This might be really dumb, I understand everything has it’s own melting and boiling points, like water boils at 100 degrees celcius. But water can be hotter than 100 degrees, it’ll just boil off faster.
What I don’t understand is how they have lower melting point waxes when it’s melted by the same wick as a normal candle. Its melting point is lower but can’t it still get hotter? There’s also the jet fuel can’t melt steel beams argument (not a conspiracy theoriest, just genuinely confused) so would more fuel make it hotter? Is there a limit to the heat something can produce? One match is hot but a house on fire is much hotter.
Edit – I can’t believe I got something we did in science classes at 12 years old wrong. As pointed out below (thanks all!) water does not go higher than 100 degrees Celsius. I think that’s where most of my confusion was coming from. Thank you guys for your help!
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Temperature is a measurement of how much particles are moving. Adding heat makes them move faster because you’re adding energy. Melting is when particles are moving fast enough that they’re spreading away from each other. This is a consistent temperature becauE the particles move a certain amount based on the structure of the molecules.
Boiling point (simplified) is when the particles are moving so fast that they spread really far away from each other.
Water boils at 100C (assuming it’s not mixed with salt or anything like that). It’ll boil at 100 or higher. Making the temperature much higher doesn’t make it boil any hotter, it just makes it boil fasterbecause it’s a whole bunch of energy added at once. This is the concept behind flash boiling.
Jet fuel gives off a certain amount of heat when it burns no matter if it’s a drop or a million gallons. So in theory adding more jet fuel does not make it melt more it just makes it able to reach more particles faster. The steel still has a limit. For how much each particle can move based on the energy the fuel is giving off.
Jet fuel can’t melt steel beams but it can soften them because the particles are moving and spreading, just not enough to liquify it. The argument for 9/11 and not melting steel beams is more complicated because it’s not just heat from jet fuel being applied to the beams, it’s pressure, distribution of weight, etc. that’s not your question so I’m not diving into that.
FYI: Melting and boiling points depend on temperature and pressure, but for your examples we stay at atmospheric pressure.
>But water can be hotter than 100 degrees, it’ll just boil off faster.
At normal pressure it can’t, liquid water will remain at 100C and just convert to water vapor more quickly. That vaport then gets hotter than 100C.
>What I don’t understand is how they have lower melting point waxes when it’s melted by the same wick as a normal candle.
Waxes are composite materials, where you can fine tune properties such as the melting point. Some oils are mixed into the wax to reduce the melting point. That is somewhat similar to ice/salt mixtures. Salt reduces the melting point of water from 0C down to -21C. (As a general rule of thumb: Adding something (that is not volatile itself) to a liquid lowers the melting point and raises the boiling point.
In general you have to distinguish between the temperature of a reaction (such as burning, of a certain material which is more or less fixed depending on how much oxygen is available), and accumulation of heat and what it does to a material. In the steel beam example you are referring to, you kind of need to compare it to ice in a glacier or forging of metal. It is not melted, but it is more ductile under all this pressure. And increased temperature might not have been able o melt the steel, but ductility was increased for a long time.
Burning is different from melting.
When something burns, it’s undergoing a chemical reaction. Once something burns, you can’t change it back.
When something melts, it’s not a chemical reaction. It’s just a change from solid to liquid. It can go back to solid again when it’s cold. And back to liquid when it’s hot. Flip flop flip flop.
Candles are actually a very special example because the wax melts and burns at the same time!
Melting and burning are fundementally different.
First, an important rule to remember, when some matyer (water, wax, etc) is going through a phase change, its temperture DOES NOT CHANGE. You can test this with bouling water. (assuming you are at sea level, with plain water) Water will boil at 100°C. So if you look at a thermometer on a pot of water, you will see the temperture slowly rise until it hits 100°, then it will start boiling. It will never go over 100°. This is why boiling water is so useful for baking and cooking. We know exactly what temperature it is. Even if you turn up your stove burner to 200%, it will not go above 100° C, it will just boil faster. This is because to change a liquid into a gas (or solid into liquid) requires energy. Any extra energy you give to the material just makes that phase change faster.
Burning is something completely different. Burning a material is not aphase change, it is actually a chemical reaction. When you burn wood you do not end up with wood in the air, you dont have wood at all anymore. Burning is generally the process of using oxygen to break the bonds of a material, whoch releases energy. Wood during the burning process turns into carbon dioxide and water while releasing heat. There is no necessary hard limit for how hot something can burn by itself, but instead you need to understand how much enery is being released, and how quickly that energy is able to dissipate.
For example, a wood campfire is likely not going to be hot enough to melt throughyour metal grill. Although the energy the qood releases will heat the metal, it will also quickly escape into the surrounding air. However, under the right circumstances, you can make a wood fired forge which WILL be hot enough to melt metal. A campfire might get to around 600-900°C, a forge fuelled with wood might reach double or triple that temperature as it turns wood into charcoal and the charcoal burns into ash. But a forge requires very specific designs, such as a small compartment, focused heat into a specific area, a constant supply of wood and usually good airflow. If you burn more wood, faster, in the same space, it will generate more heat.
Now, because of physics and the fact we live in the real world, there are pretty practical hard limits on how hot certain materials get when they burn. We can increase efficiency up to a point, but practically it can’t go past that.
Phase changes (boiling and melting) are strange, in that they don’t work like you might intuitively thing. First of all, water boils at 100ºC at normal atmospheric pressure, but the temperature it boils at depends on pressure, at a higher pressure it boils at a higher temperature. That said, for the rest we’ll ignore that bit.
For a normal substance that is not going through a phase change, if I add heat (energy) to it, the temperature goes up, if I remove heat, the temperature goes down. When a phase change is happening, though, it doesn’t work like this. If I take a bag of water that can can expand to accommodate the volume of steam, but not let any escape, and start with the water at normal room temperature and pressure, and start adding heat, the temperature of the water will go up. Once the temperature reaches 100ºC, it starts to boil. The boiling process is strange, though. If I add some heat (energy) to the water at 100ºC, the temperature does not go up, instead, some of the water becomes steam, also at 100ºC. The more heat I add, the more of the water becomes steam, but as long as there is both water and steam present, the temperature will not rise. Only when the last bit of water has boiled and become steam, so that there is no water left, does the temperature start to rise again. If I add this heat faster, the water boils faster, if I add it slower the water boils slower, but the key point is as long as boiling is taking place, the temperature does not change.
Incidentally this is why scalds from steam are so dangerous, because the reverse is also true during condensation. When steam comes in contact with your body, because your body is colder than 100ºC, it will transfer heat to your body, burning it. But as the steam loses heat and condenses to water, during the condensation process lots of heat is transferred, but the temperature is still at 100ºC until the steam is fully condensed.
In the candle example, the same thing happens. As the candle flame adds heat to the wax, its temperature increases up to the melting point. Once it is at the melting temperature, any added heat will not increase the temperature of the solid wax, instead it will go into converting the solid wax to liquid wax, so the wax below the melting zone on the candle will not increase in temperature, as all the energy is going into melting the wax.
In both of these cases, what is important to appreciate is the process of taking a solid and making it a liquid or a liquid to a gas at a fixed temperature “uses up” quite a lot of heat energy.
Now onto the second point about burning. If I take a bit of fuel and burn it, what happens is that the fuel reacts with oxygen in the air, converts to combustion products (eg carbon dioxide and water vapour) by combining with that oxygen, and the result is some heat energy release. The amount of heat energy released is determined by the chemical makeup of the fuel. You only get a finite amount of energy out. You know, instinctively, you can’t heat a whole house by burning a single piece of paper in the fireplace. It will all burn up and only release a little bit of heat. To heat the house to a higher temperature you need more fuel.
If I have a perfectly insulated box full of air and burn a little bit of fuel in it, the temperature will go up a bit. The heat release will be the result of the chemical reaction between the fuel and the oxygen, which will consume some oxygen and all the fuel, producing some water vapour and carbon dioxide (assuming the fuel is some sort of organic fuel such as gasoline or kerosine). All that heat will be distributed throughout the container, so any excess air not involved in combustion, and all the components of air that are not oxygen (air is mostly nitrogen, with about 21% oxygen, and a few other gases in small amounts), so the temperature rise will depend on how much heat is released and how much excess air that heat is spread around.
If I then repeat the process with half the amount of air (but still more than I need), that heat release will be distributed over less stuff overall, so the final temperature will be higher. If I work out exactly how much air I need for complete combustion, and not a single bit more, so that every bit of oxygen is consumed and all the fuel is burned, this will result in the smallest possible quantity of stuff for the heat to be shared around, so this will give me the highest possible temperature after the burning process is finished. I can’t get any hotter than this. If I want to burn more fuel, I need more air to burn it in, so the larger heat release from more fuel will simply be spread over more final product (combustion products and the inert gases in the air), and the final temperature will be no hotter.
If I want to get a higher final temperature, there are two things I can do. One is to burn the fuel in something with less of the inert gases in it. Instead of air, which is only about 21% oxygen, I can burn in pure oxygen, so all the nitrogen and other stuff isn’t taking its share of the energy released, and the energy is concentrated in a small amount of stuff, resulting in a higher temperature. There will still be a limit to how hot I can get, as I will have a fixed amount of heat related from the burning, and a fixed amount of combustion product that the heat has to be shared around.
The other thing I can do is start with hotter air/fuel to begin with. The heat release from burning will produce a change in temperature, going from cold to hot. If I start with hot air, the heat release will instead produce a temperature change from hot to very hot.
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