C3 Edexcel Notes

C3.1 -- Water testing:

• Flame tests can be used to identify the following ions: Ca2+ (brick red), Na+ (yellow), K+ (lilac) and Cu2+ (green-blue).
• Sodium hydroxide tests can be used to identify many other ions, and precipitation reactions occur. These include Al3+ (white precipitate), Ca2+ (white precipitate), Cu2+ (pale blue precipitate), Fe2+ (green precipitate) and Fe3+ (rust brown precipitate). Al3+ and Ca2+ can be distinguished as the aluminium ion precipitate redissolves in excess sodium hydroxide.The precipitate is always a solid hydroxide of the cation.

C3.2 -- Safe water:

• Halide ions can be identified via the use of a few drops of nitric acid (to prevent false positives) followed by silver nitrate. Cl- results in a white precipitate, Br- in a cream one and I- in a yellow one. 
• Ammonium ions can be found by reacting a substance with sodium hydroxide and adding damp red litmus paper: if NH4+ is present, the litmus paper turns blue. A distinctive smell is also produced.
• Ionic equations can be written by removing spectator ions. For the halide tests these take the simple form of Ag+(aq) + Cl-(aq) -> AgCl(s). For the NaOH tests these take the form of Al3+(aq) + 3OH-(aq) -> Ag(OH)3(s) or Ca2+(aq) + 2OH-(aq) -> Ca(OH)2(s).

C3.3 -- Ion identification (N/A: Practical):

C3.4 -- Safe limits:

• Chemists perform tests on water to ensure that it only contains certain safe amounts of potentially dangerous substances. These substances can come from the rocks over which the water runs or from how it is treated. (Fluoride, for example, is often added to water to help dental health). Patients' blood is also often treated to test for diseases, and chemists are also used in medical research to try and find connections between chemicals and certain conditions.

C3.5 -- Water solutes:

• Water with a high concentration of calcium and magnesium ions is called "hard" and will not lather: instead, they combine with the soap to form a scum, and all the ions of calcium and magnesium must be precipitated as scum before a lather will form. This makes washing difficult.

C3.6 -- Hard and soft water:

• There are two main kinds of "hard water": temporarily hard water, where the hardness can be removed by boiling, and permenantly hard water, for which an "ion exchange column" must be used.
• The ion exchange column is a fairly simple device: the column contains many plastic beads made of resin, to which sodium ions are weakly attached. When hard water is run through the column, the calcium and magnesium ions which make the water hard are exchanged for sodium ions. Eventually all the sodium ions will be used up, at which point concentrated sodium chlorine is run through the column, hence replacing the said sodium ions.

C3.7: Finding the mass of solute in a solution (N/A: Practical):

C3.8: Particles and moles (Higher):

• "Avogadro's number" is 6.02 * 10 to the 23 atoms. A "mole" of one substance is the amount which contains Avogadro's number atoms. The mass of an element…