During a soccer game on a hot day, you reach for a cup of water with ice cubes in it.  If you leave your cup out in the heat during the game, the ice might melt and turn back into water.  When you get home from your game, you see a pot on the stove with curls of steam drifting up from it.  Water, ice, steam.  These are all water, but why do they seem so different?


Water freezes when it's cold outside.  When there is less heat, the water molecules do not have as much energy and don't move as much.  They lock together to form into ice.  They stay packed together tightly, like a group of penguins bunched up together to keep warm in Antarctica.  They hardly move.  Once the water is frozen, it holds the same shape and takes up the same amount of space -- until it melts, of course.  Ice is a solid, meaning it always keeps the same shape and takes up the same amount of space. 
 

When water freezes it turns into solid ice.

Photo by Petr Kratochvil

When the sun beats down on your water bottle, it adds energy in the form of light.  This light heats the water and makes the water molecules move faster.  Eventually they move away from each other.  Think of it this way: have you ever tried running while holding your friends' hands?  Once you get moving fast, it's hard to hang on.  That's what happens with the water.  The molecules cannot stick together once things heat up.  They move over, under and all around each other.  Over time, the ice begins to lose its shape and turns into water.  Water is a liquid, which means it always takes up the same amount of space, but does not keep a fixed shape.  Liquids can squish around and take the shape of whatever is holding them, like your water bottle.

Water as a liquid is easier to drink.

Photo by Hans Braxmeier

If you left your water bottle outside with the cap off and came back a few days later, some or most of your water would be gone.  What happened?  As time passes, the sun adds a lot of heat and energy to the water.  The water molecules move faster and faster, spreading out even more than they did when they turned into liquid water.  Some of the water moves so much that it escapes the water bottle, and it moves around in the open air.  This is how water turns into a gas.  A gas does not have a set shape and does not take up a fixed amount of space.  The group of molecules in a gas will take up as much room as you give them, whether it's a water bottle or the entire sky.
 

When water evaporates it turns into a gas. If it's really hot, it's steam.

Photo by Darren Lewis

What if you kept adding heat to the water after it was already a gas?  The heat energy would make the molecules move around so much they would finally break apart into tiny pieces.  Have you ever shaken a toy until its pieces started flying off?  Maybe the wheel of a toy car flew off and kept on moving.  You would need a lot of energy for that.  Plasma is made when a gas has so much energy that the molecules separate into smaller pieces and keep on moving.  Sometimes things in this state go so crazy that they even let off light, like the lights in your classroom.

 
Stuff can be in different forms depending on how hot it is.  When you add heat to something, you add energy.  Then things move more, and move apart.  Even though ice looks strong and hard, the molecules only move a little bit.  Over time, the sun's heat changes the ice to water and even gas.  And when things are very hot, stuff can turn into plasma, which is what our sun is made of in the first place.



References:

Curtiss, C. F.  And J. O. Hirschfelder, "Gas," AccessScience. McGraw-Hill Education, 2012. Web. 12 Oct. 2013.

Hershkowitz, Noah And Joseph E. Borovsky, "Plasma (physics)," AccessScience, McGraw-Hill Education, 2012. Web. 12 Oct. 2013.

Nachtrieb, Norman H. "Liquid," AccessScience. McGraw-Hill Education, 2012. Web. 12 Oct. 2013.

"Phase State Changes." World of Earth Science. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. Detroit: Gale, 2007. Science In Context. Web. 12 Oct. 2013.

"Plasma." The Gale Encyclopedia of Science. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. 4th Ed. Detroit: Gale, 2008. Science In Context. Web. 12 Oct. 2013.

"States of Matter." World of Scientific Discovery. Gale, 2010. Science In Context. Web. 12 Oct. 2013.