Questions tagged [heat]

For questions about heat - a form of transfer of energy from one body to another or from one body to its surroundings. Also see these tags: [thermodynamics], [enthalpy], [energy], [temperature].

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49
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9answers
479k views

Why does ice water get colder when salt is added?

It is well known that when you add salt to ice, the ice not only melts but will actually get colder. From chemistry books, I've learned that salt will lower the freezing point of water. But I’m a ...
29
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5answers
22k views

Microwaving a glass of water, what happens?

Most of us here would already know the simplified idea behind microwaving food: Microwave radiation hits the water molecules present in food, which excites them and causes 'em to vibrate rapidly in ...
28
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3answers
6k views

What properties of carbon dioxide make it a greenhouse gas?

Why is it that heat from the Earth reflects back off carbon dioxide and other greenhouse gases but not gases like nitrogen or oxygen?
28
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2answers
7k views

When a candle burns, why does the top of wick glow if bottom of flame is hottest?

Totally an elementary question. Staring at a candle, it appears that the bottom of the wick is dark whereas the top glows. However the bottom of the flame (the blue) is the hottest. Is the reason ...
25
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2answers
40k views

Why does diamond conduct heat better than graphite?

Our teachers made us accept (without any explanation) that diamond conducts heat better than graphite. What is the reason behind this (alleged) fact?
22
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3answers
10k views

Why does milk overflow when boiled?

I was wondering that like whenever I boil milk I just have to keep standing in front of it to make sure that I don't waste any milk but then I was wondering like Why does it even overflow ? In case of ...
22
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1answer
2k views

If aliens lived in a hydrogen (or any flammable gas) based atmosphere, would they perceive oxygen to be flammable?

Energy, fuel, and oxygen are needed for a fire to burn. In a hydrogen chamber, if there was a pipe spewing small amounts of oxygen, could you light the oxygen on fire as it began to mix with the ...
21
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4answers
2k views

What is the origin of the maximum in rotational heat capacity?

The graph of rotational heat capacity above shows a small maximum before approaching the equipartition value. What is the origin/physical explanation of this maximum?
19
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3answers
3k views

How to evaluate the heat of formation with computational chemistry?

I'm sorry if this is a stupid question, but since I'm a beginner with computational chemistry, please, be patient. Let's consider the reaction of formation of water: $$\ce{1/2 O2(g) + H2(g) -> ...
17
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5answers
2k views

What exactly is temperature?

I've read at many places that temperature is the average kinetic energy of particles present in an object. I just don't intuitively get how kinetic energy is connected with temperature. And how is ...
16
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2answers
49k views

Why is the specific heat of water high?

How is the specific heat of water so high? I was reading the hyperphysics article on it but could not understand it properly.
15
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6answers
5k views

Is there a cheap, safe liquid that evaporates EXOthermically around room temperature?

The heating and AC system in the building where I occasionally work, works like this (basically a thermostat): The administrators set some temperature that is maintained automatically. Say, in the ...
15
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9answers
10k views

How can 30 ml of water be heated in less than 10 seconds?

How is it possible to heat a tiny amount (30 ml)[1] of water to a high enough temperature to make a coffee, in less than 10 seconds and possibly instantly? Most heaters that I know of heat water ...
15
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2answers
51k views

Heats of combustion and stability of rings

Which isomer would have the largest heat of combustion? A) Propylcyclopropane B) Ethylcyclobutane C) Methylcyclopentane D) Cyclohexane E) Since they are all isomers, all would have the same heat of ...
14
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3answers
4k views

How can a thermometer ever show the actual temperature of an object if the object loses heat to the thermometer?

We know that when two objects are placed in contact with each other, after a period of time, the two objects will have the same temperature. Thus, if a hot body comes into contact with a relatively ...
14
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2answers
7k views

Burns from boiling water and steam

Why is a burn to the skin caused by steam more serious than a burn caused by the same amount of boiling water at the same temperature? The temperature is the same, which implies that the kinetic ...
14
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3answers
66k views

Why is the enthalpy change not zero in an adiabatic process?

Which of the following must be true for adiabatic processes? $C_V = C_p$ $\Delta H = 0$ $\Delta U = 0$ $\Delta S = 0$ $q = 0$ (Source: Chemistry GRE) The answer is $q = 0$. From ...
14
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1answer
463 views

What factors promote honey's crystallisation?

Honey is a complex mixture containing, probably, more than hundred compounds. I understand that the % of those components will affect the crystallisation. However, I am wondering what factors may ...
13
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3answers
10k views

Can you heat water with additives?

I have been curious about this question for a while. If you want to warm up a large amount of water, is it feasible to do this by adding a substance that has an exothermic reaction with the water? ...
13
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2answers
189k views

What is "heating under reflux"?

When dealing with organic reactions, heating under reflux is often required, such as the oxidation of Toluene using acidified $\ce{KMnO4}$ and dilute $\ce{H2SO4}$ to Benzoic acid($\ce{C6H6O2})$ What ...
13
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1answer
1k views

How is enthalpy change equal to heat supplied at constant volume?

We know that: $$\begin{align} H &= U + pV \\ \Delta H &= \Delta U + \Delta (pV) \end{align}$$ Now, If $p$ is constant, $$\begin{align} \Delta H &= \Delta U + p\Delta V \\ &= q_p + ...
12
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3answers
28k views

Relation between constant-pressure and constant-volume heat capacities: Cp - Cv = nR

For an ideal gas, we have $$C_p - C_V = nR$$ where $C_p$ is heat capacity at constant pressure, $C_V$ is heat capacity at constant volume, $n$ is amount of substance, and $R=N_\mathrm A\cdot k_\...
12
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3answers
5k views

Which materials/compounds expand in response to cold temperatures?

Which materials or compounds expand in response to cold temperatures? This material or compound should expand at a slow (but constant rate), from about 0–20 months when in the following temperature ...
11
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2answers
1k views

Can single molecules of C and O2 react in isolation, and if so how will momentum be conserved?

I am trying to figure out how is it possible to reconcile the reality of exothermic reactions, which means that kinetic energy is transferred to (heats) the surrounding matter, with the principle of ...
11
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2answers
26k views

What is the difference between tempering and hardening metals and their end products?

I think I've heard hardening is when you dunk red-hot metal into cold water, and tempering is when you take that hardened metal, heat it slightly, and then let it cool slowly. However what is the ...
11
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0answers
176 views

Constraints on pyrotechnic explosives that won't detonate spontaneously at 500 °C when landing on Venus?

The Space Exploration SE question Premature detonation of explosive bolts when landing on hot Venus? explains the need for and widespread use of pyrotechnic fasteners and cable cutters for Mars ...
10
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2answers
17k views

How does sulfuric acid react to heating?

How would $\ce{H2SO4}$ react to heat? I assume that it would form toxic sulfur (di/tri)oxide, but I can't find any information on this. I'm concerned about fire hazards specifically, so assume very ...
10
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3answers
42k views

What is the difference between enthalpy (H) and heat (q)?

It seems that in some cases it's the same (that is, $\Delta Q_p=\Delta H$) yet in others they differ. For example, \begin{align} \ce{2H2(g) + O2(g) &->2H2O(l)} & \Delta H &= -572~\...
10
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3answers
10k views

Chemical reactions with a room-scale cooling effect

Are there chemical reactions that could cool down an average sized room by a noticeable amount (say 5 °C)? I would like to investigate if it is possible to have a situation where I can mix 2 ...
10
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1answer
2k views

Why would a ~1 cm thick layer of argon be a significantly poorer conductor of heat than air?

In yesterday's new Periodic Video, Argon (new) - Periodic Table of Videos, after about 07 min 00 sec, Sir Martyn Poliakoff says: The final, and I have to admit perhaps slightly boring application of ...
10
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1answer
1k views

What chemicals are used in receipt paper?

Most "invisible inks" turn brown when heated and they take a while to transform. I would like to make (or obtain) an "ink" that responds to the heat of an iron and permanently turns black or a ...
9
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5answers
64k views

Why is it obligatory to cool down the container of a sample to measure its mass in a lab?

We were doing an experiment about hydrated crystals and more precisely how to determine $n$ in $\ce{CuSO4.nH2O}$. After we heated the crucible we were to cool it down using a Desiccator. Then this ...
9
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2answers
30k views

How much does water expand when heated, and does salt affect that?

For this question, assume all things not mentioned are perfectly controlled, all at 14.7lbs of atmospheric pressure, and water is 100% pure of anything not mentioned, with no dissolved air. Given a ...
9
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1answer
1k views

Why does lead nitrate powder on heating starts turning yellow from the top rather than from the bottom?

Today while in school, I observed the thermal decomposition of lead nitrate. The peculiar thing I noticed was that the lead nitrate powder in the test tube starts turning yellow (due to the formation ...
9
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4answers
11k views

What happens if you autoclave HEPES?

Many molecular biology protocols advice against sterilizing HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) solutions by autoclave. Supposedly, HEPES is heat labile, and the high ...
9
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1answer
896 views

Why does a glass tube show white bands after being heated with a torch?

When I use a torch to split a glass tube, there's always a hazy white band which appears. Does anyone know what the cause of this is? It was a hydrogen-oxygen torch. The glass is fused quartz glass, ...
8
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4answers
25k views

Why won't water freeze if you put ice in it, despite ice being frozen?

Just now I got some room temperature water and added some ice to cool it, and it just came to me, like the scientist I wish I was ... why doesn't frozen (freezing point) ice added to higher ...
8
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3answers
124k views

Are all PP(Polypropylene), i.e. resin code 5 plastics, safe for microwave and steaming?

If a plastic is marked PP as in Polypropylene (i.e. resin code 5), is it automatically safe for steaming and microwaving, such that it wouldn't leach chemicals into the food(besides not melting)?
8
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1answer
7k views

Heat of combustion for octane

I'm a Mechanical Engineering student and I have been working on a school project but I want to go a bit beyond that and it has led me here. I'm designing an engine cylinder, and I figured I should be ...
8
votes
2answers
328 views

Can the human body withstand the heat from setting plaster?

There was an incident a couple of years ago where a teenage girl during an art lessen got her hands stuck in a bucket with plaster. As the plaster set, the heat from the chemical process caused her ...
8
votes
2answers
880 views

Silicone for use in casting Bismuth

I am trying to cast bismuth, which melts at 271.4 °C. Silicone rubber seemed like a good material to use for a mold, given its flexibility and the ease with which it can be cast itself, as well as its ...
8
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0answers
606 views

Does heating potato starch destroy its resistant starch?

Resistant starch, as found in potato starch, is supposed to have health benefits. There are differing opinions on whether or not heating potato starch destroys the resistant starch in it and thereby ...
8
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0answers
174 views

Why are IUPAC's definitions of exo- and endothermic disconnected from the direction of heat flow?

I'm wondering if anyone can provide a rationale for IUPAC's definitions of exothermic and endothermic, since they disconnect these terms from the direction of heat flow. Specifically, here are IUPAC'...
8
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2answers
3k views

How is work converted to heat?

My book says work can be converted into heat but I want to know how work affects kinetic energy of molecules which, when transferred, is called heat?
7
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1answer
576 views

Does all fire only emit light on its outermost shell?

This question is about light emission, which may overlap with physics, but I am most interested in combustion and types of flame (incandescence, petrochemical fuel flames, nuclear ractions). Consider ...
7
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4answers
77k views

How do I chill water without a refrigerator?

How can we chill water in a plastic bottle without a freezer or refrigerator? Scenario: I'm in the office and I have two plastic bottles of water (brought from home). I'm in the office for 3 to 4 ...
7
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5answers
20k views

Why is a tin can that appears like galvanised steel rusting?

I bought a canned pineapple and finished the content. The part of the can that is in contact with the liquid appears like galvanized steel (crystalline surface or spangles sort of appearance), while ...
7
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3answers
6k views

Why substitution and elimination reactions are favored respectively at lower and higher temperature?

I read about the competition between nucleophilic substitution and nucleophilic elimination depending on temperature here. Though the webpage clearly says higher temperature favors elimination while ...
7
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1answer
23k views

Why does a substance with an endothermic heat of solution dissolve?

How does a substance with an endothermic heat of solution dissolve?
7
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1answer
2k views

How to calculate the mass of iron with a temperature of 325 °C needed to melt ice and raise the temperature of water from 0 to 22 °C?

A large well-insulated container holds a mixture of $75\ \mathrm g$ of ice and $100\ \mathrm g$ of water at $0\ \mathrm{^\circ C}$. Using the data given below, calculate the mass in grams of solid ...

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