My blog - LouiseSchmidt

So from the link, this is the chemistry that explains the colors that occur. Let me know if this is the part you don't understand. Chemical Reactions Sodium metabisulfite and water react to form sodium hydrogen sulfite: (1) Na2S2O5 + H2O --
2 NaHSO3 Iodate(V) ions are reduced to iodide ions by the hydrogen sulfite ions: (2) IO3- + 3 HSO3- --
I- + 3 SO42- + 3 H+ When the concentration of iodide ions becomes sufficent for the solubility product of the HgI2 to exceed 4.5 x 10-29 mol3 dm-9, then orange mercury(II) iodide precipitates until the Hg2+ ions are consumed (assuming an excess of I- ions): (3) Hg2+ + 2 I- --
HgI2 (orange or yellow) If I- and IO3- ions remain, then an iodide-iodate reaction takes place: (4) IO3- + 5 I- + 6 H+ --
3 I2 + 3 H2O The resulting statch-iodine complex is black to blue-black: (5) I2 + starch --
a blue/black complex The other thing is kinetics. That is why they change color at different rates. The concentration of the chemicals in each case is different. Concentration is one factor that influences the rate of reaction. The precipitation reaction that makes the orange color from HgI2(s) is fast, but depends on the formation of I-(aq) from the second reaction. Once the Hg2+(aq) is consumed by the precipitate, the I-(aq) concentration continues to build from the same second reaction. The 4th reaction then occurs from the continued I-(aq) production. The creation of Iodine from that reaction sets of the last reaction with the starch indicator to give the blue-black color that overwhelms the still-present color from the precipitate. http://en.wikipedia.org/wiki/Chemical_kinetics