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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 7

Lesson 1: States of matter

# Specific heat, heat of fusion and vaporization example

Specific heat and phase changes: Calculating how much heat is needed to convert 200 g of ice at -10 degrees C to 110 degree steam. Created by Sal Khan.

## Want to join the conversation?

• Why is the heat of fusion so much less than the heat of vaporization? •   When a solid melts, the molecules start moving around, but still cling close together through dipole, hydrogen and van der Waals-bonds. When the liquid is vaporised, these bonds are torn apart, which requires more energy than just pushing the molecules around enough to move.
• I saw an episode of a Backyard Science show once that showed that hot water freezes faster than that of room temperature. Why is this? Considering this video shows that it would need significantly more energy. • The Mpemba effect is caused by convection currents which cause the bottom of the hot water to be colder than the bottom of the cold water, even though the top is much hotter. The hot water has much more time for convection than the cold water. Once the bottom is below 4 C (39.2 F), the convection reverses, causing the rest to freeze. In ancient times, it was attributed to the false concept that opposites amplified each other. Hope this helps!
• Shouldn't we always calculate with K in the equation not C? Especially @ because that's how the K's cancel out. •  When you're talking about change in temperature, C is actually the same as K. The difference between -10 C (263 K) and 0 C (273 K) is the same in both cases: 10 degrees. It's important to note the difference when you're talking in absolute values (0 C vs. 273 K) but when you're talking in relative values, the amounts will be the same. (Note that this does NOT apply to F. Each degree F is about half a degree C or K.)
• wait a minute, what's a joule? • The joule is a derived unit of energy or work in the International System of Units. It is equal to the energy expended (or work done) in applying a force of one newton through a distance of one meter (1 newton metre or N·m), or in passing an electric current of one ampere through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889).
• Why should temperature be measured in Kelvins, not degrees Kelvin? • Is the formula sal is using q = m*Cp*∆T ? • Why does hot water freeze before cold water? • The Mpemba effect is caused by convection currents which cause the bottom of the hot water to be colder than the bottom of the cold water, even though the top is much hotter. The hot water has much more time for convection than the cold water. Once the bottom is below 4 C (39.2 F), the convection reverses, causing the rest to freeze. In ancient times, it was attributed to the false concept that opposites amplified each other. Hope this helps!
• Shouldn't the Y axis read 'time', not 'heat'? • Its the dependence of temperature on the amount of heat you put in the system so the y axis is temperature and the x axis is heat...you could do the dependence of temperature on time i guess and if the source of heat would produce constant amount of heat in time, the dependence would look the same...i hope i answered what you asked
• I am not sure if my question is relevant to the topic , but I wish to know the answer to this question - HOW DOES NAPHTHALENE SUBLIMATE ? ( I know what sublimation is , but I want to know how that actually happens and is it because of it's structure ? ) • It is only within the confines of a pressurized environment that the liquid state exists at all. In space, only the sold and gas states exist. So, the real issue is why some substances melt instead of sublimating.

In order for a liquid phase to exist for a substance, there must be enough pressure to force the molecules to stay together after they have enough kinetic energy to cease being in the solid state. Otherwise, the substance will go straight from the solid phase to the gaseous phase -- there being just too much kinetic energy once the substance ceases to be a solid for the intermolecular forces to allow the substance to exist as a liquid.

Thus, each substance that can exist in all three states (there are some substances that cannot) has a minimum atmospheric pressure required for the substance to melt instead of sublimating. If the atmospheric pressure is too low, then the substance will sublimate. So that is why sublimation happens.

Another way of looking at this is to recall that reducing the pressure reduces the boiling point (but doesn't really affect the melting point very much). So, if the pressure is reduced enough, the boiling point can be reduced below the melting point. If that happens, the substance will skip being a liquid and go straight into being a gas.

In more technical terms, sublimation occurs when the atmospheric pressure is less than the vapor pressure of the solid phase at some temperature too cool for the substance to melt. 