Question

The cells used in a wrist watch undergo:
A. physical change
B. periodic change
C. chemical change
D. none of the above

Answer 1

We know that a physical change is the change in which no new products formed. Only the state, size, shape etc. got changed in a physical change. One common example of physical change is melting of ice, when the ice is heated it changes its state to liquid state (water) but the composition of water remains same, that is, ${{\rm{H}}_{\rm{2}}}{\rm{O}}$.

Complete step by step answer:
Now, we will discuss the function of cells. The conversion of chemical energy into electrical energy is done by the chemical cells. Cells of different compositions are made in similar size and are interchangeable mechanically. But, the composition of cells affects the voltage stability and service life. Moreover, the use of the wrong cell results in improper operation or short life.

Now, come to the question. The question is about the cells used in wrist watches. We know that the cells used in wrist watches are button cells. These cells are single cell batteries whose shape is like a button of shirts. They are small and have a long life. Some examples of button cells are silver oxide cell and mercury oxide cell. Mercury oxide and silver oxide in their respective cells behaves as cathode and zinc acts as anode in both the cells. The chemical reactions undergo by these cells are shown as below:

Anode: ${\rm{Zn}} \to {\rm{Z}}{{\rm{n}}^{2 + }} + 2{e^ - }$

Cathode: ${\rm{Mn}}{{\rm{O}}_{\rm{2}}} + 2{e^ - } \to {\rm{M}}{{\rm{n}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}} + 2{\rm{O}}{{\rm{H}}^ - }$

So, we can say that button cells undergo chemical change.

So, the correct answer is Option C .

Note:
Remember that, in a chemical change, new products formed which possess completely different properties (physical or chemical) than the original compound. A chemical change is accompanied by different observable changes such as evolution of gas, colour change of products, releases of heat etc.