Why are fine-grained structures appear to be harder than coarse grained structure?

The smaller the grain size, the more frequent is the pile up of dislocations. With decrease in grain size, the mean distance of a dislocation can travel decreases, and soon starts pile up of dislocations at grain boundaries. This leads to increase in yield strength of the material.

Why vinyl alcohol tautomerizes to acetaldehyde?

Vinyl alcohol rearranges slightly under ordinary conditions. It tautomerizes to the more stable species acetaldehyde.

Explain how emulsion is different from suspension?

Emulsions are a mixture of two or more liquids that are usually immiscible. Emulsions do not form spontaneously since they are unstable; they form upon shaking, stirring, or mixing by any means. The droplets in an emulsion can get together and form larger droplets when mixing like this. An emulsifier can be added to this to increase the stability. Surfactants can act as emulsifiers thus, increase the kinetic stability of the emulsion. Suspension is a heterogeneous mixture of substances; there are two components in a suspension, the dispersed material and the dispersion medium. There are larger solid particles (dispersed material) distributed in a dispersion medium. The medium can be a liquid, gas or a solid. If the suspension is allowed to stand still for some time, the particles can be settled down to the bottom.