Why does kinetic energy stay the same during a phase change?

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by Ron Kurtus (updated 8 February 2022)

An interesting property of matter is that its temperature will remain constant during a phase change, assuming its surrounding pressure is constant.

In other words, once the temperature of a liquid reaches its boiling point, the temperature will not increase until all the liquid has changed its phase to become a gas. Likewise, once the temperature of a liquid is lowered to its freezing point, the temperature will not decrease until all the liquid has changed its phase to become a solid. A similar effect occurs in the opposite direction.

Temperature is the average kinetic energy (KE) of the material's molecules. That means for a given temperature there will be some molecules moving faster and having a higher kinetic energy, while others move at a slower speed.

Temperature does not necessarily remain constant in transitions to exotic states.

Questions you may have include:

• What happens in a solid/liquid transition?
• What happens in a liquid/gas transition?
• What happens in exotic transitions?

This lesson will answer those questions. Useful tool: Units Conversion

Sufficiently heating a solid can cause it to become a liquid. Likewise, cooling a liquid can turn it into a solid.

Changing solid to liquid

In a solid, molecules vibrate in place according to the temperature of the material. As the solid is heated, its molecules move faster and gain kinetic energy (KE) until some of them break lose and escape the solid. Those molecules are in the liquid phase and are not as confined as in a solid.

Since the temperature of the solid is the average KE of its molecules, the solid actually loses some energy when these molecules escape. This keeps the temperature from rising beyond the boiling point.

As heat energy is added, more molecules escape, and the temperature remains relatively constant. Finally, when all the molecules have escaped the solid and transitioned into the liquid phase, the temperature can start to rise above the freezing point.

Example with ice

With ice, some of the molecules will break lose from the solid state and turn into water molecules as the material is heated. But when they do change, they take energy from the ice, thus keeping its average temperature of the ice at or below freezing. The temperature of the ice will remain near the freezing point until all of the molecules have changed their state to become liquid. Then the temperature of the liquid will increase as heat is added.

Changing liquid to solid

The opposite effect occurs when you cool a liquid to change its phase to a solid. Collections of liquid molecules will gather to form solid particles. That transition slightly increases the temperature or average KE of the liquid, which will remain at a temperature near to the freezing point until all the liquid has turned to the solid phase.

Thus water will remain at or near the freezing point until all the material has crystallized into ice. Then the temperature of the ice can start to decrease.

Liquid/gas transition

Sufficiently heating a liquid can cause it to become a gas. Likewise, cooling a gas can turn it into a liquid.

Changing liquid to gas

When a liquid—such as water—is heated at a constant pressure, some of its molecules get enough KE to escape the liquid. However, when the high energy molecules escape, they slightly lower the average energy of the liquid.

You can experience the lowered temperature when water evaporates into gaseous molecules.

Thus, more heat energy is needed to cause other molecules to escape. This keeps the liquid at a temperature slightly below that of the boiling point. Once all of the molecules have enough energy to escape into the gaseous state, the temperature of the gas can increase.

Changing gas to liquid

The opposite effect occurs when you cool a gas to change its phase to liquid. It will remain at a temperature near to the boiling point until all the gas has turned to liquid.

Exotic transitions

Heating or cooling a substance that is not among the three classical states can also cause a change in state or phase. Since the transition is more complex, temperature may not be constant in the change of state. Some examples include plasmas and glass.

(See Exotic States of Matter for more information.)

Gas/plasma transition

Heating a gas to extremely high temperatures can result in a change to the plasma state of matter. Likewise cooling a plasma will cause it to revert to a gas.

The temperature probably remains the same during the transition. However, I haven't seen any experiments to prove that.

Glass transition

Cooling molten glass results in a solid form of the material. Likewise, heating glass can melt the material.

When heated, glass becomes a liquid and cooling that liquid glass creates

Summary

The temperature of a solid, liquid or gas will remain constant during a phase change, assuming its surrounding pressure is constant.

In other words, once the temperature of a liquid reaches its boiling point, the temperature will not increase until all the liquid has changed its phase to become a gas. Likewise, once the temperature of a liquid is lowered to its freezing point, the temperature will not decrease until all the liquid has changed its phase to become a solid. A similar effect occurs in the opposite direction.

Temperature does not necessarily remain constant in transitions to exotic states.

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Physics topics

As condensation forms on a glass of ice water, the temperature of the air surrounding the glass __________.

Possible Answers:

is impossible to determine

Correct answer:

increases

Explanation:

As a substance condenses from the gas phase to the liquid phase, it loses energy in the form of heat loss. Heat is transferred from the water to the air, resulting in an increase in the temperature of the air. 

How much energy is required to boil 9 moles of liquid water at its boiling point, and what is the temperature of the water vapor product?

Possible Answers:

Correct answer:

Explanation:

The enthalpy of vaporization gives the amount of energy required to evaporate a liquid at its boiling point, in units of energy per mole. The total energy requirement to heat a given amount of steam is found by mulitplying the the number of moles to be vaporized by the energy of vaporization per mole.

The temperature remains constant throughout a phase change, thus the final temperature would still be 100°C.

How much heat must be added to raise a sample of 100g of water at 270K to 280K?

The specific heat capacity of water is

Possible Answers:

Correct answer:

Explanation:

The following fomula gives the heat needed to generate a given temperature change for a substance of known specific heat capacity:

where

However, in the event of a phase change (water melts at 273K), the heat of fusion or vaporization must be added to the total energy cost. The formula becomes:

Why does water boil at a lower temperature at high elevation?

Possible Answers:

The atmospheric pressure is lower at high elevations

There is a lower heat of fusion at higher elevation

Water has a higher vapor pressure at high elevation

The higher the elevation, the denser water is. 

Hydrogen bonds are easier to disrupt at high elevation

Correct answer:

The atmospheric pressure is lower at high elevations

Explanation:

Increasing temperature means that vapor pressure increases as well. When vapor pressure is equal to the atmospheric pressure, water boils. The atmospheric pressure is lower at high elevation, so water boils at a lower temperature. 

Using the heat curve, define the segment time(s) that the kinetic energy of the substance is increasing.

Possible Answers:

Correct answer:

1, 3, and 5

Explanation:

Remember, temperature is a measure of the average kinetic energy of molecules. Therefore the kinetic energy increases whenever the temperature is increasing. So, the kinetic energy is increasing during segments 1, 3, and 5.

Using the heating curve, determine which segment(s) relate to an increase in potential energy.

Possible Answers:

Explanation:

Remember, temperature is a measure of the average kinetic energy of molecules. So, the potential energy of the molecules will increase anytime energy is being supplied to the system but the temperature is not increasing. Therefore the potential energy is increasing during segments 2 and 4.

The heating curve represents a substance in phases solid, liquid, and gas. Which segment represents only the liquid phase?

Possible Answers:

Explanation:

When the kinetic energy is increasing (the temperature is also increasing) the substance is not going through a phase change. Therefore only the segments that are at an incline will have the substance in just one phase. The flat areas of the graph represent areas in which heat is being added, but there is no corresponding increase in temperature. Rather, this added heat energy is used to break the intermolecular forces between molecules/atoms and drive phase changes.

Finally, because liquids are higher in energy than solids, and lower in energy than gasses the middle slanted line must be the liquid phase. In this case it is labeled as segment 3.

In the given heating curve, which segment(s) correlate to a mixture of phases?

Possible Answers:

Explanation:

When kinetic energy is increasing molecules are simply moving faster. However, when the potential energy is increasing molecules are changing phases. Therefore, when the potential energy is increasing is when the molecule is changing phases. Therefore there is a mix of molecules during segments 2 and 4.

The given heating curve represents a substance in phases solid, liquid, and gas. Which segment represents the substance as it is boiling?

Possible Answers:

Explanation:

Boiling is a phase change from liquids to gas. Therefore we are looking for a segment that is flat (because the potential energy is increasing) and that is between the liquid and gas phases. 

In this case, gas phase is the highest energy phase, and liquids is the next highest. Therefore the substance is boiling during segment 4.

In the heating curve shown above, at what point do the molecules have the highest kinetic energy?

Possible Answers:

The beginning of segment 5

The beginning of segment 1

Correct answer:

The end of segment 5

Explanation:

Remember, temperature is a measure of the average kinetic energy. Therefore the kinetic energy will be the highest when the temperature is the highest.

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