Question

How will you distinguish between $1,3$ -butadiene and But-$1$-yne?

Answer 1

The organic molecule $1,3$ -butadiene has the formula ${(C{H_2} = CH)_2}$ . It's a colourless gas that condenses quickly into a liquid. It is used as a predecessor to synthetic rubber in industry. The molecule is made up of two vinyl groups joined together. It's the most basic conjugated diene there is.
1-Butyne has the chemical formula $C{H_2}C{H_2}C{H_3}$ and is an organic molecule. It's a colourless and flammable gas. Alkyne metathesis, hydrogenation, and condensation with formaldehyde are all processes that 1-butyne participates in. It is significantly less stable than its isomer 2-butyne in terms of heat of combustion.

Complete answer:
But-1-yne is a terminal alkyne. Therefore, it gives white ppt. of silver but-1-ynide with Tollen's reagent and red ppt. of copper but-1-ynide with ammonical cuprous chloride.
$C{H_3}C{H_2}C \equiv CH$ $+$ $[Ag{(N{H_3})_2}]$ $^ +$ $+$ $OH$ $^ -$ $\to$ $C{H_3}C{H_2}C \equiv C - Ag$ $+$ $2N{H_3}$ $+$ ${H_2}O$
But-$1$ -yne Tollen's reagent silver-But-$1$ -ynide
$C{H_3}C{H_2}C \equiv CH$ $+$ $2[Cu{(N{H_3})_2}]$ $O{H^ - }$ $\to$ $C{H_3}C{H_2}C \equiv C - Cu$ $+$ $2N{H_3}$ $+$ ${H_2}O$
But-$1$ -yne Diammine copper $(I)$ hydroxide Copper but -$1$ -ynide (Red ppt)
But-2-yne, on the other hand , being a non-terminal alkyne does not give these reactions.

Additional information:
Isomerism occurs when distinct atoms in an organic molecule are coupled to each other with different types of bonds. A kind of structural isomerism is functional isomerism. There are five further types of structure besides functional isomerism. Chain isomerism occurs when two compounds have the same chemical formula but distinct branching architectures.

Note:
Keep in mind that when two compounds have the same chemical formula but different functional groups, they are functional isomers. Position isomerism occurs when the position of the double/triple bond differs while the chemical formula remains the same.