Identify the term or substance based on the descriptions given below:

Identify the term or substance based on the descriptions given below:

(i) Ice like crystals formed on cooling an organic acid sufficiently.

(ii) Hydrocarbon containing a triple bond used for welding purposes.

(iii) The property by virtue of which the compound has the same molecular formula but different structural formulae.

(iv) The compound formed where two alkyl groups are linked by $\overset{\underset{\text{||}}{\text{O}}}{\text{-C-}}$ group.