The Chemistry of Water
In this section we will have a brief overview about one of the important ingredients of life, which is water (H2O). We will have a discussion around the properties of water and some anomalous properties of water. Lastly, we will have a brief look about the latest technology to address the problem about clean water.
To begin with, we will see some facts and figures about water. Water covers almost 70% (2/3 of Earth's surface) and the Earth contains approximately 1,360,000,000 km3 with the distribution of 97.2% in oceans, 1.8% in glaciers, 0.9% in ground water, 0.02% in rivers and lakes, and just 0.001% as water vapour. However, this massive amount of water only accounts for less than a fiftieth of 1% of the Earth's mass. Besides that, water makes up around 60% of the human body. Water is colourless, tasteless, and non-toxic liquid and it is the only naturally occurring inorganic liquid on Earth. Another facts about water is it is a near-universal solvent for many ionic salts and some say it has around 63 anomalous properties. Furthermore, the physical properties of water is shown below.
Then, the next thing we should know is about the phase diagram of H2O. Phase diagram generally describes about the phase transition of a substance. The phase diagram of water is shown below.
The point where 3 phase boundaries meet is called the triple point and at this point the solid, liquid, and gas phases are co-exist. Besides that, water is an excellent solvent for polar or ionic compounds and salts due to its polarity, high dielectric constant and small size. Furthermore, its unique hydration dictates the structure and function of DNA and proteins.
As most of us acknowledge, the chemical formula of water is H2O, where oxygen is an electronegative atom and carries 2 lone pairs. Because of this, the structure of water is non-linear, bent molecule with a dipole moment of 1.85 D. Furthermore, the hydrogen bonded water dimer in gas phase is shown below.
In ice, each water molecule is hydrogen-bonded to approximately 4 other water molecules in average to form a tetrahedral arrangement. The lattice structure of ice is a tessellated (plane-filling) hexagonal lattice and it has 15 forms of ice and type 1h is the most common one. Besides that, the open hexagonal lattice structure of ice as shown below leads to a relatively low density for ice.
As ice melts to form liquid water, its 3D tetrahedral structure breaks down as thermal motion disrupts, distorts, and breaks up the hydrogen-bonded lattice. The relative positions of molecules in liquids cannot be determined (x-ray diffraction only works well for solids), so there is no unambiguous method to determine the detailed structure of liquid water. Furthermore, bulk water is a seething mass of molecules in which hydrogen-bonded clusters are continually forming, breaking apart, and re-forming. After discussing the main properties of water, we will see 6 out of 63 anomalous properties of water.
Firstly is water has a relatively high boiling point compared to similar molecules as illustrated in the table below.
NH3, H2S and HF are gaseous at 20 °C due to their reduced intermolecular forces. The hydrogen bond interaction strengths for H2O and HF are comparable at around 24 kJ mol-1 but molecule forms 4 hydrogen bonds whereas HF has only 2 H bonds per molecule. Meanwhile, the hydrogen bonds in NH3 and H2S are significantly weaker at 16 and 8 kJ mol-1, respectively. Hence, high boiling point of water is due to both strength of its hydrogen bonds and also the large number of H bonds formed by each water molecule.
Secondly is water has a relatively high melting point compared to other hydrides. In general, melting point normally increases for higher molar mass compounds and the melting point of water is over 100 K higher than other group 16 hydrides. As comparison, data for group 14 hydrides also shown below.
Melting point of water is also much higher than that of O2 (54 K) or H2 (4 K).
The melting point of water means the amount of energy needed to break hydrogen bonds to melt the ice and it only gets small volume change when ice melts, so the enthalpy is almost unchanged. Therefore, melting only occurs if change in entropy provides sufficient energy to break hydrogen bonds. However, liquid water has relatively low entropy (due to its high level of organisation), thus ice has a relatively high melting point.
Thirdly is water has a relatively high surface tension compared to other liquids as shown below.
Water molecules at air-water interface have fewer hydrogen bonds, hence they experience a net attractive force that pulls them toward the underlying strongly hydrogen-bonded bulk liquid, leading to a skin effect. This anomalous property is used by pond skater to walk on the water.
Besides that, water has a high specific heat capacity compared to most other liquids as the fourth anomalous properties of water. Specific heat capacity is defined as the energy required to raise a known mass of a substance by 1 °C. Furthermore, specific heats of some common substances is shown below.
Then, the question is what high specific heat capacity actually means. When water is heated, water takes a long time to cool down or it takes a lot of energy to make water hot. This property has important consequences for the UK climate as the high heat capacity of ocean acts as a heat reservoir: gulf stream carries tropical warmth to Europe.
The fifth anomalous properties of water is ice melts at high pressure. Higher pressure usually causes freezing, since solids are more ordered than liquids as illustrated by positive gradient for the liquid/solid boundary in CO2 phase diagram.
For water, the liquid/solid boundary has a negative gradient; thus at higher pressure, ice melts to form water.
The last anomalous properties of water that we will discuss in here is most solids sink in liquids, but ice floats on water. As a solid melts, molecules move further apart in liquid, so the density of solid is higher than the density of liquid. Therefore, most solids sink if immersed in their own liquid.
In water case, it has a local maximum in density at 4 °C, so cold water is more dense than ice. This is not the case for most other pure liquids. The consequence of this property can be seen at the freezing of rivers, lakes and oceans is always from the top down. Then, the ice surface insulates liquid water from further freezing and allows rapid thawing and this allows aquatic ecology to survive and is essential for life on Earth.
We move to another discussion as the impure water is a major problem in the developing world. Around 1.2 billion people have no access to clean water, so they have to use such as impure water that might be contaminated by bacteria, viruses, parasites, pesticides etc. Besides that, collecting (contaminated) water is a task usually given to children and in remote rural areas, this daily task can take up to 5 hours per day. The consequence is very obvious that such children cannot attend school which means no educational opportunities. The clean water crisis set to get worse with predicted global warming over next century and 1.8 million children die each year because of diarrheal illnesses e.g. cholera, dysentry, typhoid; and this death toll exceeds HIV/AIDS and malaria combined.
P&G has developed PUR technology that uses the same techniques, principles and chemicals used for municipal water treatment. One 4.0 g sachet treats 10 litres of water via flocculation, coagulation and disinfection; and one sachet costs just 2 cents. The PUR water purification technology in action is shown below.
The formulation of P&G PUR technology is composed of:
Then, PUR technology works as bleach and oxidant kills bacteria, viruses, parasites, and destroys pesticides. Besides that, synergistic effect of Fe2(SO4)3 and anionic polyacrylamide (molecular weight approximately 107 g mol-1) causes rapid aggregation of particulates, removes heavy metals, dying bacteria and viruses, and quickly clarifies the water. This purification of 10 litres of water takes just 10 minutes and diarrhea (often deadly for malnourished children) reduced by more than 90%.
The impact of this technology is it can kills 99.99999% bacteria and 99.99% of viruses that causes cholera, typhoid, hepatitis etc. 464 million sachets has been distributed worldwide and purified 4.6 billion litres of water. Besides that, 190 million days of diarrhoea averted and more than 25 000 lives saved. This technology was used by aid agencies and charities for many major global disasters, including Japanese tsunami, Indonesian earthquakes, cholera outbreaks in Africa, Haiti earthquake, and Pakistan floods.
Lastly, as this year is chosen as International Year of Water Cooperation, together we can make our world better with water: water sustains life, builds peace, eradicates poverty, and protects environment.
- Molar mass = 18.01524 g mol-1
- Density = 1.00 g cm-3 at 4 °C
- Melting point = 273.15 K
- Boiling point = 373.15 K
- Triple point = 273.16 K
- Critical point = 647 K at 22.1 MPa
- Latent heat of steam = 40.7 kJ mol-1
- Latent heat of ice = 6.01 kJ mol-1
- Viscosity = 0.001 Pa s at 20 °C
- Surface tension = 72.8 mN m-1 at 20 °C
- Specific heat capacity of ice = 2 060 J kg-1 K-1
- Specific heat capacity of water = 4 186 J kg-1 K-1
- Specific heat capacity of ice = 1 970 J kg-1 K-1
Then, the next thing we should know is about the phase diagram of H2O. Phase diagram generally describes about the phase transition of a substance. The phase diagram of water is shown below.
 |
| The phase diagram of water |
 |
| The structure of water molecule |
 |
| The hydrogen bonded water dimer in gas phase |
 |
| Tetrahedral arrangement of water in ice |
As ice melts to form liquid water, its 3D tetrahedral structure breaks down as thermal motion disrupts, distorts, and breaks up the hydrogen-bonded lattice. The relative positions of molecules in liquids cannot be determined (x-ray diffraction only works well for solids), so there is no unambiguous method to determine the detailed structure of liquid water. Furthermore, bulk water is a seething mass of molecules in which hydrogen-bonded clusters are continually forming, breaking apart, and re-forming. After discussing the main properties of water, we will see 6 out of 63 anomalous properties of water.
Firstly is water has a relatively high boiling point compared to similar molecules as illustrated in the table below.
Secondly is water has a relatively high melting point compared to other hydrides. In general, melting point normally increases for higher molar mass compounds and the melting point of water is over 100 K higher than other group 16 hydrides. As comparison, data for group 14 hydrides also shown below.
 |
| The melting point of group 14 and 16 hydrides |
The melting point of water means the amount of energy needed to break hydrogen bonds to melt the ice and it only gets small volume change when ice melts, so the enthalpy is almost unchanged. Therefore, melting only occurs if change in entropy provides sufficient energy to break hydrogen bonds. However, liquid water has relatively low entropy (due to its high level of organisation), thus ice has a relatively high melting point.
Thirdly is water has a relatively high surface tension compared to other liquids as shown below.
 |
| The bulk water and pond skater |
Besides that, water has a high specific heat capacity compared to most other liquids as the fourth anomalous properties of water. Specific heat capacity is defined as the energy required to raise a known mass of a substance by 1 °C. Furthermore, specific heats of some common substances is shown below.
The fifth anomalous properties of water is ice melts at high pressure. Higher pressure usually causes freezing, since solids are more ordered than liquids as illustrated by positive gradient for the liquid/solid boundary in CO2 phase diagram.
 |
| The phase diagram CO2 of and water |
The last anomalous properties of water that we will discuss in here is most solids sink in liquids, but ice floats on water. As a solid melts, molecules move further apart in liquid, so the density of solid is higher than the density of liquid. Therefore, most solids sink if immersed in their own liquid.
 |
| An iceberg, ice floats on water |
 |
| The freezing ocean ecology |
We move to another discussion as the impure water is a major problem in the developing world. Around 1.2 billion people have no access to clean water, so they have to use such as impure water that might be contaminated by bacteria, viruses, parasites, pesticides etc. Besides that, collecting (contaminated) water is a task usually given to children and in remote rural areas, this daily task can take up to 5 hours per day. The consequence is very obvious that such children cannot attend school which means no educational opportunities. The clean water crisis set to get worse with predicted global warming over next century and 1.8 million children die each year because of diarrheal illnesses e.g. cholera, dysentry, typhoid; and this death toll exceeds HIV/AIDS and malaria combined.
P&G has developed PUR technology that uses the same techniques, principles and chemicals used for municipal water treatment. One 4.0 g sachet treats 10 litres of water via flocculation, coagulation and disinfection; and one sachet costs just 2 cents. The PUR water purification technology in action is shown below.
 |
| PUR technology in action |
The formulation of P&G PUR technology is composed of:
- CaOCl as bleach and anti-microbial,
- KMnO4 as oxidant and disinfectant,
- Fe2(SO4)3 as coagulant,
- anionic high molecular weight polyacrylamide as flocculant,
- bentonite clay particles as seed for coagulation and to improve self life.
Lastly, as this year is chosen as International Year of Water Cooperation, together we can make our world better with water: water sustains life, builds peace, eradicates poverty, and protects environment.
Comments