Changes of State
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The example we will use here is ice melting into water. Immediately after the molecular bonds in the ice are broken the molecules are moving vibrating at the same average speed as before, so their average kinetic energy remains the same, and, thus, their Kelvin temperature remains the same.
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Below is a picture of solid ice melting into liquid water. The molecule of ice and the molecule of water the black balls are moving with the same rate of vibration in this diagram. This is meant to show that they have the same average speed and thus the same average kinetic energy since they have the same mass and thus the same Kelvin temperature. The motions are, though, greatly exaggerated. Actually, the motions of the molecules should be considered tiny vibrations. In the ice the molecules are strongly bonded to one another, thus forming a rigid solid.
When heat is added to the ice these bonds are broken and the ice melts.
Change of Phase/State (Phase Transition) with Examples
The molecules afterward bond to one another with less strength and a different geometry, and water is formed. Now, before the melting, the molecules were actually moving when in the solid state. They were vibrating back and forth. They had an average kinetic energy. So they had a Kelvin temperature proportional to this average kinetic energy. After the melting the water molecules are still vibrating.
And they have the same average kinetic energy as they had before the melting. So, the water is at the same temperature at the moment after the melting that the ice was at the moment before the melting. Heat came into the situation, but it was not used to change the kinetic energy of the molecules. It was used to change the bonding between the molecules. Breaking the bonds between the molecules of the ice requires energy, and this energy is the added heat.
Energy Changes That Accompany Phase Changes
In a similar way heat enters a liquid to change the molecular bonding when the liquid boils or evaporates into a gas, and heat enters a solid to change the molecular bonding when it sublimates into a gas. In an inverse way heat leaves a gas to change the molecular bonding when the gas condenses into a liquid, and heat leaves a liquid to change the molecular bonding when it freezes into a solid. In none of these changes of state is the heat energy that is input or output used to change the speed of the molecules.
The average speed of the molecules is the same before and after a phase change, and so is the average kinetic energy. At this point the solid melts into liquid. The temperature at which this change from solid to liquid happens is called the melting point.
Each solid has a set melting point at normal air pressure. At lower air pressure, such as up a mountain, the melting point lowers. This change from liquid to solid is called freezing or solidifying.
Phase Changes - Chemistry LibreTexts
It is the opposite process to melting. When a liquid is heated, the particles are given more energy.
They start to move faster and further apart. At a certain temperature, the particles break free of one another and the liquid turns to gas. This is the boiling point.
The boiling point of a substance is always the same; it does not vary. This is the temperature at which water turns to steam. Steam is an invisible gas.
When it reaches the lid it cools back to a liquid. Even without boiling water in a kettle, some of the liquid water changes to gas. This is evaporation. It occurs when a liquid turns into a gas far below its boiling point.
There are always some particles in a liquid that have enough energy to break free from the rest to become a gas. Water that is present as a gas in the air cools down and changes into tiny drops of liquid water on leaves and windows. This change from gas to liquid is called condensation. History Government U.