Chemical Science Lesson Plan

Title: Hydrogen Bonding and Surface Tension

Grade Level: 2-5

Prepared by: Grace Choe
Science background for teachers

Introduction

Activity procedure

Materials

Summary and discussion


Objectives:
Students will observe the affects of hydrogen bonding on surface tension. They will see that certain substances interact strongly with water and reduce the interaction between water molecules.

Science background for teachers

A water molecule is comprised of one oxygen atom with two hydrogen atoms bonded and has the molecular formula H2O. The oxygen and hydrogen atoms share electrons, but since oxygen atom is more electronegative, which means it has a stronger pull on the electrons than hydrogen atoms, the electrons spend more time near the oxygen atom. This results in a partial negative charge around the oxygen atom while having a partial positive charge on the hydrogen atoms.


Polarity is a term to describe the separation of electric charge leading to a molecule having an electric dipole. It results from the uneven partial charge distribution between atoms in a molecule. Atoms such as nitrogen, oxygen, and halogens, tend to have partial negative charges because they are more electronegative, the atom’s ability to attract electrons toward itself.

In a compound like water, electrons are not shared equally between two bonded atoms, hydrogen and oxygen, therefore resulting in partial positive and negative charges. This separation of the charges creates a dipole, meaning two poles. Thus, when a molecule contains polar bonds as such, the molecule is said to be polar.

This is why a water molecule is polar, and this uneven distribution of electron density enables the oxygen from one water molecule to attract to the hydrogen atom of another and form bonds called hydrogen bonds.



When many water molecules form hydrogen bonds with other water molecules, they form a lattice of water molecules, which is strong and flexible. This creates a high surface tension. Surface tension is what allows water striders to walk across the surface of water. The water molecules at the surface do not have other water molecules to form hydrogen bonds like other water molecules that hydrogen bond with other water molecules on all sides, so they attract more strongly to the water molecules around them. In other words, water tends to have the smallest surface possible in order to get more stable by trying to be in the interior of the liquid. The surface tension creates a surface film, and it is responsible for bubbles, water droplets, and capillary action.

The forces experienced by a water molecule at the surface and on in the bulk liquid are different, and this is how surface tension works. A water molecule at the surface is attracted to its neighboring water molecules including the ones that are below and to either sides, but there are no other water molecules that it can interact above the surface. As a result, this molecule at the surface tends to be pulled into the bulk of the liquid and thus minimizes the surface area of a liquid.



A detergent, on the other hand, destroys the surface tension of water. Soap is a molecule that has one end polar with the other end non-polar, which means one end attracts to water and the other end is attracted to oil or grease, and this is how soap makes oil and water to mix and clean grease off of dish. This occurs because detergent literally interferes physically with water molecules and weakens hydrogen bonds between them.

The detergent molecules arrange themselves to form a ball-shaped cages, called micelles, in water. The "water-loving" outside of the micelle interacts strongly with water and this reduces the water-water interaction at the surface. Other substances that interact strongly with water have the same effect on surface tension.

Introduction to the Activity

Water molecules are the smallest bits of stuff that behave like water. In water vapor, or steam, there are individual water molecules that move around by themselves. In liquid water though, the water molecules are connected strongly to other water molecules in all directions. At the surface of some water, water is connected to other water molecules in the liquid and along the surface but there can't be any connections above the surface.

Do you think this makes water at the surface behave differently than water in the middle of the liquid?

What does detergent do to water at a surface?

Today we're going to explore some properties of water at surfaces. It is important to watch and make careful observations on your lab sheet.

Materials

water plastic wrap drinking straw
dish soap salt, pepper sugar
plastic cup 50-60 pennies talcum powder


Activity procedures

Download lab sheet here.


Activity 1

Fill a cup all the way to the top with water.

What do you think would happen if you were to add pennies to it?

Try adding pennies one at a time. What happens to the water in the cup?

How many pennies can you add without causing the water to overflow?

Activity 2

  1. Take some water with a straw and put a few drops on plastic wrap.
      (a) What is the shape of the drop?
      (b) Move a drop around with your straw. Does the drop change?

  2. Move one of the drops close to another one with your straw. What happens when 2 drops meet?

  3. Put a small amount of one of the solids (salt, pepper, sugar, talc) on one of the drops. Does the shape change?

  4. Try this again with another of the solids.



Activity 3

  • Put some water in your cup.

  • Sprinkle black pepper all over the surface. What does the pepper do? Record your observations.

  • Add a drop of dish soap to the water. What does the pepper do? Record your observations.

Summary and discussion

  1. Why does water form drops rather than spreading out?

    (The teacher can illustrate that a drop has less surface area than other shapes by putting some water in a ziploc bag. Lying flat, spread out, the whole bag is at the surface of the water. Holding the bag up so the water is more drop-like, only part of the bag is at the water surface. A drop has the smallest surface area.)

  2. Why is it better for water to have the smallest possible surface area?

    (In the middle of liquid water, all the water molecules can connect to other water molecules in all directions. At the surface, connections in one direction are missing.)

  3. What affects the surface of water: salt, sugar, pepper, talcum powerder, dish soap? Why?

    (The substances that connect to water molecules like water connects to itself can make the water-water connections on the surface weaker.)


EnLIST Chemistry Workshop, University of Illinois, 2010