General Knowledge - Physics - Discussion

@Sumone, this one is for you!

The temperature of boiling water at sea level is 212 degrees Fahrenheit, or 100 degrees Celsius.

The atmosphere of air surrounding the earth creates a pressure against us and all objects on earth. This pressure at sea level is one atmosphere, (14.696 pounds per square inch (psi)), or if measured in mercury - 760mm (approx. 30 inches of mercury) in a column. It is the density of the atmosphere, or air, that causes the pressure.

As you increase altitude, the density of the air becomes thinner, and this thinner or less dense air then exerts LESS pressure. So, the higher the altitude the less dense the air and pressure decreases, until in space - no air, no density, no pressure.

Now, to boil water requires energy, this energy is in the form of heat and may have been produced by gas flame, electrical, solar, burning wood etc. As the water molecules are heated, the energy of the water molecules is increased, and they will vibrate or become more agitated, until finally the water molecules will break loose from the surrounding liquid water and rise up as steam. The water will also move quite violently due to the expanding dissolved gases that are contained in the water - hence the bubbles seen moving rapidly to the top of the boiling water.

So, you have introduced energy into the water to such a point that the bonding energy between the water molecules has exceeded the bonding threshold, and they have broken away and coalesced into steam.

What have been the influences on the water?

Pressure from the atmosphere at sea level - this has helped compress the water molecules together into a liquid.

The bonding energy between each water molecule.

As you increase altitude, the external pressure on the water is decreased, therefore it will take less energy to break the water molecules free from their bonded energy. If it takes less energy, then it will take less heat, as this is the energy we are introducing. If less heat is required, then less temperature is required, then the water will boil at a lower temperature

How about that egg we want to cook on top of a high mountain, say at 10,000 feet?

We now know that the water will boil at a lower temperature on top of the mountain at lets say, 185 degrees Fahrenheit. To hard-boil an egg at sea level takes say, five minutes at 212 degrees Fahrenheit.

Now, there is a very good law in chemistry that states "You cannot get something for nothing" (the left-hand value of an equation must equal the right-hand value). Time multiplied by temperature equals a hardboiled egg. That is to say, 212 times 5 equals a hardboiled egg.

If the temperature of the boiling water on the mountaintop is 185 degrees Fahrenheit, then the time taken to cook the egg will have to INCREASE to get our hardboiled egg. Our equation to equal a hardboiled egg cannot change. (185 times extended time equals hardboiled egg.) It is no different to cooking a piece of steak or cooking the potatoes. You can cook at a low temperature for a long time, or a high temperature for a short time.

It is time and temperature that does the cooking. It has nothing to do with whether the water is boiling. That is only a physical phenomenon that you can see. You have to measure for temperature and time, as these are the two factors that determine when the egg is hardboiled.

The given answer is correct.

I would like to give you an example "Pressure Cooker".

The pressure level inside the Cooker is increased to 15 PSI(Pounds per square inch). So the rise is cooked at the temperature 120 degree Celsius. Therefore, the rise is cooked in 15 minutes.

If you cook the rise in an open vessel, it will take up to 30 minutes to cook the rise. Because the rise is cooked in 100 degree Celsius (because of the normal atmosphere which is less than that inside the Pressure cooker). That's why it takes much time to cook.

Similarly, in the hills the atmospheric pressure is lower than that in the plains and therefore water boils at a temperature lower than 100 degree Celsius causing an increase in cooking time.

I hope my explanation will clear your confusion with other options.

Have a nice day!

This given answer is incorrect because we know that if we will go to top then pressure will decrease and melting point will increase.

So, definitely decrease the boiling point, then we needs of more than 100 degree temp for boils of any liquid material. In above statement we can say B option should be for this question. Your explanation supports answer B Mr. Sundar. So read Answer carefully.

Since p = p(atom)-qgh, hence pressure decreases while moving in upward direction.

At the hill as p is less, the water in the pressure cooker starts boiling at a lower temperature then 100 degree celsius hence rice doesn't get sufficient amount of heat for cooking purpose.

Hence option A is correct.

Option A is correct because at high altitude, pressure is low, and this low pressure causes water to boil at a low temperature, less than 100 Celsius. So, it will take longer time

But if water boils at less than 100*c, then shouldn't the cooking time be lessened as less than 100*c is obtained faster than 100*c, right? please explain.

The answer is B because when we will be in high place pressure. Will be less therefore the water takes high heat to boil means more than 100°.

Very correct @M. Sahu.

Because temperature and pressure are indirectly proportional.

Thus in hills, less temperature would mean high pressure.

If water boils at a temp less than 100°C, why is there an increase in cooking time??

Sundar has explained nicely the phenomenon.