Question

On heating, the colour of zinc oxide changes from white to yellow. This is a/an:
a.) physical change
b.) chemical change
c.) photochemical change
d.) irreversible change

Answer 1

Hint: Zinc oxide is a chemical compound that is white in colour. When it is heated at high temperatures, it does not decompose to form new compounds, rather it just changes its colour.

Complete answer:
A chemical change is a change in which one chemical substance is said to have transformed into one or more other chemical substances. These changes occur through chemical reactions, in which the molecules and the ions rearrange themselves to form different structures and thus have different properties.
They are mostly irreversible reactions but some changes are said to be reversible in nature.
A physical change is a change in which the changes affect the form of a chemical substance but does not change its chemical composition. These changes are used to separate mixtures into their constituent compound but it does not separate compounds into chemical elements or simpler compounds.
A physical change is a change that is subjected to the physical properties of a substance.
Photochemical reaction, a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules’ absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules.
In an irreversible reaction, the reactants react to form the products, which cannot revert back into reactants or in simpler words the chemical reactions in which the products formed do not combine to give the reactants are known as irreversible reactions.
Now, in the reaction given, zinc oxide is a white powder and when it is heated strongly (above 800 degree Celsius) it turns yellow, but again after cooling down, it changes back again into white color. The hot zinc oxide absorbs some of the blue wavelengths of light making the reflected light more yellow. Also, when it is heated at high temperatures a very minute amount of oxygen evaporates from the lattice. Hence, it is a reversible chemical change.
So, the correct answer is “Option B”.

Note: There are basic differences between a chemical change and a physical change.
For a chemical change, few things need to be kept in mind.
● Colour change
● Formation of a precipitate
● Formation of a gas or gas bubbles
● Odor or taste change
● Volume change
● Production of light and many such changes.

For a physical change, few things need to be kept in mind.
● Change in size and volume
● Change in density and mass
● Change in pressure
● Change in temperature
● Change in color
● Change in texture and flexibility