mmm, soap at pH 1
May. 26th, 2008 01:14 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
baratronToday's "fantastic" exam paper answer of the day:
Question - State one large-scale use of sulphuric acid.
Student's answer - "Soap".
Yes, I wash my face in sulphuric acid every day. That's why my skin is so red and dry. But it's great for removing grease and oils!
I am currently quite annoyed with this student, because she's been giving me 3 or 4 past papers per week, each of which is so appallingly rushed that it gives no insight as to her ability or the grade she'll achieve in the actual exam. When a 60 mark paper loses 10 marks through careless mistakes and another 10 through vague wording (when I'm sure she understands the concepts really), that's not going to get more than a C unless the grade boundaries are ridiculously low due to Extreme Hardness. An exam of one hour should be attempted in one hour, and if she finishes in 35 minutes then the remaining time should be used for checking. Argh!
My latest blitz has been on the students' habit of handing in exam papers where they've failed to write anything on some questions. In the past few days, I've marked papers with anything from 3 to 20 of the marks left completely blank. Wasting 20 of 75 marks really does reduce your maximum possible grade - to get a B, you'd need to get 100% of what you did answer correct. Also, if you leave a question completely blank, then you will, of course, obtain zero marks for it. But if you write down whatever you remember that's related to the question, you stand a chance of scoring 1 or 2 of the marks. This applies doubly when it comes to calculations. The calculations are always structured to give the marks step-by-step, so even if all you can do is calculate the molar mass of the compounds, or work out the number of moles of the first compound, that will give you 1 or 2 marks.
This advice doesn't work so well if you're sitting an exam where they take off marks for incorrect answers, but here, in the GCSE and A-level exams, you only get marks deducted if something you write is so blindingly, appallingly wrong that it contradicts the rest of your answer. For example, if you are writing about why metals conduct electricity then suddenly decide they have covalent bonding. This type of bonding contradicts what you've written about electrons and positive ions, so you'd lose all the marks on that part of the question for it.
I am also annoyed with having to tutor kids who are only sitting in-school exams rather than public exams. They just don't take the exams seriously, and will sit and argue with me that they've never learned topics even though they're on the syllabus for their age group. When I have a child in the 10th grade at an international school argue with me that he's never learned about digestive system enzymes, I laugh a lot because it's on his school's syllabus for 8th grade. Hmmm. As for the kidney, his textbook devotes a whole chapter to the nephron and ADH, and he is supposed to have covered everything in the textbook. He thinks I'm going to teach him International Baccalaureate Chemistry. I'm totally not - the IB Higher Level is ridiculously hard, and the options chosen by his school contain vast quantities of 1st and even 2nd year university chemistry (organic reaction mechanisms From Hell). It requires a lot of hard work and maturity, and I don't think he can manage either.
Blargh!
Question - State one large-scale use of sulphuric acid.
Student's answer - "Soap".
Yes, I wash my face in sulphuric acid every day. That's why my skin is so red and dry. But it's great for removing grease and oils!
I am currently quite annoyed with this student, because she's been giving me 3 or 4 past papers per week, each of which is so appallingly rushed that it gives no insight as to her ability or the grade she'll achieve in the actual exam. When a 60 mark paper loses 10 marks through careless mistakes and another 10 through vague wording (when I'm sure she understands the concepts really), that's not going to get more than a C unless the grade boundaries are ridiculously low due to Extreme Hardness. An exam of one hour should be attempted in one hour, and if she finishes in 35 minutes then the remaining time should be used for checking. Argh!
My latest blitz has been on the students' habit of handing in exam papers where they've failed to write anything on some questions. In the past few days, I've marked papers with anything from 3 to 20 of the marks left completely blank. Wasting 20 of 75 marks really does reduce your maximum possible grade - to get a B, you'd need to get 100% of what you did answer correct. Also, if you leave a question completely blank, then you will, of course, obtain zero marks for it. But if you write down whatever you remember that's related to the question, you stand a chance of scoring 1 or 2 of the marks. This applies doubly when it comes to calculations. The calculations are always structured to give the marks step-by-step, so even if all you can do is calculate the molar mass of the compounds, or work out the number of moles of the first compound, that will give you 1 or 2 marks.
This advice doesn't work so well if you're sitting an exam where they take off marks for incorrect answers, but here, in the GCSE and A-level exams, you only get marks deducted if something you write is so blindingly, appallingly wrong that it contradicts the rest of your answer. For example, if you are writing about why metals conduct electricity then suddenly decide they have covalent bonding. This type of bonding contradicts what you've written about electrons and positive ions, so you'd lose all the marks on that part of the question for it.
I am also annoyed with having to tutor kids who are only sitting in-school exams rather than public exams. They just don't take the exams seriously, and will sit and argue with me that they've never learned topics even though they're on the syllabus for their age group. When I have a child in the 10th grade at an international school argue with me that he's never learned about digestive system enzymes, I laugh a lot because it's on his school's syllabus for 8th grade. Hmmm. As for the kidney, his textbook devotes a whole chapter to the nephron and ADH, and he is supposed to have covered everything in the textbook. He thinks I'm going to teach him International Baccalaureate Chemistry. I'm totally not - the IB Higher Level is ridiculously hard, and the options chosen by his school contain vast quantities of 1st and even 2nd year university chemistry (organic reaction mechanisms From Hell). It requires a lot of hard work and maturity, and I don't think he can manage either.
Blargh!
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no subject
Date: 2008-05-26 01:05 am (UTC)Soap is formed from the interaction of fats with BASES, not ACIDS.
In my high school (tenth grade, when I was fifteen years old or so), we had a chemistry teacher who was Old Skool.
I mean, our exam questions involved us knowing things that we were also instructed never, ever to use.
Such as how to tell the difference between an acid and a base by feel. (Answer: bases feel slick when you rub them between your fingers, because they're dissolving the oils on your fingers and turning you into soap.) And the fact that ALL salts taste salty, even the deadly poisonous ones.
no subject
Date: 2008-05-26 04:51 am (UTC)Usually, handmade soaps recipes are intended to end up slightly acid to leave a bit of oil un-converted. This protects against the opposite—unconverted lye—and the leftover oil makes the soap feel a bit nicer on your skin.
But if you're making a liquid soap, the leftover oil is a nuisance. So the usual procedure is to make the soap come out with a bit of extra lye and then neutralize it with a bit of acid.
But the acid is usually acetic, not sulphuric. And this is hardly what one would call "large-scale.")
no subject
Date: 2008-05-26 08:23 am (UTC)*grrs on your behalf*
no subject
Date: 2008-05-26 05:53 pm (UTC)For those who are struggling, I'm having a blitz on unanswered questions because when I have sat with them going through the answers and asking them the questions orally (many of my students are dyslexic, and often have a better chance of answering the question if they hear it), most of the time they've been giving me at least enough to get half of the marks. And then saying "but I didn't know how to start". So I point out that most of the time, you don't need to write full sentences, just coherent bullet points ideally in a logical order, and that if they'd written the thing they'd just told me they would have done better. Also, on a lot of these A-level exams, the grade boundaries can be just 6 marks apart. So it's possible to write only a little more than you did and go up a whole grade - or possibly two grades. The psychological difference between a D and a B is enormous, yet the mark number difference isn't.
For those who should be doing well, like the girl I was moaning about - she handed in a difficult A2 paper that scored 65/75 (a good A), and two easy AS papers that each managed 40/60 (a B on the first paper because it was hard so the grade boundaries were lowered, a C on the second paper because it was about normal difficulty). How can I be sure what her ability and probable grade is from that? I can't be certain that she even did the A2 paper herself because it was just too good - if she's that bright, why is she barely scraping Bs on easy AS papers that contain much of the same work at a lower level? Also, if she keeps writing oversimplistic, imprecise answers to 4 mark questions that come up regularly, then she will eventually learn the wrong version off by heart. I had a go at her today and she did squirm and have no excuse to offer, so I think my half hour estimate was right.
A lot of the problem comes from the students simply not understanding the purpose of doing past papers. I do try to explain to mine how attempting exam questions is the best way to practise the skill which you will be examined on, but this means that some, like this one, decide to rush their way through every paper that exists so they've seen them all. It would be far better simply to READ all the papers that exist and then do a handful of them properly to test your knowledge. Sigh.
no subject
Date: 2008-05-26 05:57 pm (UTC)I have no clue where that "use" came from, either. She insisted it was in one of the textbooks, but when we looked - strangely enough! - it wasn't. Hmmm.
no subject
Date: 2008-05-26 06:02 pm (UTC)Actually, now I am wondering how the stomach would cope with sulphuric acid. I mean, it's lined with special cells that can deal with the hydrochloric acid at pH 2 that it produces. Does that mean it could also cope with other strong acids of similar pH? I kinda want to do an experiment, but I can't think of anything safe to try in vivo, and I don't know whether a stomach removed from a dead creature would still have the acid-protective lining. (I'm pretty sure it has to be constantly regenerated.)
Also a random thought of the day for any listening biologists - is a human vegan a herbivore if they also eat fungus? Which most of us do - mushrooms maybe, yeast almost certainly. Fungi aren't plants...