(a) Why do the densities of most liquids increase as they are cooled and solidified? How does water differ in this regard?
(b) Rank the following compounds in order of decreasing surface tension at a given temperature, and explain your ranking.
CH3CH3, CH3OH, H2C=O
(c) Rank the following compounds in order of increasing viscosity at a given temperature; explain your ranking.
HOCH2CH2OH, CH3CH2CH2OH, HOCH2CH(OH)CH2OH
(a) The densities of most liquids increase as they are cooled and solidified due to a decrease in molecular motion. When a liquid is cooled, the molecules lose kinetic energy and slow down. This reduction in molecular motion brings the molecules closer together, leading to an increase in density. In the solid state, the molecules are packed tightly together, resulting in a further increase in density.
Water, however, exhibits a unique behavior. As water is cooled below its freezing point, its density decreases instead of increasing. When water reaches its freezing point, the water molecules rearrange themselves into a hexagonal lattice structure. This arrangement creates open spaces or gaps between the molecules, causing the solid form of water (ice) to occupy a larger volume than the liquid form. As a result, ice is less dense than liquid water, allowing it to float on top of liquid water.
(b) The surface tension of a liquid is a measure of the force that holds the molecules together at the surface, causing it to behave like a stretched elastic skin. It depends on various factors such as intermolecular forces, molecular shape, and the presence of functional groups.
To rank the compounds in decreasing order of surface tension at a given temperature, we need to consider their molecular structures and properties. Generally, compounds with stronger intermolecular forces and more compact molecular shapes tend to have higher surface tension.
Ranking in decreasing order of surface tension:
1. CH3OH (methanol): Methanol has the strongest intermolecular forces of the three compounds. It can form hydrogen bonds due to the presence of an -OH (hydroxyl) group, which leads to stronger attractions between molecules and increases surface tension.
2. H2C=O (formaldehyde): Formaldehyde has weaker intermolecular forces compared to methanol since it lacks the -OH group. However, it still has a somewhat polar bond, which allows for some dipole-dipole interactions and contributes to moderate surface tension.
3. CH3CH3 (ethane): Ethane consists of non-polar C-H bonds, resulting in weak intermolecular forces. It has the lowest surface tension among the three compounds.
(c) Viscosity is a measure of a liquid's resistance to flow and is influenced by factors such as molecular size, shape, and intermolecular forces. Compounds with larger molecular size and more complex structures tend to have higher viscosities.
Ranking in increasing order of viscosity at a given temperature:
1. CH3CH2CH2OH (1-propanol): 1-propanol has the smallest molecular size among the three compounds. It consists of a linear chain of carbon atoms with three hydrogen atoms and an -OH group. This relatively simpler structure allows for easier movement of molecules, resulting in a lower viscosity.
2. HOCH2CH2OH (ethylene glycol): Ethylene glycol, also known as anti-freeze, has a slightly larger molecular size compared to 1-propanol. It contains two -OH groups and has a slightly more complex structure. This increased molecular size and added -OH groups contribute to higher intermolecular forces and, hence, increased viscosity.
3. HOCH2CH(OH)CH2OH (glycerol): Glycerol has the largest molecular size and the most complex structure among the three compounds. It consists of three -OH groups attached to a three-carbon backbone. The larger size and greater number of -OH groups result in stronger intermolecular forces, leading to a higher viscosity compared to the other compounds.