$A$ change in the physical state can be brought about:

Which of the following represents $1 \, u$?

Give one example each of:
$(a)$ Triatomic element
$(b)$ Monoatomic molecule
$(c)$ Polyatomic element
$(d)$ Polyatomic ion

Find the ratio by mass of the combining elements in the following compounds. (You may use Appendix $-III$)
$(a)$ $CaCO_{3}$ $(b)$ $MgCl_{2}$
$(c)$ $H_{2}SO_{4}$ $(d)$ $C_{2}H_{5}OH$
$(e)$ $NH_{3}$ $(f)$ $Ca(OH)_{2}$

$(a)$ In this crossword puzzle (Fig),names of $11$ elements are hidden. Symbols of these are given below. Complete the puzzle.
$1. Cl$
$2. H$
$3. Ar$
$4. O$
$5. Xe$
$6. N$
$7. He$
$8. F$
$9. Kr$
$10. Rn$
$11. Ne$
$(b)$ Identify the total number of inert gases,their names,and symbols from this crossword puzzle.

The chemical formula of the oxide of an element $X$ is $X_2O_5$. Write the chemical formula of its chloride.