I have a question about distillation.

Or heating in general. When something says heat for 1hr or 1/2 hr do you start counting when you turn on the hotplate? Or do you start counting when the mixture is boiling?

I have a feeling that this issue is affecting my yields as I usually think, "oh this didn't start to actually boil or reflux and so I need to heat it for awhile longer or, maybe it would be better to not heat it for so long even though it wasn't boiling or refluxing untill 15min after I started heating the mixture"

which is correct?

Thanks

If it is to boil, start counting when the solution starts to boil. If it says heat for 1/2 hour and some temperature is specified, (such as a real T as in 50 C, then start counting when the T reaches 50 or so but a moderate T heat up to about 40 or 50 and count about 1/2 hour).

what if no temperature is specified?

For example the reaction that I'm doing it says:

"For compound A, the alluminum block temperature should be about 140C, and for compound B, the temperature should be about 120C. Be careful to adjust the reflux ring, if one is visible so that it remains in the lower fourth of the condenser. Violent boiling will cause a loss of product. Continue heating the reaction mixture containing A for 75 minutes.Heat the mixture containing B for 60min."

1-For measuring the temperature of the alluminum block I think that I have to have the thermometer in the aluminum block hole that is exposed to the hotplate itself right?

2- So for this I guess assuming I use compound A, I would heat the reaction mixture for 75 min starting when the alluminum block reaches 140 and the mixture starts boiling? But what if the hotplate reaches the temperature and the mixture is not boiling or vice versa?

Thanks Dr.Bob

The first statement, what if no temperature is specified is another question. (not supposed to be refering to the bottom example which has temperatures given)

If no T is specified I would count from the time the hot plate reaches its approximate T and the mixture in the flask has been heated somewhat. At least I wouldn't count from a cold start. For #1 above, yes. For #2 yes. Assuming the mixture is to boil, I would count from the time it starts boiling.

Thanks very much Dr.Bob

When a procedure instructs you to heat something for a specific amount of time, such as 1 hour or half an hour, the timing usually starts when you turn on the hotplate or apply heat to the mixture. It doesn't typically start when the mixture begins to boil or reflux.

The reason for this is that the time it takes for a mixture to reach boiling or refluxing can vary depending on different factors like the starting temperature, volume, and composition of the mixture. By starting the timer when you initiate heating, you ensure a consistent and controlled duration of heating across different experiments.

If you wait for the mixture to start boiling or refluxing before starting the timer, it may lead to inconsistencies in your results. For example, if you start the timer when the mixture begins boiling and later realize that it took 15 minutes to start boiling, your total heating time would be shorter compared to someone else who started the timer right away.

To avoid any confusion or inconsistency, it's best to follow the instructions and start the timer as soon as you apply heat to the mixture, regardless of whether it's boiling or refluxing. This will help ensure accurate and reproducible results in your distillation or heating process.