English II
Page 53
Form 001
50° Celsius (41°, 59°, 68°, 95° and 122° Fahrenheit). Nathan
avoided water at 0 °C (32 °F) since ice doesn’t drip well. He
also avoided boiling water, which could get dangerous. The teen
dripped water at each temperature and weighed it to determine
the size of his drops, running each temperature test three times.
9 As the temperature of the water increased, the size of the
drops decreased, which suggested the surface tension was also
decreasing. Nathan concluded that cleaning a dish with hot
water is probably more efcient than cold.
10 Next, he tested soap—or rather a main ingredient in it, sodium
lauryl sulfoacetate (or SLSA). This chemical is a surfactant—a
chemical that decreases the attraction between water molecules,
which lowers surface tension. Nathan tested pure water, water
with 0.01375 percent, 0.01275 percent, 0.055 percent,
0.1 percent, 0.2 percent and 0.5 percent SLSA. (All were held
at the same temperature of 20 °C.) He dripped and weighed his
mixtures again.
11 The addition of the surfactant decreased surface tension more
than simply heating water, Nathan found. After reading more
about the chemical, the teen discovered that SLSA is a chemical
with two ends. One is hydrophobic—it is repelled by water. The
other is hydrophilic, or attracted to water. When SLSA with its
two ends is placed in water, the molecules of the chemical line
up at the water’s surface. Their hydrophobic heads stick out into
the air while their hydrophilic tails stay comfortably in the water.
This lm of molecules means water can’t form its usual bonds at
the surface. The whole mixture has lower surface tension.
12 At high concentrations of SLSA, something else happens. The
surface of the water is lled with the chemical, and the rest
of the molecules are stuck under the water. There they form
clumps called micelles. These clumps are very good for cleaning
up oil, Nathan notes. Oil is also hydrophobic, and a micelle
isolates the oil from the water.
13 Finally, Nathan wanted to see what would happen if he added
regular old table salt. He thought that salt might decrease
surface tension, because adding the tiny molecules to the water
would stop the water bonding to itself at the surface. But he
found that salt made very little difference.