Why water H2O is liquid at room temperature while HF is gas?

First of all, recall that physical states (solid,liquid and gas) are decided by strength of intermolecular forces. So in first glance, by seeing F in HF we guess F being most electronegative in periodic table would have strongest hydrogen bonding (intermolecular force)  so, HF should be liquid. This is not fact HF is Gas at room temperature although F has greatest electronegativity value i.e 4.

Here in this case, not the strength of hydrogen bonding is only playing a role rather number of hydrogen bonds per molecules is deciding the physical states of H2O ( liquid) and HF ( gas).

In case of HF there is one H atom and three electron lone pairs on F  in a molecule. Presence of three electron pairs cannot make three hydrogen bonds as there is one hydrogen atom in a molecule. Hence HF can make one hydrogen bond per molecule and exist in linear geometrical shape. 

Hydrogen bond between HF molecules.

While in case of water there are two lone pairs on O atom and two H atoms per molecule so, H2O produces two hydrogen bonds per molecule. And water has bend geometrical shape. Each molecule attach two more H2O molecules by two hydrogen bonds.so H2O exists in liquid state at room temperature.

Hydrogen bonding between H2O molecules.