If equimolar amounts of iodine and propanone are reacted in acidic conditions, the [propanone] vs time graph would be expected to show an exponential decay as expected from first order kinetics. The [iodine] vs time graph vs time must therefore also be an exponential decay in order to obey the stoichiometry. But this is a zero order reaction which is associated with a straight line decay - not a curve - how to rationalise this? Most school experiments use a large molar excess of propanone in quenching (eg 25x) or colorimetric (100-400x) methods to illustrate the zero order behaviour of iodine. The [iodine] vs time graph indeed shows an apparent straight line indicating zero order behaviour. But this cannot be a true straight line as the reaction stoichiometry is then not obeyed. Surely the consumption of iodine must mirror that of propanone (ie exponential decay) or else the mass of iodine is not conserved at all times throughout the reaction. I presume therefore that the apparent straight line is simply a small section of an exponential curve, reflecting the very small extent of propanone reaction. Has anyone investigated this reaction on an equimolar basis – if so did it produce an exponential decay for iodine as I suggest above?! A related concern is the use of these methods to show zero order behaviour. Typically, a time series of iodometric titrations on a single reaction mixture is used to generate a “straight” line, which is therefore subject to my queries above. As argued above, the [iodine] vs time progress curve only appears straight because the other reactants are in excess (= 25x in our school method) and only a small extent of reaction is measured. It seems it is more valid to use the method where different iodine concentrations are reacted with excess propanone/acid and the iodine absorbance followed, which produces a series of apparently parallel “straight” lines, again for the reasons outline above. This initial rates method is arguably better evidence to establish the kinetics, as several initial concentration runs are employed compared to a single run ie that simply more data is collected. With the levels of excess propanone/acid used in typical school procedures (100, 200, 300, 400x), this again equates to pseudo-zero order conditions in propanone/acid, and any effect on rate of changing [iodine] should be apparent (approx 4x, 3x, 2x based on these excesses). When my students conducted these experiments the lines were parallel within a 25% spread, which is reasonably good. I suppose my concern is that teaching a [ ] vs time straight line is diagnostic for zero order is incorrect. It cannot be true for equimolar reactants and apparent linearity needs explaining when used in practice with pseudo-zero order co-reactants. For other than zero order behaviour, then graphing a single progress curve I’m sure is a more effective method. So my conclusion is that a zero order reactant concentration cannot have a linear decay over time - an apparently linear fit must be due to an excess in other reactants. The only time a zero order reactant progress curve can be truly linear is in the case of catalysis by eg enzymes or metals, where active sites determine rate, which will be truly zero order when saturated. Does anyone agree? Does anyone care?!!