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Chemistry Review: States of Matter

Hot and cold

Drops of blue and green food coloring are added to two plastic cups filled with water. The first cup contains cold water while the second cup contains hot water. The color disperses through the hot water much more quickly than the cold water, yielding a uniform green hue before the coloring in the cold water mixes appreciably.

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  • If you put food coloring in cups of hot and cold water, the food coloring mixes faster in the hot water than the cold.
  • Why does this happen?

Heat affects the motion of molecules

Two plastic cups filled with water that have had food coloring added to them.  In the cup with warm, water the colors disperse through the liquid much more quickly than in the cup that contains cold water.
  • Adding heat energy to water increases the motion of the water molecules and the molecules of food coloring.
  • The faster-moving molecules cause the food coloring to mix into the water faster.
  • The water molecules and food coloring molecules in cold water don’t move as quickly as in hot water. The slower moving molecules result in slower mixing of the water and food coloring.

Water's attraction

Several molecules of water associate with one another by aligning according to their corresponding areas of slight negative and positive charge. These small areas of charge are produced by unequal sharing of electrons in their bonds between oxygen and hydrogen.

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  • It’s the motion of the molecules and their attractions for one another that determine whether a substance is a solid, liquid, or gas.
  • Water molecules are attracted to each other based on their slight positive and negative areas.

Water changing to ice

Molecules of liquid water turn to ice. The motion of water molecules slow down sufficiently to allow them to associate with many other water molecules and become incorporated a large lattice structure.

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  • If water is cooled enough, the molecules slow down so much that their attractions overcome their motion.
  • The molecules arrange themselves in a repeating pattern in a rigid structure. The water has changed from liquid water to solid ice through the process of freezing.
  • The water molecules in ice are further apart than the molecules in liquid water. In the vast majority of substances, molecules are closer together in the solid state than in their liquid state. But this is not the case for water.

Ice melting

As heat is added to water molecules in ice, the heat increases the motion of the molecules. Eventually, when enough heat is added to the system, water molecules can be sufficiently energetic to break away from their surrounding water molecules in the ice crystal. As water molecules break away from the crystal structure, the ice melts.

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  • As ice, the water molecules are held in fixed positions and cannot move past one another but they do vibrate.
  • When heat energy is added to ice, the motion of the water molecules increases. This motion competes with the attractions between the molecules.
  • When the motion of the water molecules overcomes the attractions between the molecules, the ice melts, changing from a solid to a liquid. This video appears courtesy of Roy Tasker.
    © Roy Tasker, VisChem Project.

Liquid water

In the liquid phase, water molecules associate with one another because of their corresponding areas of positive and negative charge, but can still slide past each other.

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  • As a liquid, the combination of the attractions between the water molecules and their motion keep the molecules close together but sliding past each other.
  • The molecules still feel the attractions from other molecules but their added motion allows them to move more freely.

Evaporation and condensation

When some water molecules in liquid water take on energy, they can break away from the other water molecules that make up the liquid and become water vapor. This is evaporation. When molecules that are in the gas phase as water vapor approach liquid water, they sometimes associate with the molecules of liquid water and return to the liquid phase. This is condensation.

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  • Some water molecules have enough energy to overcome the attractions keeping them together.
  • These molecules break away to become the gas, water vapor. This process is evaporation.
  • Some molecules of water vapor contact the liquid water and are attracted by the molecules in the liquid.
  • These water vapor molecules can form associations with other molecules in the liquid and join the liquid water. This process is condensation. This video appears courtesy of Roy Tasker.
    © Roy Tasker, VisChem Project.

Heat vs. temperature

  • “Heat” and “Temperature” mean different things, though sometimes you may hear them used interchangeably.
  • Temperature is a measure related to the motion of the atoms, ions, or molecules of a substance. Temperature can be measured with a thermometer.
  • Heat is the energy transferred from one substance or object to another. Adding or removing heat energy increases or decreases the motion of molecules, resulting in a higher or lower temperature.
  • Heat is always transferred from the warmer substance to the cooler one based on the temperature difference between them.

Heat vs. temperature: An example

A drop of water in an open hand
  • When you place a drop of room temperature water on your hand, the heat energy will move from your hand into the water.
  • This is because your body temperature (around 98°F) is higher than the temperature of the water.
  • The temperature of the water will increase while the temperature of that part of your hand will decrease. Your brain will register that your hand feels cooler.

Summary

  • Matter exists in three states or phases: solid, liquid, and gas.
  • A combination of the motion of molecules and their attraction for one another determines whether a substance is a solid, liquid, or gas.
  • Adding heat energy increases the motion of molecules.
  • Removing heat energy, or cooling, decreases the motion of molecules.
  • If heat is added to a solid, the molecules can move fast enough to change from a solid to a liquid. (Melting)
  • If heat is added to a liquid, the molecules can move fast enough to change from a liquid to a gas. (Evaporation)
  • If heat is removed from a gas, the molecules can slow down enough to change from a gas to a liquid. (Condensation)
  • If heat is removed from a liquid, the molecules can slow down enough to change from a liquid to a solid. (Freezing)
  • Heat always moves from an object at a higher temperature to an object at a lower temperature.

Molecular Animation Downloads

The animations featured in the slideshow above are available for download by clicking on the "Download this animation" link below each file.

Each video is offered for download in both Quicktime Movie (.mov) and Windows Media Video (.wmv) format.

To play these videos, you will need either Quicktime or Windows Media Player.