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These liquids should be placed at the same time on a surface like a brown paper towel so that students can tell when each liquid evaporates. The dark spot on the paper towel made by the alcohol will turn lighter faster than the dark spot made by the water.
This indicates that the alcohol evaporates more quickly than the water. Read more about counting molecules in the teacher background section. Note : This test is fine for middle school students but there is something about the test that does not make it completely fair. There are many more water molecules in a drop of water than alcohol molecules in a drop of alcohol.
The test would be more fair if the same number of water and alcohol molecules are placed on the paper towel. Determining the number of particles in a sample is a basic concept in chemistry, but is beyond the scope of a middle school chemistry unit.
Even if the same number of water and alcohol molecules were used in this activity, the alcohol would evaporate faster. Project the image Water and Alcohol Molecules. Tell students that understanding about polarity can help explain why water evaporates more slowly than alcohol. Remind students that the oxygen-hydrogen O—H bonds in water make it a polar molecule. This polarity makes water molecules attracted to each other.
Explain that the oxygen-hydrogen O—H bond in the alcohol molecule is also polar. But, the carbon-hydrogen C—H bonds in the rest of the alcohol molecule are nonpolar. In these bonds, the electrons are shared more or less evenly. Because there are both polar and nonpolar areas on the alcohol molecule, they are somewhat less attracted to each other than water molecules are to each other.
This makes it easier for alcohol molecules to come apart and move into the air as a gas. This is why alcohol evaporates faster than water. You know that water and alcohol have different characteristics because of the molecules they are made of and how these molecules interact with each other.
Project the image Water and Alcohol Boiling. The American Chemical Society is dedicated to improving lives through Chemistry. Skip Navigation. Lesson 5. Project the video Water Balloon. Project the video Water Fountain. Explain Show molecular model animations that illustrate why water molecules are attracted to each other. First Frame of the Animation Electrons are shared between atoms in a covalent bond. Remind students how the shared electrons in a water molecule are attracted to the protons in both the oxygen and the hydrogen atoms.
These attractions hold the atoms together. Water molecules are neutral. Be sure students realize that no protons or electrons are gained or lost. The water molecule has a total of 10 protons and 10 electrons 8 from the oxygen atom and 1 from each of the two hydrogen atoms.
Since it has the same number of protons and electrons, the water molecule is neutral. Tell students that another way to see the difference in where the electrons are is by using the electron cloud model.
Unequal sharing of electrons makes water a polar molecule. Tell students that the oxygen atom attracts electrons a little more strongly than hydrogen does. So even though the electrons from each atom are attracted by both the oxygen and the hydrogen, the electrons are a bit more attracted to the oxygen.
This means that electrons spend a bit more time at the oxygen end of the molecule. This makes the oxygen end of the molecule slightly negative. Since the electrons are not near the hydrogen end as much, that end is slightly positive. Video from: Noel Pauller. Why does anion of oxygen have a charge of negative 2? Oct 7, Oxygen ions have a charge of Explanation: Oxygen atoms have 8 protons and 8 electrons.
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