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.
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.
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.
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.
Activity 1Fill 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?