OPEN FOR DISCUSSION
By Michael Tinnesand
A Healthy Dose of Skepticism
“Adding salt makes pasta cook faster”
“Soda is so acidic it can dissolve a tooth overnight”
This myth got its start from a nutritionist who made the
claim in the 1950s. Sodas contain acids, such as phosphoric, citric, and carbonic acid. But their concentrations
are lower in soda than in natural drinks, such as orange
or cranberry juice. When left in soda, a tooth will not
completely dissolve overnight, or even over a few days. Also, when we
drink soda, we don’t tend to hold it in our mouths for long periods of
time, and the saliva in our mouths helps protect the enamel.
But this does not mean that soda is harmless to teeth. High-sugar
drinks can contribute to tooth decay, and acidic drinks can erode
tooth enamel over time. The reason is that although enamel is hard,
the substance that makes up most of it, hydroxyapatite [Ca5(PO4)3OH],
is in equilibrium with its dissolved form, like any ionic solid in the presence of water. At equilibrium, most of hydroxyapatite is in solid form:
Ca5(PO4)3OH (s) ⇌ 5 Ca2+ (aq) + 3 PO43– (aq) + OH– (aq)
But when an acid is added, its free hydrogen ions (H+) neutralize
some of the hydroxyde ions (OH–), as follows:
H+ + OH– ➞ H2O
This shifts the equilibrium to the right, causing more hydroxyapatite
to dissolve, thus dissolving the tooth enamel.
“Swallowed gum stays in your stomach for
seven years”
Many cooks believe that adding
salt to water will make their pasta
cook faster. There is a good basis
for this, at least in theory. Adding
salt, or any nonvolatile solute, to
water will raise the boiling point.
This is called a colligative
property. Hotter water will cook
foods, including pasta, faster.
The problem is that the typically
small amount of salt added to
pasta water makes little difference in the boiling temperature.
You would need to add a lot of
salt to make much of a difference, which would likely make
the pasta inedible.
Interested in more topics that are open
for discussion? Check out additional
“Open for Discussion” articles at:
www.acs.org/chemmatters
(under “Open for Discussion”)
4 Chemmatters | FEBRUARY/MARCH 2015
It seems logical to believe this
myth based on our own experience with chewing gum. Take a
stick of gum, and you can chew
it for hours, with little change in
its texture, although the flavor
tends to fade. This is because
while the gum resins are indigestible, the sugar and flavorants
are both soluble and digestible.
If the gum is swallowed, it
flows through your stomach
and intestine with the digestive
fluids, bile, and other undigested foods. Your stomach and intestines are smooth, and there is no
real way for gum to get stuck in your stomach. Under your school desk,
maybe. In your stomach, no.
www.acs.org/chemmatters
jack spellman; shutterstock
Being thoughtful about what we do and why we do it is essential in the
study of science. You may have heard of widely believed facts that seem
to make sense, but then, on second thought, you realize that such facts
do not hold up to scrutiny. This thoughtfulness is sometimes referred to
as “healthy skepticism,” or the ability to know whether an explanation
makes sense, based on the evidence observed. Here are three examples
of widely believed facts which are not borne out by the evidence (and
may surprise you!)