Acid

These lemons are a form of acid because they hold a high acidity content level.

An acid would be considered any compound that contains either hydrogen ions or hydronium ions when it is dissolved in water. Acids can be measured using the pH scale to determine how concentrated or diluted the acid is. A strong acid with the pH level of 1 or 0 would be a substance such as battery acid. A weaker acidity such as a level 6 acid on the pH scale would be found in normal range precipitation. The range of acidity levels on the pH scale are measured from 0 to 6, 0 being the strongest level of acidity.

Sources:
Ophardt, E. Charles. Acids and Bases. elmhurst.edu. Retrieved August 24, 2011, from http://www.elmhurst.edu/~chm/vchembook/184ph.html