Does sitting or running water have higher levels of oxygen? Write down your guess, often called a hypothesis, in your notebook.
  5. Next, carefully read the instructions that come along with your dissolved oxygen test kit. Testing oxygen levels is not too difficult, but smart scientists always make sure they're following the instructions correctly. A skipped step can result in false conclusions!
  6. Use the kit to test the oxygen in your sample.
  7. Record the results in your notebook.
  8. Now use the measuring cup to pour water from one dishpan into the other. Do this for five minutes before taking a water sample from the second dishpan.
  9. Quickly use the dissolved oxygen test kit to determine the oxygen level.
  10. Once again, make sure you record the results.
  11. Look over your findings. Was your hypothesis correct?

Results:

The running water should have higher levels of oxygen than the sitting water. 

Why?

Isn't water just water? How does more oxygen get into running water than sitting water? To understand this, let's take a look at the different ways oxygen is able to mix with water. First up, we have photosynthesis, or the process that allows plants to make food. Using photosynthesis, plants take in carbon dioxide and sunlight while releasing oxygen. Plants like algae, that are underwater, release this oxygen directly into the water.

However, in your experiment, no algae plants were present. That means we need to move on the second way oxygen can get into water. Sometimes oxygen in the air mixes with water. This can happen when there is an increased amount of movement -- specifically in running water! Think of the molecules in water and in air as marbles. The air molecules can be tiny white marbles, and the water molecules larger blue marbles. If you tried to pour the blue marbles through a layer of white marbles, some of marbles would get mixed. That same exact thing happens with running water. Since molecules like to move from an area of higher concentration, which describes how tightly packed in molecules are, to an area of lower concetration, some of the air molecules transfer into the water.

How do you think plants and sea creatures "breathe" underwater? Try using your dissolved oxygen test kit to see if water with living organisms in them have higher amounts of oxygen than water without. You can test sea water or even water from an aquarium -- or somewhere entirely different. Just don't let your oxygen exploration stop now! Continuing to guess and test scientific theories is what being a scientist is all about.

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