Swede Posted December 2, 2009 Posted December 2, 2009 I can generally help her, but this is a weird one. They have a mystery substance - alum - KAl(SO4)2 - 12H2O and are doing things like mixing with barium nitrate (yeah!) and heating to drive off water, etc. But one of the discussion questions has me stumped: Why must objects be cooled before their mass is found on a sensitive balance? All I can think of is that excess heat might interfere with the balance mechanism, by literally radiating heat upwards, creating a minute interfering force. Any thoughts? Otherwise, temperature of an anhydrous material should not affect mass. Thanks!
TYRONEEZEKIEL Posted December 2, 2009 Posted December 2, 2009 unless the material will react with the surface the scale is made out of, I think it is probably a safety thing. I remember in my high school chem class, we had to do weird things like that. I really doubt it will interfere with the mass reading on the scale. The teacher is probably wanting to minimize any kind of minute chance of burns, or like you said damaging the equipment.
scarbelly Posted December 2, 2009 Posted December 2, 2009 http://answers.yahoo.com/question/index?qi...25135037AAzqpZf Looks like the original poster may have been working on the same problems!
Mumbles Posted December 2, 2009 Posted December 2, 2009 I don't know if it's the air currents or what, but it is real. Our first lab in analytical chemistry dealt with common errors. Hot glassware, finger prints, water bound to the glass, error propegation from pipetes, etc.
psyco_1322 Posted December 2, 2009 Posted December 2, 2009 My guess would be just that. Heat moving the surrounding air could cause enough currents to affect the balance.
Swede Posted December 2, 2009 Author Posted December 2, 2009 (edited) http://answers.yahoo.com/question/index?qi...25135037AAzqpZf Looks like the original poster may have been working on the same problems! Haha, that is AWESOME! It is the exact same experiment, but dated three years ago. Obviously we are dealing with identical laboratory texts. I tried to show my daughter the effect on a milligram scale, but it was not sensitive enough to show the effect. By waving air around the scale, it rippled and demonstrated that even 0.001 grams is sensitive to air, and of course a uGram scale will ripple from footsteps and vibrations near the machine, let alone air currents. She turned her assignment in today, and what I told her, that the moving air will interfere with an accurate reading, should pass muster. Thank you all who answered. It is interesting and a bit sad that she is asking simple questions that I can no longer answer. Like "Why does dissolving some compounds heat the water, and others cool the water?" and I wracked my brain for heats of formation, dissociation, etc, and could not come up with a reasonable answer. Edited December 3, 2009 by Swede
Mumbles Posted December 3, 2009 Posted December 3, 2009 You would be suprised at how sensitive some scales are. In an enclosed space, even a centigram scale oscillates up to .5g or so from movement. One of the several reasons I hate working in a glove box. Being bone dry, static is also a PITA. If only they'd let me bring some static guard in there, the world would be better.
BurritoBandito Posted August 11, 2014 Posted August 11, 2014 I know this is an OLD thread, but apparently the question has been recycled so maybe someone will find this useful. If not I still appreciate the theoretical discussion. Could it possibly have anything to do with E=mc^2? I realize the air currents would be affecting the scale to a larger degree, but let's assume the scale was in a vacuum. There is still a direct relationship between an objects energy and mass. Right?
Mumbles Posted August 12, 2014 Posted August 12, 2014 There is likely more than just heat currents affecting this, though it likely plays a role too. The electronics to very sensitive scales (1mg or 0.1mg) are sensitive to the heat as well to the point that there is a measurable apparent mass difference. I might see if I can do a demonstration of this. I think we have sensitive enough scales at work. If it was just heat affecting the electronics, putting an insulator between the weighing pan and the hot item should negate it. If it was the heat currents then the mass should be the same whether or not the insulator was there. As far as the E=mc2 stuff, I doubt it would matter. If you rearrange it, the change in mass is going to be proportional to the change in energy/c2. Given that it's therefore proportional to 1/c2 the change in energy is going to have to be extremely large to have any noticeable impact on the change in mass.
BurritoBandito Posted August 12, 2014 Posted August 12, 2014 (edited) Another case of me displaying my ignorance. Damn it Mumbles... How dare you have the audacity to use common sense when evaluating my hypothesis. I'll let it slide... This time, but next time I am blatantly wrong about something I expect you to agree with me. Capiche? Edit: Though I think it is obvious, I thought I should clarify that I was joking. Edited August 12, 2014 by BurritoBandito 1
sora Posted September 8, 2014 Posted September 8, 2014 (edited) If its a mechanical scale, the hot object could cause part of the scale to expand unevenly,leading to a change in the equlibrium position and which could lead to measurement errors. Edited September 8, 2014 by sora
gregh Posted September 8, 2014 Posted September 8, 2014 As far as the E=mc2 stuff, I doubt it would matter. If you rearrange it, the change in mass is going to be proportional to the change in energy/c2. Given that it's therefore proportional to 1/c2 the change in energy is going to have to be extremely large to have any noticeable impact on the change in mass.You are correct. It takes an increase of 90 Megajoules of energy to an object just to raise its mass by 1 microgram. One megajoule is equivalent to the kinetic energy of 1 tonne (2000kg) traveling at 100 mph(160kph). Or an example more relevant to fireworking, is with chemical energy. 90 megajoules would be about the equivalent of all the chemical energy contained in about 10 kilograms of flash powder.
BurritoBandito Posted September 8, 2014 Posted September 8, 2014 What an embarrassing thread... I clearly didn't think my response through, and thought I was so damned clever.
dagabu Posted September 8, 2014 Posted September 8, 2014 Are you kidding? I laughed out loud and called the boy#3 to look at your post too. I just didn't have anything to add.
gregh Posted September 8, 2014 Posted September 8, 2014 Don't worry BB. At least you were right about there being a direct correlation between mass and energy. However, unless we are making thermonuclear salutes...... hmmmmmm.
gregh Posted September 8, 2014 Posted September 8, 2014 Sorry BB, wasn't trying to be an ass. (My wife says it happens naturally.) We are all familiar with that equation, but rarely truly understand the magnitude of the relation. All the destruction of the first atomic bombs was created by converting about 1 gram of mass to energy. The Trinity bomb started with 6.15 kg of plutonium. About 1kg of that actually underwent fission. The total weight of the lighter elements it created was only 1 gram less than starting weight of plutonium. That is just astounding to me.
BurritoBandito Posted September 8, 2014 Posted September 8, 2014 (edited) It's ok Greg, I wasn't really offended, just giving you a hard time. I should've given my answer a bit more thought before posting. Feel free to tease me about this, or any other ridiculous posts that I might make. Edit: Check out my signature... It says it all. Edited September 8, 2014 by BurritoBandito 1
Depup Posted September 12, 2014 Posted September 12, 2014 My guess would be just that. Heat moving the surrounding air could cause enough currents to affect the balance. That is why scales are enclosed ,door shut while takeing a measurement . One lab where I worked one scale was on a pillar of steel that went down to bedrock ,there were red lights in the rooms and hallways for 50 ft in all directions on all four floorsthat came on (nobody move ) while a measurement was in progress. The sensitivity was so good I could tell you the weight of a pencil dot on the label on a 20lb sample of materiel. 1
TheExplosionist Posted September 19, 2014 Posted September 19, 2014 I recently learned that G is still only known to 4 decimal places and barely improved since the 19th century experiment. I'd say science has a long way to go when it comes to measuring gravity.
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