Moreover, the melting curve of water in its phase diagram has an unusual negative slope. The melting point of water decreases when the pressure increases. This is due to the fact that high pressure tends to reduce the volume of water and this can be achieved by changing ice to liquid water. (The density of ice is lower than that of water) The phase diagram of water is shown below.
Water is a good solvent because of its polar nature. It can dissolve many ionic substances like sodium chloride by the salvation of irons in the solution.
Water is a poor electrical conductor. It can only slightly ionize in water to form H2O+(aq) and OH (aq). The ionic product of water Kw is only mol2 dm-6 at 25ºC.
Water has a low viscosity and a high specific heat capacity (4200 J kg-1 °C-1) comparing with other substances.
Occurrence
Water is a naturally occurring substance and 80% of the earth’s surface is sea. However, most water occurred naturally has some impurities, for example, sea water is made up of water, sodium chloride and other ions. We can obtain pure water from sea water by simple distillation and the setup is shown below.
Chemical properties
Water is involved in many chemical reactions and some reactions and some examples are given below.
Reaction with metals
Water is involved in many chemical reactions and some examples are given below.
Reaction with metals
Water can react with reactive metals such as potassium, sodium and calcium to from metal hydroxide and hydrogen gas.
2K(s)+2H2O(I) → 2KOH(aq)+H2(g)
2Na(s)+2H2O(I) → 2NaOH(aq)+H2(g)
Ca(s)+2H2O(I) → Ca(OH)2(aq)+H2(g)
When water is in gaseous form, it can also react with magnesium, aluminium, zinc and iron to form metal oxide and hydrogen gas.
Mg(s)+H2O(I) → Zn(s)+H2O(g)
Zn(s)+H2O(g) → ZnO(s)+H2(g)
Reaction with s-block metal hydrides
All s-block metal hydrides can react with water to form hydroxides and hydrogen.
NaH(s)+H2O(1) → NaOH(aq)+H2(g)
CaH2(s)+2H2O(1) → Ca(OH)2(aq)+H2(g)
Reaction with ethene
Water can react with ethene to form ethanol. This reaction is catalytic hydration of ethane which is the industrial preparation of ethanol.
Reaction with ester
Water can undergo a reversible reaction with ester to form alkanol and alkanoic acid
Importance of water
Water is very important in our daily lives and some examples are given below.
Coolant
As mentioned before, water has a high specific heat capacity comparing with other substances. It can absorb a large amount of heat with only a small rise in temperature. Hence, water is often used as coolants in car engines and air conditioning units.
Solvent
Water can commonly used as solvent since it can dissolve many polar substances.
Essential component in living things
Water is essential to living things. Our body is made up of 70% of water. Water governs many cellular reactions inside our body and one cannot live without water. Water is also important to plants. It provides turgidity in the plant cells and supports food for plants and animals.
HYDROGEN PEROXIDE H2O2
Structure
A hydrogen peroxide molecule consists of two hydrogen atoms and two oxygen atoms which are joined together by covalent bonds. The structure of hydrogen peroxide molecule is shown below.
Hydrogen peroxide has a simple molecular structure and its molecules are held by hydrogen bonds. The slightly positive charged hydrogen atom is attracted by the lone pair electrons on the oxygen atoms. The hydrogen bonds should be stronger due to the presence of an extra oxygen atom.
Physical properties
Hydrogen peroxide has a high melting point and boiling point comparing with other substances having simple molecular structure due to the presence of hydrogen bonds between the molecules of hydrogen peroxide.
Synthesis
Hydrogen peroxide can be prepared by autoxidation of 2-ethylanthraquinol.
Chemical properties
Decomposition
Hydrogen peroxide is readily decomposed into water and oxygen in the presence of MnO2 or catalase.
Unusual oxidation number of oxygen
The oxidation number of oxygen in hydrogen peroxide is instead of . This property enables it to be an oxidizing agent (in which the oxidation number of oxygen decreases from to ) and reducing agent (in which the oxidation number of oxygen increases from to 0).
Oxidizing property
Hydrogen peroxide is a powerful oxidizing agent. It is readily reduced by the following half equation. The oxidation number of oxygen decreases from to .
For example, hydrogen peroxide oxidizes iodine to iodine.
Reducing property
Powerful oxidants such as permanganate ions or chlorine will oxide hydrogen peroxide into oxygen gas by the following half equation. The oxidation number of oxygen increases from to 0.
For example, permanganate ions will oxidize hydrogen peroxide.
Hydrogen peroxide is also a good reducing agent in alkaline solution.
Radical formation
Hydrogen peroxide can undergo homolytic bond fission to form radicals.
Uses
Antiseptics
Hydrogen peroxide can kill bacteria and micro-organisms and hence it can be used as antiseptics and contact lens cleaner.
Catalyst in addition polymerization
Hydrogen peroxide can be used as a catalyst in addition polymerization of chloroethene to form polyvinyl chloride.
Bleaching agent
Hydrogen peroxide is a mild bleaching agent and it can be a bleaching agent for textile, for example, wool and silk.
