chemistry question?

Question

Propane, C3H8, is a gas at room temperature; hexane, C6H14, is a liquid; an dodecane, C12H26, is a solid.

could someone explain why this statement is true? thanks in advance.

Answer 1

"Kurniawan *" is correct, but the real answer goes much deeper than that....his is just a generalization of what occurs, not really an explanation.

The melting point and boiling points of substances is determined by the strength of the intermolecular forces of attraction occurring within the substance. On a molecular level, the molecules of a substance are naturally drawn together by various forces...the stronger these forces, the higher the melting point and boiling point.
Several intermolecular forces of attraction exist and each has their own mechanism for why they attract, but in the case of the hydrocarbons you listed (propane, hexane, and dodecane) the IMFoA which is pertinent is the one in which all molecules have....London forces, the weakest of all IMFoA.
The London force of attraction grows stronger with larger molecules since it works by inducing a partial charge on the atoms which cause them to stick together somewhat. The larger the molecule and the more atoms sticking out, the stronger the London force is.
Propane is a (comparatively) small molecule, so the London force is (comparatively) weak.
Hexane is 3 Carbons (and 6 Hydrogen atoms) bigger than Propane and has a stronger London force causing its boiling point to be high than room temperature.
Dodecane is much larger than both Hexane and Propane and correspondently has much stronger London forces at work, causing the melting point to be higher than room temperature.

So, in short, the statement is true due to the fact that the larger the molecule, the higher the Intermolecular forces of attraction, the higher the melting / boiling point.

Answer 2

As the number of Carbon atoms in the organic compounds increases, it's boiling point increases.