Water is an exceptional chemical, especially due to its ability to sustain life. Without water, nothing can live, and thus this attribute makes it one of the most important chemicals in the world.
The first important aspect of water is the fact that it can dissolve many compounds, with the following table being very important in chemistry:
- All compounds of the ammonium ion (NH4+), and of Alkali metal (Group IA) cations, are soluble.
- All nitrates and acetates (ethanoates) are soluble.
- All chlorides, bromides and iodides are soluble EXCEPT those of silver and lead.
- All sulfates are soluble EXCEPT those of silver, lead and barium.
- All carbonates, sulfites and phosphates are insoluble EXCEPT those of ammonium and Alkali metal (Group IA) cations.
- All hydroxides are insoluble EXCEPT those of ammonium, barium and alkali metal (Group I) cations.
- All sulfides are insoluble EXCEPT those of ammonium, Alkali metal (Group I) cations and Alkali earth metal (Group II) cations.
The boiling point and melting point of water are 1000C and 0oC respectively while these two characteristics can be used to test for the purity of water since any impurities will affect the boiling and melting temperature. If impurities are present the melting point would be lowered while the boiling point would be increased.
Definitions
Water of crysallisation: This is when some water molecules are trapped in between larger molecules, such as Cobalt (II) chloride. This number is normally constant for different molecules.
Deliquesence: This occurs when a molecule absorbs water from the air, with the effect that it absorbs so much water that it produces a solution and it practically dissolves in the absorbed water such as Sodium Hydroxide.
Hygroscopic: This is any chemical that absorbs water from the air without dissolving in it such as silica gel.
Effloresence: This is a word used to describe any chemical that is able to lose water to the surroundings such as hydrated Calcium sulfate and hydrated Copper (II) sulfate.
Gases in water
Gases do dissolve in water, although the rate of solution is dependant on the surface area of water available. It must be noted that not all gases dissolve at the same rate and therefore the Oxygen to Nitrogen ratio is different in water then in air.
Water hardness
Water hardness is a big problem for humans since this can produce kidney stones which are very painful.
There are two types of water hardness, temporary and permanent, with the temporary water hardness being due to Magnesium Hydrogen Carbonate and Calcium Hydrogen Carbonate while permanent water hardness is due to Calcium Sulfate and Magnesium Sulfate.
As the name implies temporary water hardness is easy to remove due to the fact that upon heating the Hydrogen Carbonate would decompose to form the Carbonate which is insoluble in water.
Ca(HCO3)2 → CaCO3 + CO2 + H2O
The permanent hardness cannot be removed simply by heating, and there are only two ways of removing permanent hardness, which is either by the use of soap or by ions exchange systems.
In ion exchange systems the sulfate ion would be removed and the introduction of Cl– would normally take place.
With the use of soap the reaction would be:
Normally the cation bonded with the soap is sodium so that when it reacts with the hard water it would form sodium sulfate which is soluble in water.
Washing with soap in hard water is a wasteful activity. The first thing that happens when you add soap to hard water is that the soap reacts with chemicals to form scum. In a way, you are just throwing away the first batch of soap you add because it cannot be used to clean anything. Once all the chemicals in hard water are used up, then any additional soap can be used for cleaning something.
Syndets do not have this problem. When a syndet is added to hard water, no precipitate is formed. The syndet is ready to go to work immediately to start cleaning something.