1. Fresh water freezes at O0C. This is called the freezing point of water.
2. Ice melts at O0C. This is called the melting point of water.
Yes, the freezing and melting point of water is the same temperature, which is O0C.
Important:
1. Liquid water and ice can both exist at the freezing point of water, which is O0C.
2. The change of state from liquid to solid or solid to liquid occurs at O0C
Change of State: Liquid to Solid
Without changing temperature, liquid water loses energy, moves slower, and when enough energy is lost solid ice is formed.
Change of State: Solid to Liquid
Without changing temperature, ice gains energy, moves faster, and when enough energy is gained the structure of ice breaks open and liquid water is formed.
The diagram uses cartoon people to represent the changes of state. Amount of clothing represent energy. The boy on the right is dressed with a warm coat and other warm clothing. This boy is very active. He can move around faster than the others in the diagram.
The boy in the center has on some protective clothing, but not enough for him to be very active.
The group on the left have on the least amount of clothing. They are moving very slow and are grouping together. Like ice crystals, which are made up of six water molecules, this group is made up of six cartoon characters.
Think About This
1. What would happen if the boy with the most clothes gave his clothes to the the boy with medium clothes?
The answer is that the two boys would change places.
The answer is that the two boys would change places.
2. What would happen if some of the kids in the group were given warm winter clothing?
The answer is that the group would break up and each kid would be warmer. The kids would have more energy, but the the temperature would still be O0C.
Wait a minute! Where did all the warm clothes come from?
While not shown, there are other kids dressed in warm winter clothes and other groups of kids huddled together without winter clothing.
The answer is that the group would break up and each kid would be warmer. The kids would have more energy, but the the temperature would still be O0C.
Wait a minute! Where did all the warm clothes come from?
While not shown, there are other kids dressed in warm winter clothes and other groups of kids huddled together without winter clothing.
Remember: The total amount of clothing being swapped back and forth does not change.
The swapping of clothing represents the exchange of energy between liquid water molecules and water molecules making up ice crystals at the freezing point of water, O0C. The temperature remains constant because the total amount of energy being swapped back and forth between liquid water and crystalline ice remains the same. The energy is being used to change the physical structure of crystalline ice to liquid water.
Think About This
1. At the surface of an ice cube, there is a thin layer of liquid water. This water layer as well as the layer of solid ice beneath it are both at the same temperature,
the freezing point of water, O0C.
2. Liquid water molecules are more energetic than are the water molecules making up ice crystals.
3. To be more energetic, the liquid water molecules must have more energy than the water molecules bound together in the ice crystals.
4. At the surface of the ice cube, there is a constant exchange of energy from the liquid water to the water in ice crystals. This means that ice crystals break up and change to liquid water. At the same time the liquid water molecules that have lost their energy form an ice crystal.
3. To be more energetic, the liquid water molecules must have more energy than the water molecules bound together in the ice crystals.
4. At the surface of the ice cube, there is a constant exchange of energy from the liquid water to the water in ice crystals. This means that ice crystals break up and change to liquid water. At the same time the liquid water molecules that have lost their energy form an ice crystal.
Ice Crystals + energy —–>Liquid water
Liquid Water -energy —->Ice Crystals
Liquid Water -energy —->Ice Crystals
If the ambient temperature is at the freezing point of water, O0C. you might see ice in liquid water puddles.
As long as there is an equal exchange of energy between the water and ice crystals, the amount of liquid water as well as the amount of ice crystals does not change. This means the energy exchange is at equilibrium.
Challenge:
Ice has formed on highways, patios, and walkways. The weather man says the air temperature is not going to rise above freezing. What can be done to melt the ice?
Fact:
1. There is always a thin water layer on the surface of ice as long as the ice is above -150C. Ice above -150C is referred to as wet ice. This means there is a thin layer of water on the surface of the ice.
2. While the thin layer of water and thin layer of ice below it are at O0C, the ice below the surface is generally colder if the air temperature has been below O0C..
1. There is always a thin water layer on the surface of ice as long as the ice is above -150C. Ice above -150C is referred to as wet ice. This means there is a thin layer of water on the surface of the ice.
2. While the thin layer of water and thin layer of ice below it are at O0C, the ice below the surface is generally colder if the air temperature has been below O0C..
Answer
1. When table salt is sprinkled over ice it melts in the thin layer of water on the surface of wet ice. A solution of salty water now covers the surface of the ice. Salty water freezes at a temperature below O0C. This is because the presence of the salt particles prevents the water molecules from combining to form ice crystals.
2. The ice crystals below the now salty water layer continue to absorb energy from the water layer. The water molecules losing energy cannot combine to form ice because the salt particles are blocking them.
Think About This
1. Energy from the water molecules is being transferred to ice crystals.
2. The presence of salt lowers the freezing point of water. The salty water does not freezes at O0C, thus the salty water gets colder as its energy is transferred to ice crystals.
3. There is no longer an equal change of liquid water to Ice crystals. When the ice crystals absorb energy from the salty water, the crystals melt forming liquid water. This fresh water mixes with the salty water and increases the amount of water on the surface of the ice. Layer after layer of ice absorbs energy from the salty water and melts. As the energy is taken away from the salty water its temperature decreases.
