Objective

The goal of this project is to investigate how added salt and added detergent affect the surface tension of water.

Introduction

Water molecules—good old H₂O—are made of one oxygen and two hydrogen atoms. The single oxygen and two hydrogen atoms are held together because they share electrons—this is called a covalent bond. The hydrogen atoms don't line up on opposite sides of the oxygen atom, as you might think. Instead they are at an angle of about 105° (if they were on opposite sides of the oxygen atom the angle would, of course, be 180°).

The oxygen atom tends to hold on to the shared electrons from the hydrogen atoms more tightly, so each end of the water molecule ends up with a partial charge. The oxygen portion of the molecule has a partial negative charge, and the hydrogen ends of the molecule have a partial positive charge. Another way of talking about the partial charges is to say that water molecules are polarized. Like a magnet, with a north and south pole, a water molecule has electrical poles. The oxygen atom is the negative pole, and each hydrogen atom is a positive pole.

These partial charges cause water molecules to interact with one another. Because opposite charges attract, water molecules tend to 'stick' to one another. The partial positive charges of the hydrogen atoms tend to align themselves with the partial negative charge of the oxygen atoms of neighboring water molecules. You can see models of this alignment in several of the references in the Bibliography section (Wiseth, date unknown; Hipschman, 1995a; Kimball, 2006). This tendency of water molecules to stick together due to the partial positive and negative charges is called hydrogen bonding.

Hydrogen bonding between water molecules leads to many interesting consequences at the visible, macroscopic level. For example: the boiling point of water, its surface tension, and it's ability to dissolve salts are all related to hydrogen bonding.

The boiling point of water, 100°C, is unusually high for a molecule with such a low molecular weight. The boiling point is so high due to hydrogen bonding. On average, each water molecule interacts with about four others (each hydrogen atom interacts with the oxygen atom of separate water molecules, and each oxygen atom interacts with the hydrogen atoms of two more water molecules). In water vapor, the molecules are too far apart for hydrogen bonding to occur, so boiling water means breaking up all of the hydrogen bonds in liquid water. Breaking those bonds takes energy, thus the high boiling point for water.

Hydrogen bonds also give liquid water a high surface tension. The water molecules on the surface have partners for hydrogen bonding only within the liquid; above the water surface there are no more molecules available for hydrogen bonding. This means that molecules at the surface experience a net force pulling them inward. If you fill a glass right up to the rim and then carefully add a few more drops of water, you can see that the glass can be overfilled without spilling. The surface tension of the water holds on to the 'extra' water as if there were a skin on the surface of the water.

Water is an excellent solvent for charged (polar) molecules like table salt, NaCl. In water, salt dissociates into positively charged sodium (Na⁺) and negatively charged chloride (Cl⁻) ions. The partial positive charge of the hydrogen ends of the water molecules surround the negatively charged chloride ions, and the partial negative charge of the oxygen ends of the water molecules surround the positively charged sodium ions. What effect will dissolved salt ions have on hydrogen bonds between water molecules?

Water behaves very differently when mixed with uncharged (nonpolar) molecules. An example of a nonpolar molecule is cooking oil. You may have heard the saying "oil and water don't mix," and this is why. Oil molecules are uncharged. Water molecules, as you have learned, are partially charged. The uncharged oil molecules disrupt the hydrogen bonding between water molecules. So when you try to mix oil and water, the oil ends up forming droplets within the water. The nonpolar oil molecules stick together and the polar water molecules stick together. Eventually, you get two layers, with the less dense oil floating on top of the denser water.

Nonpolar substances are sometimes called 'hydrophobic' (meaning 'water fearing'), and polar molecules are sometimes called 'hydrophilic' (meaning 'water loving') because of the two different interactions illustrated by salt and cooking oil.

Liquid detergents have dual properties. One end of the molecule is oily, and the other end is charged. In water, the oily ends of detergent molecules stick together, with the charged ends sticking out, into the water. Detergents can form small blobs in water (called micelles) and can also disperse, like oils, into a layer on the surface of the water (for illustrations, see Hipschman, 1995b). How do you think added detergent will affect the surface tension of water?

One way to find out is to count how many drops of water you can 'pile up' on top of a single penny. The Experimental Procedure section shows you how to do this with plain water, salt water, and water with detergent.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• surface tension,
• chemical structure of water,
• covalent bond,
• hydrogen bonds,
• polar solvent,
• non-polar solvent,
• hydrophobic,
• hydrophilic.