When two or more compounds differ in position of substituent atom or functional group on the carbon skeleton, they are position isomers. In the given structures, I and II; III and IV, and VI and VII are position isomers.

When two or more compounds have similar molecular formula but different skeletons, these are referred to as chain isomer.

In the following structure

On adding dilute $\mathrm{H}_2 \mathrm{SO}_4$ for testing halogens in an organic compound with $\mathrm{AgNO}_3$, white precipitate of $\mathrm{Ag}_2 \mathrm{SO}_4$ is formed. This will interfere with the test of chlorine and this $\mathrm{Ag}_2 \mathrm{SO}_4$ may be mistaken for white precipitate of chlorine as AgCl . Hence, dilute $\mathrm{HNO}_3$ is used instead of dilute $\mathrm{H}_2 \mathrm{SO}_4$.

The given structure is of allene $\left(\mathrm{C}_3 \mathrm{H}_4\right)$

$$\mathrm{H}_2 \stackrel{1}{\mathrm{C}}=\stackrel{2}{\mathrm{C}}=\stackrel{3}{\mathrm{C}} \mathrm{H}_2$$

In allene, carbon atoms 1 and 3 are $s p^2$-hybridised as each one of them is joined by a double bond. And, carbon atom 2 is $s p$-hybridised as it has two double bonds at each of its side. Therefore, the two $\pi$-bonds are perpendicular to each other, in allene, as shown below.

$\mathrm{H}_a$ and $\mathrm{H}_b$ lie in the plane of paper while $\mathrm{H}_c$ and $\mathrm{H}_d$ lie in a plane perpendicular to the plane of the paper. Hence, the allene molecule as a whole is non-planar.

Electronegativity of carbon atom, also depends on the hybridisation of the carbon atom. Since, $s$-electrons are more strongly attracted by the nucleus than $p$-electrons, thus, electronegativity increases with increase in s-character of the hybridised orbital i.e.,

Thus, $s p$-hybridised carbon is the most electronegative carbon.