Why at room temperature is H2O a liquid, but H2S is a gas?
This question is basically just asking you why water has a higher boiling point that H2S. So you need to ask yourself what is the factor which means water has a higher boiling point? If you remember back to electronegativies of the elements in the periodic table, you may recall that the elements in the top right hand corner are the most electronegitive (i.e. they pull electron density towards them). Oxygen sits above sulfur in row 6 of the periodic table and so is more electronegative. Furthermore, you may also recall that F, N and O are the three most electronegative atoms in the table. This property means that when they are attached to a hydrogen (which has a much lower electronegativity) there is a difference in electronegativity. This different is particularly large for H-F, H-N and H-O, resulting in a special intermolecular interaction called hydrogen bonding which is exclusive to these bonds. Therefore, H2O has a higher boiling point than H2S because H2O has the ability to hydrogen bond whereas H2S does not. Hydrogen bonds are stronger than other intermolecular interactions such as van der Waals interactions or dipole-dipole interations, so require more energy to break them. Hence, water has a higher boiling point (or is a liquid at room temperature) whereas H2S is a gas at room temperature.
