expansion and contraction of metals is utilised.
W Chemical changes are irreversible. Justify the statement by giving two examples.
A Give an example of a change caused by mixing of things.
(d) Why melting of hutter is said to be anhyical and reversible change?
Answers
Answer:
(A) Expansion or contraction of metals on heating or cooling (1) Physical Change
Metals expand on heating and contract on cooling. During the expansion or contraction, the chemical properties of the metal remains unchanged. In both processes, the material remains the same and no chemical change or reaction takes place.
The only change is the intermolecular spacing which is a physical change.
(B) Action of heat on zinc hydroxide (3) Combination of physical and chemical change
When zinc hydroxide is heated, it leads to the formation of zinc oxide and water
Zn(OH)
2
→ZnO+H
2
O
It is a chemical change as well as a physical change. When zinc hydroxide is heated, it changes from colorless to yellow. Heat changes the chemical composition of the substance thus giving a yellow color. But simultaneously on cooling, it changes back to colorless.
Thus, it is also reversible reaction. Therefore, physical change.
The color changes from colorless to yellow. Therefore, chemical change.
(C) A stone placed under sunlight. (4) Neither a physical nor a chemical change.
When a stone is placed under sunlight, no change occurs.
(D) Shaping of glass by heating (1) Physical Change
When a glass is heated, there are changes in the physical state, since the shape of the glass changes making it a physical change.