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