How to follow the rate of reaction?

I'm doing AS chemistry and was wondering if anyone knew how I would be able to follow what happens during a chemical reaction (no particular one in mind). This would be a way of measuring what happens during a reaction as time goes by...like a pH change or a change in gas given off. Also if you have any examples of reactions and the way they are monitored it would be a HUGE help...thanx!How to follow the rate of reaction?
Lots of different ways. If hydrogen ions are produced or lost in the reaction, you can use a pH electrode, but this is not very sensitive as it responds to the log of the changing concentration. If a gas is given off or absorbed, you can measure changes in volume and/or pressure.



One very common method is spectroscopic. At its simplest, you choose a wavelength where only one of the reagents absorbs light, and measure how optical density at that wavelength changes over time.



An exciting variation of this, which won Norrish and Porter (UK scientists) the Nobel prize, is flash photolysis. Here you use a very short pulse of bright light to generate unstable reagents photochemically, and then use spectroscopy to see how quickly they disappear.How to follow the rate of reaction?
For my A-level chemistry course work I followed the chemical reaction of different amounts of hydrogen peroxide and yeast and measured the amount of oxygen produced.
don't u mean ';optical intensity';. Or more specifically, transmittance or absorbance (absorbance has a logarithmic relationship i think).