Water and Surface Tension
Brooklyn International High School
Summer Research Program for Science Teachers
Note to Teachers: This is a good lesson for biology, chemistry, physical science, and environmental science. I suggest teachers start by introducing molecular kinetic theory so students have a solid understanding that matter is made of particles (atoms and molecules). It's useful to use water to help them understand this concept further. This is a simple and inexpensive lesson and lab.
Aim: What’s so special about water?
Do Now:(I have a beaker of water boiling at the front of the room, and I ask them): What are the bubbles coming off the beaker?
After students complete the Do Now ask them questions about what we did in the previous one to two classes:
· What is an atom?
· What is a molecule?
· What is an example of a molecule?
· What keeps atoms together?
· What is a covalent bond? Give an example.
· What is an ionic bond? Give an example.
· What do we use salt for?
· What is the chemical formula for salt?
· What happens when you put salt in water?
· Is that a physical or a chemical change? Why?
· How can you isolate salt from water?
Chemical and Physical Changes: I want to go over chemical and physical changes in relation to bonding. When a physical change happens, the molecule remains the same. (Draw water, for example: ice, liquid, gas, still water). But when a chemical change happens (e.g., CH3COOH and Na(CO3)2). When you combine these two molecules, you get a different molecule. How does that happen? Well the bonds have to break. What causes that to happen? Energy.
Mystery letter (Note: Find a picture of a water molecule online and write “You depend on me. I help you exist,” on the other side. Then make copies of it. Students love the sense of mystery): Tell students when I woke up in the morning I had a strange piece of paper slid under my door. It said, “You depend on me. I help you exist.” It was a bit scary. I don’t know how someone could have gotten in. What do you think it could be? (I hand out the sheet). Some students will immediately say water. Water? How do you know? Which one is the oxygen and which one is the hydrogen? Okay. So what are these positive and negative signs for? Students will say that the positive and negative charegs attract one another.
They do? Oh right. What causes something to be more positive or negative? In a previous class we defined a covalent bond as a bond in which two atoms share electrons. We said water is covalently bonded. How could there be a charge?
It’s covalent. But there’s more to water than meets the eye. In water, it’s true the oxygen and the hydrogen share electrons. However, it turns out there is a slight inequality.
Think of it this way. Two people are sleeping in the same bed. It’s winter. They have a nice electric blanket that keeps them warm. But the blanket just barely covers them. The bigger person needs more of the blanket, so they take more of it than the little person. This is like the relationship between oxygen and hydrogen in a water molecule. The oxygen tugs at the electrons more.
What do you think happens when the oxygen tugs at the electrons a bit more? Think in terms of overall charge. The oxygen becomes slightly negative while the hydrogen becomes slightly positive. (Draw it on board).
Electronegativity is the tendency for one atom to pull the electrons of another atom. Which atom in oxygen is more electronegative? Oxygen. Good. Oxygen is electronegative. It is like the person in bed who tugs more at the blanket.
So what do you think this means in terms of a bunch of water molecules together? What will happen in terms of attraction? Yes, the hydrogen atoms in one water molecule are attracted to the oxygen atom of another molecule. So the result is water molecules are attracted to other water molecules.
The water molecules form weak bonds with one another. These are called Hydrogen bonds.
We are going to look at an important property of water called surface tension in a lab today. When we’re done, we’ll come back and talk about what you discovered, and what that means in terms of how water helps sustain life on earth.
The tendency of molecules to stick together is called cohesion, which is stronger for water than for most other liquids. Cohesion is very important in plants. Trees depend on cohesion to help transport water from their roots to their leaves. Cohesion helps water rise against the force of gravity.
Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid. Hydrogen bonds give water an unusually high surface tension, making it behave as though it were coated with an invisible film.
Break students into groups and start lab using the worksheet (Willy the Water Strider) (PDF file).
Each Group Gets:
· 1 dropper
· 1 small beaker with detergent
· 1 paper with pepper on top
· 50 pennies
· Sheet of wax paper
· 2 needles
· (Make sure paper towel is out)
· Clear plastic cup
· Beaker with alcohol
After the lab review the questions on the back of the worksheet.
Discuss with students about the importance of surface tension, and it’s effect on life. For example, water striders are able to walk on water because of water’s surface tension. Plants rely on the surface tension to transport water inside themselves.
National Science Standards:
Content Standard B: All students should develop an understanding of structure of atoms, structure and properties of matter.
State Science Standards:
Scientific Inquiry (Commencement Level)
1-1. The central purpose of scientific inquiry is to develop explanations of natural phenomena in a continuing, creative process.
1-2. Beyond the use of reasoning and consensus, scientific inquiry involves the testing of proposed explanations involving the use of conventional techniques and procedures and usually requiring considerable ingenuity.
1-3. The observations made while testing proposed explanations, when analyzed using conventional and invented methods, provide new insights into phenomena.
Physical Setting (Commencement Level)
1-3. Matter is made up of particles whose properties determine the observable characteristics of matter and its reactivity.