We all know that water is this colourless liquid that we have to drink to survive. Most of us even know that its chemical formula is H2O. But what exactly is water made of, and how does this influence its effects?
Chemical composition and properties of water
Water is a molecular compound, with molecular formula H2O. This means that a molecule of water consists of 2 hydrogen atoms and 1 oxygen atom. The atoms in a water molecule are held together by strong covalent bonds, which are very difficult to break and therefore, requires a large amount of energy to break it.
Structure of water
The dot-and-cross diagram for a water molecule shows that there are two bonding pairs of electrons and two non-bonding pairs of electrons. The four pairs of electrons repel one another, forming a tetrahedral pattern. The molecule itself (the spatial distribution of atoms) is described as ‘bent’, ‘angular’ or ‘non-linear’.
The two electrons in each oxygen-hydrogen bond within the water molecule are not shared equally. They are more strongly attracted to the oxygen atom due to the electronegativity of the oxygen atom as compared to the hydrogen atom. As a result, the oxygen-hydrogen bond is polar as it has a ‘negative end’ (the oxygen atom) and a ‘positive end’ (the hydrogen atom).
A hydrogen bond forms between a non-bonding pair of electrons on the oxygen atom (‘negative end’) of one water molecule and the hydrogen atom (‘positive end’) of another water molecule. The hydrogen bond is about ten times weaker than a single covalent bond.
Uses of Water
1. Water acts as a solvent.
Most compounds with ionic bonding such as metal salts, dissolve in water. The oxygen atoms of water molecules are attracted to cations (ions with a positive charge) of the metal salts and water molecules surround it. These water molecules attract more water molecules and hydrogen bonds form between them. The result is a cluster of water molecules around the ion. We say the ion is hydrated.
Similarly, anions (ions with a negative charge) become surrounded by clusters of water molecules. This time it is the positive ends of the water molecule, the hydrogen atoms, that are attracted to the anion of the metal salts.
A wide range of molecular compounds also dissolve in water, including sugars, amino acids, small nucleic acids and proteins. All these molecules are polar. This means they have a positive end and a negative end as the result of polar covalent bonds within them.
In addition, water acts as a solvent for chemical reactions and also helps transport dissolved compounds into and out of cells.
Another important property is that many compounds dissolve and transfer a proton (a hydrogen ion) to a water molecule. The result is an acidic solution with pH < 7. Compounds that release a proton in this way are called acids. For example,
CH3COOH (aq) + H2O (aq) 
CH3COO– (aq) + H3O+ (aq)
H3O+(aq) is called a hydroxonium ion and is responsible for the acidic properties of the solution.
Some molecules receive a proton from a water molecule. The result is an alkaline solution with pH > 7. Compounds that accept a proton in this way are called bases. For example, OH–(aq) is called a hydroxide ion and is responsible for the alkaline properties of the solution.
2. Water acts as a temperature buffer.
Cells host a huge range of chemical reactions. Many of these are catalysed by enzymes. Enzyme activity is sensitive to temperature changes and reactions only occur in a narrow range of temperatures. Water helps to buffer temperature changes because of its relatively high specific heat capacity (the heat required to raise 1 kg of water by 1 oC). It also has a relatively large enthalpy of vaporisation (heat energy required to convert a liquid to a gas) and enthalpy of fusion (heat energy required to convert a solid to a liquid). This is reflected in the unusually high boiling and melting points of water:
| Liquid | Molecular formula | Bpt / oC | Mpt / oC | Specific heat capacity / kJ kg-1 oC -1 |
| Water | H2O | 100 | 0 | 4.18 |
| Ethanol | C2H5OH | 79 | -117 | 2.46 |
| Benzene | C6H6 | 80 | 6 | 1.05 |
| Tetrachloromethane | CCl4 | 77 | -23 | 0.86 |
These properties are a consequence of hydrogen bonding.
3. Water acts as a metabolite.
Chemical reactions take place in cells. Collectively, all these reactions are called metabolism, i.e. all the chemical and physical processes within a cell. The chemicals involved are called metabolites. Water is a metabolite in many reactions, either as a reactant or as a product of reactions. For example, it’s involved in photosynthesis, digestion and aerobic respiration.
For example, photosynthesis is the process by which plants, some bacteria and some protistans use the energy from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed.
Photosynthesis may be summarised by the word equation:
carbon dioxide + water
glucose + oxygen
The conversion of usable sunlight energy into chemical energy is associated with the action of the green pigment chlorophyll.
When water reacts with a chemical to break it into smaller molecules, the reaction is described as hydrolysis.
When water is formed as one of the products when two molecules join together, the reaction is described as condensation.
Harmful effects of water
1. Reaction of aluminium with water.
Aluminium may negatively affect terrestrial and aquatic life in different ways. Regular aluminium concentration in groundwater is about 0.4 ppm, because it is present in soils as water-insoluble hydroxide. At pH values below 4.5, solubility rapidly increases, causing aluminium concentration to rise above 5 ppm. This may also occur at very high pH values.
Dissolved Al3+ ions are toxic to plants; they affect roots and can lead to a decrease in phosphate intake. As mentioned above, when pH values increase, aluminium dissolves. This explains the correlation between acid rain and soil-aluminium concentration.
Aluminium is mainly toxic to fish at pH values 5.0-5.5. Aluminium ions accumulate on the gills and clog these with a slimy layer, which limits breathing.
2. Reaction of iron with water.
When high concentrations of iron are absorbed, for example by haemochromatose patients (accumulation of iron in the body from any cause), iron is stored in the pancreas, the liver, the spleen and the heart. This may cause damage to these vital organs. Healthy people are generally not affected by an iron overdose, which is also generally rare. It may occur when one drinks water with iron concentrations over 200 ppm.
Iron compounds may have a more serious effect upon health than the relatively harmless element itself. Water-soluble binary iron compounds such as FeCl2 and FeSO4 may cause toxic effects when their concentrations exceed 200 mg, and are lethal for adults upon doses of 10-50 g. Nerve toxin iron penta carbonyl is known for its strong toxic mechanism. Iron containing compounds such as iron chelates may be toxic, and iron dust may cause lung disease.
References
Water and living organisms. (n.d.). Retrieved March 21, 2017, from http://www.rsc.org/Education/Teachers/Resources/cfb/water.htm
Water Treatment Solutions. (n.d.). Retrieved March 21, 2017, from http://www.lenntech.com/periodic/water/iron/iron-and-water.htm
Water Treatment Solutions. (n.d.). Retrieved March 21, 2017, from http://www.lenntech.com/periodic/water/aluminium/aluminum-and-water.htm