What would be some strengths and limitations in a experiment?
What could be considered a strength or a limitation?
Here's some examples stolen from first year chemistry experiments (note: their theory ain't that different to yours)
Experiment is heating hydrated copper sulphate to determine how much of the compound is water by mole (i.e., determine x in CuSO4.xH2O). All you do is weigh copper sulphate, heat it until its colour stops changing, weigh it again, and do maths based on how much weight is lost.
Pro: Methodology is simple
Con: Some of the copper sulphate may convert to copper oxide if you heat it too hard. However, if you don't heat it enough, not all the water will be removed.
Way to fix the experiment: Add a dessicating, or drying, agent to help remove the water - but make sure it's added in a way so that it isn't mixed in with the copper and is easily removed.
Next, how about an experiment where you try to make an ester, but there are 10 steps involved:
1. Add chemicals
2. Mix things in a bowl
3. Transfer things to another bowl
4. Mix things in bowl with more chemicals
5. Transfer things to another bowl
6. Filter
7. Add precipitate to another bowl, add in more chemicals
8. Mix things
9. Filter
10. Ya done
Con: This is so many steps. The more steps you have, the less efficient things are. Particularly when multiple reactions (note: every time I say mix things, that's a separate reaction) happen at once. This particular transformation involves 4 reactions - if all of those reactions have a 95% yield (note: this is normally a REALLY good yield, see below), then your yield is now only going to be (0.95)^4=81%, which is still good, but not great any more.
Pro: potentially this is low cost method. Let's say that no matter what, each step is 95% yield. This includes the 4 steps from above, but also a theoretical reaction with only one step. If that one step reaction costs $1000, then you're LOSING 0.05*$1000 = $50 per product. What if the 4 step method only costs $200? Even though that yield ends up being 81%, you only lose 0.19*$400 = $36 per product.
Way to fix the experiment: Use a different method. But note, if you're going to play this card, it is 100% better to offer an alternative. Your alternative doesn't have to worry about costs or anything - but if you've said something takes too many steps, tell me how to do it in less steps. Maybe you made ethyl butanoate. You know a faster way to make ethyl butanoate that takes less than 10 steps? Mix ethanol and butanoic acid with some sulfuric acid. It may sound scary to suggest a whole new synthesis - but you won't be asked to suggest one if you haven't learned about it
Now, here's a list of cons that I personally wouldn't accept -
but bear in mind that I'm now 7 years out of high school, so it's worth taking these to your teacher to see if they will accept these responses:
1. Everyone will perceive colour changes differently - yes, but this is a philosophical argument. Not only is red, green, and blue all different enough that it doesn't matter if we see them slightly differently, but the colour change will typically happen when less than a drop is added. So it doesn't matter if you can't tell if this is "blue enough" yet, because that situation will never actually *happen*.
2. We need to repeat the experiment more times. Like, this is obvious - if we could do the experiment more times, we would. BUT, that doesn't make this experiment pointless. If an experiment is flawed, we're not going to do it 4 times before deciding if it's flawed - that's a waste of time. We're going to design the experiment to do once, then check if we can find any flaws. Then we'll run the experiment, look at the result, and ask again if there are any flaws. We'll worry about doing it more times AFTER we've figured out if that experiment is okay.
3. Solution wasn't mixed well enough. Unless they've given you an experiment that explictly says "solution wasn't mixed", then they did mix it. If this is an experiment you did yourself, then why DIDN'T you mix it? But also: do you need to mix it? If all you're doing is drying copper sulphate for example, mixing the lump of solid isn't going to do anything. Mixing is only going to help if something is dissolved into solution (see collision theory - if the solid is in a lump, you're only going to get collisions on the outside of the solid lumps, which is less than 1% of the molecules in that lump)
4. The reaction only has a 95% yield. Yes, it's not 100% - but that doesn't matter. You will lose some material in transfer (note: this isn't a con either, unless your reaction has 20 steps or something when it could be done in 1. Transfer loss is usually only about 1%, even in 10 step reactions), so it's expected not to get 100%. In fact, 95% is REALLY, REALLY good.
Can't think of any more at the moment - but if I do before you come back, I'll add them.