The question library on water related issues

### How are water molecules bond together?

Hydrogen atoms are attracted to other atoms such as oxygen atoms, because the electrons are pulled closer to the oxygen atom, due to its greater attraction for electrons. As a result the oxygen atom has a slightly negative partial charge and the hydrogen atoms have a slightly positive partial charge. The slightly positively charged hydrogen atoms are then attracted to the slightly negatively charged oxygen atoms of other water molecules. These forces of attraction are called hydrogen bonds. The forces holding two molecules together in a hydrogen bond are much stronger than those between molecules and hydrocarbons. The attraction between hydrogen and oxygen atoms results in a much higher boiling point of water than anticipated for a different molecule with the same mass.
Hydrogen bonds can also be formed between atoms of hydrogen and sulphur or nitrogen, typically SH- and NH2- groups.

In this picture you can see what hydrogen bonds in water look like:

Hydrogen bonds

### How does water evaporate?

To make water evaporate, energy has to be added. The water molecules in the water absorb that energy individually. Due to this absorption of energy the hydrogen bonds connecting water molecules to one another will break. The molecules are now in the gaseous state; this is called water vapour. The phase change from liquid to vapour is called evaporation. During evaporation a molecule of water absorbs latent heat.

### What are thermal properties of water?

Thermal properties of a liquid are properties that have everything to do with heat transfer through this liquid. Thermal properties can be divided up into specific heat and latent heat.

The specific heat is the amount of heat per unit mass that is required to raise the temperature of a liquid by one degree Celsius. The relationship between heat and temperature change is usually expressed by the upcoming relationship, where c is the specific heat.

Q = c*m*dT

c = specific heat
m = mass
dT = change in temperature

The specific heat of a liquid is expressed in kilo Joules per kilogram, per degree Celsius. The specific heat of water is 4.18 kJ/ kg * oC at 0oC.

Latent heat means energy that is absorbed by water molecules, in order to evaporate. It is heat that is hidden in the water molecule and that is used, when water is heated. When the water cools down the energy is hidden inside the molecule again.
Latent heat is expressed in kilo Joules per kilogram (kJ/kg). The latent heat of water is
2250 kJ/ kg at normal pressure and at a temperature of 100oC.

### How do we determine the solubility of gasses in water?

Henry's Law determines the solubility of gasses in water. This law links the partial pressure of a gas to its mole-fraction in the liquid phase, and thus declares how much of the gas is dissolved. According to calculations from Henry's Law oxygen is more water-soluble than nitrogen.
Henry's Law describes as follows:
P = H * x
In which p is the partial pressure of a gas, H is a constant that differs for each gas and x is the mole-fraction of the gas in the liquid phase.
Some gasses have a specific way of acting when they are dissolved. Carbon dioxide, sulphuric acid and various volatile agents, such as hydrogen chlorine, dissolve and then combine. Their solubility coefficient is much higher than that of other gasses.

### How do we determine the solubility of liquids in water?

Water molecules are polar. That is why the solubility of a liquid is determined by its polarity. Polair substances often contain OH-, SH- and NH2- groups. The more of these groups a liquid contains, the more water-soluble it is. This is because the groups in question can form hydrogen bonds with water, which are very strong. Because these bonds are very strong a molecule that contains more OH-, SH- and NH2- groups is more water-soluble.

### What is oxidation and reduction?

Water can take part in oxidation and reduction reactions. Water can be a donor of electrons; this is called a reducing agent. The type of reaction where a compound takes up electrons is called an oxidation reaction. The acceptor of electrons is called the oxidant. Usually oxygen originates during such reactions. Water can also act as an acceptor of electrons, an oxidant. The type of reaction where a compound accepts electrons is called a reduction reaction.

An example of a redox reaction of copper (Cu) and zinc (Zn):
Cu2+(aq) + Zn(s) -> Zn2+(aq) + Cu(s)

When we spread this reaction into two parts we can see the oxidation and reduction of electrons (e-) separately:
Zn(s) -> Zn2+ + 2e- (oxidation)
Cu2+(aq) + 2e- -> Cu(s) (reduction)

In this reaction zinc is a donor of electrons; zinc is the reducing agent. Copper is the oxidizing agent, because copper is the acceptor of electrons.

For water terminology check out our Water Glossary or go back to water FAQ overview