Why must you wait for an object to cool before you mass it ?

Question

Its one of my lab questions and i cant find the answer anywhere. My only guess i that the rising heat causes an upward thrust that would make it seem like it would have more mass. Any ideas... thanks.

my chemistry teacher said not to ever measure an object's mass when it is hot. If temperature does not ffect its mass in anyway, why would he tell us to never mass it when its hot?

Answer 1

It is true that the mass is constant regardless of the temperature within the realms of normal measurement(although Einstein did tell us that matter is really just a form of energy, and a hot object does indeed have more mass because it has more energy, however the difference is imperceptible with normal lab instrumentation).

The first response is correct, though. A hot object causes convection currents, which push up on the balance pan slightly and cause a change in the APPARENT mass of the object.

Put a hot object on the pan of a sensitive balance(that goes to .001 or .0001 grams) and watch the mass reading go crazy. The mass will appear to change by several milligrams every second.

Thus, you get an incorrect mass reading even though the mass hasn't changed.

(EDIT: Yes, Norrie, I gave you a thumbs down-While it's true that the mass remains unchanged, the apparent mass does indeed change, which gives an incorrect mass reading. Thus your answer is misleading).

EDIT 2:
Norrie,
This debate is seemingly getting us nowhere, but I'll continue to offer my insight on the subject.

If we assume that mass is a measure of the amount of matter in an object, which it is in the realm of chemistry(although most physicists don't like that definition), it is absolutely correct that mass does not change by a perceptible amount with changes in temperature.

Using this principle, though, the assumption is made that both the object being massed and the references masses both have the same density, and thus have the same buoyancy in air.

In fact, if you purchase calibration weights from the National Bureau of Standards, their mass will be reported in two ways-mass as weighed against brass, and mass in a vacuum.

In fact, in very precise work, measurements are often made in a vacuum, or else a mathematical compensation is applied to account for the difference in density.

If the object being weighed is warmer than ambient, it will increase the temperature of the air in immediate proximity, giving greater buoyancy to the object being weighed. Thus, the balance will give a mass reading that is in fact less than the actual mass of the object.

Furthermore, the convection currents are variable enough that it will often give an unstable balance reading. Put a hot crucible on an electronic analytical balance, for example, and the reading will probably be so unstable that the balance won't even give you a reading.

And, I said apparent change in mass for a reason. Most of the time, we simply base mass upon whatever a balance tells us. An apparent change in mass causes a change in the balance reading. Since we are generally accustomed to just writing down what the balance says, unless applying a specific compensation as described above, an apparent change in the measured mass will definitely cause an error in our experiment.

I have an old analytical chemistry textbook which describes a simple experiment that can be performed with a good(analytical) two-pan balance. If you take two equal mass 1 gram weights and weigh them on the two-pan balance at room temperature, the balance should show them as having equal mass. If you then take one and hold it in your fist so as to warm it up, and do another comparison weighing, the balance will show the room temperature weight as being significantly heavier. I've tried it myself and know it to be true.

EDIT 3:
No, of course mass isn't being destroyed. I never suggested anything of the sort. In formal argument, I believe this would be called a straw-man argument.

I also never suggested that we were talking of anything other than mass in every day terms. Chemist universally recognise mass as the amount of matter in an object, and that's what we are speaking of. Physicists commonly use other definitions of mass, however in practice, they are the same.

If you'd actually bother to read everything I said, I think you would find an adequate explanation of why you shouldn't weigh hot objects.

It seems obvious to me that you've never actually done any hands-on work in the lab. I'm pretty sure that anyone who has would have seen and been familiar with this effect.

Answer 2

You have to wait for it to cool because a hot object heats up the air around it, causing a convection current that will mess up your measurement. Also, many of the more expensive scales can be easily damaged by hot objects.

Answer 3

Not in the least. Mass is constant whatever the temperature.
Your Teacher is obviously being pedantic like the 2 'thumbs downers'
With increased or decreased temperature however, the object will expand or contract. This will change its Density only. Density is mass/volume = g/cc or kg/L.
If temperature changes then the volume will change to greater or less than 1cc or 1L..etc. The mass however will not change.
e.g. Water has a density of 1g/cc or 1kg/L at 4°C. (Its maximum Density).
When water cools to below 4°C it begins to expand and continues to do so when freezing and cooled below freezing point. The 'SIZE' or the VOLUME of water and ice will increase (greater than 1cc or 1L) but, it's MASS will not change.
The change in volume will therefore be different and, the mass divided by a greater volume will result in a Lower Density.
(By the way, Water is the only substance that expands both on Heating or Cooling above or below 4°C. Other substances Expand on heating and Contract on cooling).

(Whoever gave the 'thumbs down' is totally wrong .. as usual. If you have a more correct or better answer...Post it).
Ben H, As we're not going into Eisteinian theories or hypotheses, I believe that the statement of 'No change in mass' is the more acceptable. What is meant by 'Apparent mass change' ??. 'Apparent' means 'Seemingly' not 'Actually'. If however you or the other thumbs downer guy wish to be pedantic, (even though you, yourself stated that mass does not change), then it's OK by me. However I do not take revenge).
I believe that we are talking in Normal, Everday terms of Mass.
So despite everything ever taught in Schools and Colleges, and Universities, Matter (Mass, Energy), CAN be destroyed...???