Two entries about the same afternoon in Testing for Bond Type. One of them tells me nothing I did not already know.
The weak version
The entry that took two minutes and was worth about that
Today we tested three substances to find out if they were ionic or covalent. Sample C was ionic because it conducted and A was covalent because it didn’t. B didn’t do anything. The lab worked. Next time I will be more careful measuring.
Everything in it is true. None of it is information. There is not a single number in it, “didn’t do anything” hides the most interesting result of the afternoon, and “be more careful” is a resolution nobody has ever kept, because it does not name a thing to do differently.
The strong version
I predicted that any sample whose solution conducted would be ionic. The reasoning: an ionic solid is a lattice of charged particles held by electrostatic attraction, and if water pulls that lattice apart the ions end up separate and mobile in solution. Mobile charges are exactly what a current needs. A molecular substance that dissolves goes into solution as whole neutral molecules, so there is nothing for a current to move.
Each sample: one spatula tip into 10.0 mL of distilled water, stirred 30 s, then the conductivity probe. Distilled water on its own read 2 µS/cm, and I took that as my zero.
A — dissolved completely, solution read 3 µS/cm. That is one unit above the water blank, and the probe’s last digit is not stable to better than about 1 unit, so this is not a reading above zero. Not ionic. C — dissolved completely, solution read 1180 µS/cm. Not close to anything; ionic. B — did not dissolve at all, even after two minutes of stirring. On the hot plate at maximum it did not soften or melt.
So B is the one I got wrong in my prediction, because I had assumed “does not dissolve” would point one way. It does not. A covalent network solid does not dissolve and does not melt at anything a hot plate can reach. An ionic solid of low solubility also does not dissolve, and also melts far above a hot plate. My two candidate explanations predict my observations equally well, which means the data I collected cannot separate them.
The test that would separate them is conductivity of the molten sample: molten ionic compounds conduct because the ions are free, and a covalent network has no ions to free. We cannot reach those temperatures in this room, so this is a limit of the equipment rather than of my procedure — worth writing down, because “get a better result next time” is not available here and I should not pretend it is.
What I actually learned: I went in thinking the job was to sort three samples into two bins. The job is to work out which sorting my evidence is entitled to make. Two of them it decided cleanly, and the third it did not decide at all, and noticing that is not a failure of the afternoon — it was the afternoon.
Why the second one earns what it earns
| Move | Where it shows up |
|---|---|
| Prediction stated with a mechanism | ”mobile charges are exactly what a current needs” |
| Real numbers with units, not adjectives | 2, 3, and 1180 µS/cm |
| A blank measured, and used | distilled water read as the zero |
| A small difference tested against the instrument | ”not stable to better than about 1 unit” |
| A prediction admitted wrong, specifically | the assumption about sample B |
| What the evidence cannot decide, and why | both explanations predict the same observations |
| A named test that would decide it | conductivity of the melt |
| A limitation owned as equipment, not carelessness | the temperature we cannot reach |
| Something learned about thinking, not just bonding | the closing paragraph |
Nine moves, under 400 words. Length is not the thing.
The move that is new this year
Look at the third row from the bottom. Sample A read 3 against a blank of 2, and the entry does not treat that as a small positive result — it treats it as no result, because the difference is inside what the instrument can resolve.
Last year “about the same” was an acceptable sentence. This year it is replaced by a comparison against a number: the resolution of the instrument, the spread across trials, or both. That habit is worked through in Significant Figures in Practice, and it is the single thing most likely to move an entry from good to strong.
The test to apply before you close the notebook
Could someone who was not in the room reconstruct what you did, what you found, and why you believe it? If the answer is no, one more sentence usually fixes it — and it is almost always the sentence giving the number.
Now check yours against Journal Checklist. More on limitation sections in Writing a Lab Report, and on unexpected results in Mistakes Are Data.