The reaction between zinc and hydrochloric acid is Zn + HCl = H2 + ZnCl2. It is a single replacement reaction where zinc metal displaces the hydrogen to form hydrogen gas and zinc chloride, a salt.
Zinc reacts quickly with the acid to form bubbles of hydrogen. Carrying out the experiment in a closed sidearm flask allows collection of the gas in a balloon. Exposing the balloon to the heat of a lighted candle causes a rapid reaction, as the hydrogen reacts with oxygen to form water.
The zinc and hydrochloric acid reaction is an example of oxidation-reduction. The zinc metal begins the process as a pure metal with an ionic charge of zero and loses an electron as it reacts with the acid to become a positively charged zinc ion through oxidation. At the same time, hydrogen undergoes reduction by gaining an electron to become a neutral molecule.
Zinc is an important nutrient for animals and crops, including rice. In an application of the zinc and hydrochloric acid experiment, scientists have suggested adding weak hydrochloric acid to irrigation water. The acid reacts with zinc metal, found in the soil, to form zinc ions in a type the plant is able to use to improve crop yields.