Why did the needle float easier on the water than on alcohol?

Answer 1

Water has a much stronger surface tension than alcohol does. Because water molecules are highly polar, they have a relatively strong attraction for each other (as compared to the attraction that the molecules of most other liquids have for each other) and this creates a kind of limited solidity on the surface, called surface tension. The alcohol molecule is much less polar and has much less attraction for other alcohol molecules, and correspondingly less surface tension. It is the surface tension that can support a needle on the surface of the water, despite the fact that the needle is denser than water and is not buoyant enough to float.

Answer 2

Technically, a needle is heavier than water, so it doesn't really float in the usual sense of the word. It is suspended by surface tension between the water molecules on the surface, which makes the surface of the water act as an elastic membrane. This membrane is strong enough to support objects of low weight, especially if that weight is spread out over a large area. (As a counter-example, if you dropped that same needle, point first, into the water, it would immediately break through the surface and sink to the bottom, because steel is heavier than water.)

Different substances have different degrees of surface tension. Water exhibits enough surface tension to "float" a needle (if it is slowly, carefully laid down, on its side on the surface). Hexane does not have enough surface tension to support a needle, regardless of how it is placed on the surface.