Physics of Boiling
Boiling is a widely known phenomenon, mostly linked with the boiling of water. We see it every day when, for example, cooking.
Boiling is the phenomenon when water molecules are jiggled around enough to start turning into water vapor, a gas. When this gas gets hot enough, the pressure the gas exerts is big enough to overcome the pressure of water, and a bubble forms. This bubble quickly bubbles up to the surface of the water, where it is released into the air.
Given this, which of the following is not correct ?
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3 solutions
Discussions for this problem are now closed
so, what is the other method to boil the water?
when we lower the pressure of the liquid, boiling point decreases. So at a certain low pressure, we can even boil a liquid at 0 Degree Celsius.
if pressure goes on reducing water will boil even at room temperature
yeah like on hills
instead of heating water and increasing its pressure......... u can also reduce the atmospheric pressure by reducing ur room pressure
doing work on it ! that's called Joule's method !!!!!
reduce the pressure
by increasing the pressure of the liquid, water can be heated which is practical in all thermal powerplants
Uh.. Isn't it the other way around? Water is heated, and the expansion created by heating builds up the pressure, because the water is in a closed container. If thermal powerplants would work like you sugested, utilizing thermal power would be useless, because it would take even more pressure to contain the liquid.
but isn't the water gaining heat too..from the source ??
Yes, it's gaining heat. However, heat is a different from temperature in that heat is defined as the total amount of kinetic energy within a substances and temperature is defined as the average amount of kinetic energy within a substance. Water is in a liquid form because of the bonds(hydrogen bonds) between the water molecules and these bonds are what keep them together and not flying around through space(what gases are). The process uses the energy in order to break the bonds rather than to increase the temperature, which is why the temperature does not increase when you boil it. The bonds are then broken and the liquid water turns into gas water.
If the temperature is actually doesn't increase, why is it hot? (question from a newbie :) )
@Bravadi Ismail – it's about phase, change, basically when you add heat to water it increases temperature. However, when the temperature reaches the boiling point, the water stops increasing in temperature even if you add heat. So during boiling, the water is at 100 degree Celsius (pretty hot), but it won't get hotter than that.
What this means in real life is that water that has been boiled for five minutes would be as hot as water boiled for an hour. So if you want to save fuel, turn off the stove the moment the water starts boiling, keeping on longer won't make it hotter.
@Bravadi Ismail – oh god understanding its meaning.firstly we heat to take it to the boiling point.but here we are talking "during boiling".during boiling temp doesn't rise.
@Gautam Sharma – During boiling, temperature can never rise because as you heat the water, some molecules gain more heat than others. These high energy molecules escape, leaving the molecules with less heat behind. This keeps the average temperature of water at a constant value which is 100°c at 1atm.
Well how abt increase in gas temp increases pressure? Pressure is a phenomenon based on the overall volume of the gas and the volume available in the container and the temp at which it is kept.... here fixed container volume is not mentioned.... so we look at the molecular level.... where pressure is the adhesive/cohesive force of interaction between the intended surface and the gas.... but actually, here due to increase in volume of gas, lesser and lesser molecules come in contact with the intended surface.... so it is not necessary that inc. in temp inc. pressure.... one also needs to have a closed container... otherwise pressure remains constant.......
Boiling point is defined as when the vapor pressure exerted by the gaseous phase of a solution equals the atmospheric pressure , then that temperature is considered the boiling point at certain atmospheric pressure. So if you want to boil something one way is to increase temperature till it reaches boiling temp which eventually leads to increase in vapor pressure. AND pressure doesn't remain constant.
exactly well done
Boiling can be thought of as a cooling process because when boiling happens high energy molecules leave the water (in this case) leaving lower energy particles behind which lowers the overall energy ( temperature) of water.
I'm still not convinced. Who says energy must be conserved here? Boiling involves the system receiving energy from an external source, hence the energy of the system rises. There does not need to be a loss of energy in the liquid water to compensate for the rise in the gaseous water molecules. That doesn't make any sense, because if the liquid molecules lose energy, they have to gain it back again to continue the boiling process. Also, gaseous water is still water; you cannot simply say that if the liquid water cools, all the water does. Lastly, the reason water stays around 0 and 100 degrees when freezing or boiling is because the energy being put in is no longer being used to raise the temperature, but to break the attractive forces between the water molecules that make it whatever state it is currently in. The temperature does not necessarily rise. The reason the correct answer is correct is because boiling is the passive phenomenon of the water changing from a liquid to a gas; it does not itself heat the water, but is a result of the water being heated.
A method to boil water without adding heat from a fire would be to decrease the external pressure of the gas outside the liquid. Thus water will start to boil at lower temperatures than 100 Celsius (which is the normal boiling temperature at 1 atm outside pressure).
Your question has two solutions. It is important to note that boiling water does NOT increase the pressure of the system if the water is vented to the atmosphere. In this situation the system will remain at atmospheric pressure.
There are two equally valid solutions to this problem.
as u r telling that it is a cooling process..can u touch the water wen it is boiling?
yes, in the same way that you can heat up an iceberg with a candle. The iceberg is heating up, but it's still cold, in the same way that water is cooling down, but it's still hot.
No. Even though it is a cooling process, the water is still too hot to touch!
No because all the heat energy we provide to water is not taken away by those particles which leave and cause boiling but majority of heat remains with those particles which are left behind.
At boiling point temperature does not change it converted into latent heat
But as room temperature is not at all 100C, less than that & at the boiling point as temperature increases the how can you say water isn't getting heated up? still liquid water molecules might be losing energy but there is a constant energy source (geyser or burner) which may be providing the energy at least 100 times faster than they lose energy. At least, this is MY point of view. Might be wrong, right?
but the heat is added to latent heat of water and thus the temperature not rises right? if the heat you are supplying is steady, can you say that water molecules looses energy which is absorbed to convert into gaseous state. well my doubts are not clarified yet and pardon me sir if i m offending u by any way!
ya of course at lower the press boiling point will be low. but at 0 Degree also we are providing heat. therefore no other methods wihout heating to boil the liquid
Water evaporates even if it is sold (without becoming first liquid). This process is called sublimation.
I feel the answer should be increased temperature of gasses increases its pressure as by the basic equation PV=nRT here volume and no of gas atoms are not kept constant.... moreover after attaining certain energy there is a phenomenon of phase change takes place and again on heating temperature goes on increasing.....
wrong .no cooling takes place because cooling implies fall of temperature. so it is not cooling as temperature remains constant. The heat energy supplied goes into breaking the bonds between water molecules rather than increasing the thermal agitation of the molecules which is macroscopic measure of temperature.
boiling does not heats up the water????
to heat up is to increase in temperature.
when water is boiling it remains at boiling point temperature (usually around 100 degree Celcius), it won't get past that (it can't go 101 degrees), since temperature is not increasing then it is not "heating up"
Boiling does not heat up water. During boiling the temperature of the water remains constant, and that is why the heat that is transmitted to water by the fire during boiling is named LATENT heat. Again, this heat does not increase the temperature of the water. More correct would have been to say "Boiling does not increase the water temperature" instead of "HEAT UP" since in physics heat and temperature are two different things. The previous answer here is incorrect since heat is constantly added to water, water is not cooled. It just remains at the same temperature and the extra heat added (by the fire) serves to break up the bonds of between liquid molecules and transform then into to gas molecule.
what is the other method to induce boiling ?
The other method is to lower the pressure of the gas outside water. Lowering the outside pressure lowers the boiling point.
by reducing the pressure.....The same phenomenon due to which water doesn't exist in liquid state on Mars.....The Pressure on Mars is very low to sustain Water in Liquid State
Pl. try this simple experiment. Take some boiling water in a clear glass bottle, nearly half full. In the bottle water is hot but not boiling. Tight the cap of the bottle and pour some cold water on the outside of the bottle. You can see on cooling the water inside the bottle will start boiling. The reason is steam which was inside the bottle, gets condensed, reducing the pressure inside the bottle.
not always boiling increases temperature of water when it is at boiling point only state of water changes to vapor but its temperature remains constant at constant pressure
The incorrect (or I suppose correct) answer is "Boiling heats up the water".
In fact, boiling is a cooling process. Don't believe me? Note that when water changes from a liquid to a gas, the water molecules gain energy. In response, the still-liquid water molecules around it lose energy. This means that they are cooled.
Still not convinced? Note that when water boils, the mean temperature stays at about 1 0 0 C ∘ . If boiling was not a cooling process, then the temperature would still steadily increase.
This was the first physics problem I posted, and I like it because it's pretty tricky :P