"I see."

Annoying, but reasonable.  She'll double-check the mass, relative to her initial volume measurements, before she signals her readiness, but at this point she is ready.

One advantage of letting each of the students proceed through the steps of their first potion at their own pace is that they generally do end up doing it at wildly different speeds, allowing him to hover supportively for each of them individually when they first turn on their burners. This is useful because every so often someone manages somehow to light themselves on fire doing that, although Ophelia has been so diligently careful so far that he really doubts she'll be among them. 

He'll give her steps-so-far a once-over, checking that nothing seems to be amiss, and then nod supportively.

Her burner flares to the precise temperature indicated, because she will be very cross with it if it does not.

"I believe I am ready to continue, Professor."

"Well done, Miss Prince. Yes, you're doing very well so far, go right ahead."

The next thing she is supposed to do is monitor the cauldron over its brewing period as it comes to temperature, and turn it off pronto if it does any of the following list of things this potion shouldn't do (emit smoke, turn a color other than this nice shade of lavender, make high pitched shrieking noises, etcetera).

And so she shall do.

Ophelia has been meticulous in her work and so her potion will not do anything it is not supposed to do, as it heats. She may notice, however, a behavior that is unremarkable to wizards but quite unusual to a mundane chemist: as she monitors its temperature, it heats up nonlinearly, as though the perfectly smooth liquid, which is not in any way freezing or vaporizing, has a dozen different melting-like phases.

Around her, meanwhile, various other members of her class will, with various degrees of struggle, perform the same procedure. Many of them are doing it badly, but Slughorn, it transpires, is actually quite good at gently catching people before they do anything dangerous; despite the ponderous speed at which he moves about the classroom, he seems to always be in exactly the right place to cheerfully remind someone that they are about to skip a step, or that they have miscounted, or in one case to smoothly catch, with a levitation charm, a dropped bowl of ground snake fangs before it hits the ground. The kindly and reassuring smiles that Professor Slughorn offers the students who need a lot of this type of help are not, quite, perfectly sincere - he would, perhaps, prefer to spend a larger fraction of his time answering interesting questions - but handholding children through following the directions is, he is aware, a necessary component to getting to have more competent older students, so he is doing his best, and his best is fairly impressive.

Hmm.

She takes notes on the Interesting Data as best she can, and makes concurrent notes of everything that occurs along with it such as changes in color, scent - she carefully wafts the air - oh rates of change is a good one, she's going to time each phase and phase-change as best she can - perhaps texture, though she is loath to do anything not on her list of known safe procedures such as 'insert any objects into the potion', not when she understands so little - hmm, she might be able to get mass by backforming from volume if she can do anything like 'determine weight of contents of cauldron', it's a shame she doesn't have a proper way to measure that as she goes - and, in fact, is the potion increasing, decreasing, or staying the same volume, as best as she is able to determine?  Does it look to be steaming anything off?  Does this correlate with anything else she is actually able to determine at this time, if so?

There is a bit of attempted color-component-divining she tries with a carefully-controlled Lumos Spectra.

...Really, she wants to do this again, but properly.  Like an experiment, rather than a recipe.

...It seems she may have a chance to get some data even now, though - one of her neighboring table's cauldrons seems to be going - too fast, is all she can say about it with any confidence.

"...Excuse me.  I think you might have set your fire too hot - I couldn't help but notice that, judging by the rate of progress of my own potion, yours is going through the various phase-shifts that I've so far identified rather quickly.  ...I think it should be possible to recover from this - maybe by drawing out the stage four phase shift, it's in the right, direction, and if the rate of change is independent of the wattage - rate of energy transfer, I mean - I think it'll get back on track, somewhat - but if that's something you want to try, we should ask Professor Slughorn about it."