Explain what happens to molecules during a phase change?
By: Preston G
What are molecules?
Molecules are tiny little particles that make up a substance. Since all substances are matter, they are the things matter is made of. Molecules also take the properties of the substance they make up. Molecules are constantly in motion. Even the molecules of ice are in motion. As the temperature increases, the motion of the molecules also increases. The hotter the substance, the faster the motion of the molecules is. Molecules of gas move very fast in comparison to the molecules of liquids and solids. Some substances have special kinds of molecules. For example, a chemical might have a mixture of molecules within itself. Each substance has its own molecules. People can’t see them. Some molecules can even dissolve. It takes more than speed for water molecules to escape the liquid state.
Figure 1: One of Molecules Images

Molecule phase changes (evaporation and melting)

A phase change is what happens when a matter changes state to another. During evaporation, these faster molecules also must be near the surface, heading in the right direction, and they must avoid hitting other water molecules as they leave. Molecules like ice when melted absorbs thermal energy and its temperature rises, then the temperature stops rising and the molecule would change into a liquid. As a liquid molecule like water cools it loses its thermal energy so its particles start to slow down and come closer together. Attractive forces begin trap liquids particles so the crystals of a solid begin to form. Evaporation cools the liquid and anything near the liquid. Melting also happens to molecules during a phase change. As a molecule like dry ice is heated, its temperature rises until it reaches 100 degrees Celsius, then the molecule starts to melt. It boils the molecule and turns it to a liquid state. For example: when ice is melted it is turns into water; a liquid state.

Molecule phase changes (freezing and condensation)
Molecules make up the chemical elements. The chemical elements are the substances of which all matter is made. The molecules of pure elements contain only one type of atom. Liquid molecules like water which can come from rain when heated inside of a plastic or glass container of any sort would start to build condensation. As a gas cools, its particles slow down. When particles move slowly enough for their attractions to bring them together, droplets of liquid begin to form. As a gas condenses to a liquid, it releases the thermal energy it absorbed to become a gas. After the change of state is complete, the temperature continues to drop. Some molecules can change from solid state to the gas state without ever becoming a liquid. While this takes place surface particles of the solid gain enough energy to become a gas. Like dry ice, this is used to keep materials cold and dry. The molecules absorbs thermal energy from the objects around it, and changes directly into a gas. When the molecule becomes a gas, it absorbs thermal energy from water vapor in the air. As a result, the water vapor cools and condenses into liquid water droplets, forming fog.

Home-made diagram.jpg
Figure 2: four different types of phase changes

Atom- a part of particle considered to be indestructible
Chemical- a substance with distinct molecular composition
Liquid State- to become fluid or melt by condensation
Molecules- Molecules are tiny little particles that make up a substance
Substances- Something that has mass and occupies space
Thermal Energy- produced by or caused by heat

Encyclopedia Britannica Online School Edition, Inc., 2012 Web. 27 Sept. 2012 Br Molecules
Ganeri, Anita, Indoor Science New York Dillon Press 1993. Print.
Glencoe, McGraw-Hill, The Nature of Mature New York Dillon Press 2008
Marvelous Molecules, New York Website 2012