Video Transcript
Calcium carbonate will produce a
fizz of carbon dioxide when added to lemon juice. Based on this observation, what can
be said about the pH of lemon juice?
Calcium carbonate is an example of
a metal carbonate, and the question tells us that mixing calcium carbonate and lemon
juice produces carbon dioxide. This might remind you of the
classic reaction of a metal carbonate with an acid, which produces a metal salt,
carbon dioxide, and water. There are no observations related
to the production of the metal salts or the water because they stay in the
liquid. But we do see bubbles of carbon
dioxide gas. So the question is pointing us
towards the conclusion that lemon juice is acidic.
pH is the measure of acidity of a
solution. Pure water at 25 degrees Celsius
has a pH of seven and is considered neutral. A solution with a pH of zero is
very acidic. And a solution with pH 14 is very
basic. In order to react with a carbonate,
lemon juice must have a pH less than seven and therefore be acidic. Therefore, since lemon juice does
react with calcium carbonate as confirmed by the production of carbon dioxide, we
know that the pH of lemon juice must be less than seven. With only this observation, this is
as precise as we can be.
In practice, we’d expect the pH of
lemon juice to be meaningfully below seven because producing carbon dioxide at a
visible rate will require a relatively high acidity. For now, less than seven is the
best that we can do.
