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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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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