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Chapter 3.2 Changes in States of Matter

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David Thaggard

on 6 October 2016

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Transcript of Chapter 3.2 Changes in States of Matter

photo credit Nasa / Goddard Space Flight Center / Reto Stöckli
Chapter 3.2 Changes in State
Grade 8 Physical Science
Thaggard

The three (3) states of matter are, of course:
Solid
Liquid
Gas
As we look at how you switch between the three states, keep in mind that each state has the same physical properties (melting point, boiling point) but there is a little bit of variation--solid water melts at 0C, while solid platinum melts at 1768C (over 3000F for you imperials out there).
Ok, so recall that when you change matter, you can change it physically or chemically. When you change matter's state, you've only changed it physically--states of matter are physical properties.
When you melt a solid, you're actually changing the energy in that substance. When something melts, energy is released (exothermic) and the particles increase in speed, eventually breaking free from their fixed positions that they had when they were solid.
Melting: Solid-->Liquid
Freezing is the opposite of melting. When something freezes, it absorbs energy (endothermic change) and its particles begin to slow down--so much that they fall into fixed positions and become solid. Kinda like you, after a day at the park.
Freezing: Liquid-->Solid
Picking up where we left off with Melting, if a substance continues to release energy (exothermic change) after it melts, it will eventually begin to vaporize. The molecules move faster and faster until they break the bonds that hold them together and begin bouncing around as a gas.
Vaporization: Liquid-->Gas
Sometimes, when conditions are proper, a solid can jump right over the liquid phase and go straight to gas, kinda like when you get one of those "Go Directly to Go" cards in Monopoly, except this is Science and not a board (bored, ha! get it?) game and you definitely don't get 200$ for this. But seriously, what can you do with 200$ these days? I spend 200$ just washing my lab coat. Sheesh!
Sublimation: Solid-->Gas
We won't spend much time on it now, but atmospheric pressure plays a pretty big part in states of matter and how quickly one state changes to another--and even if an element can take a particular state in our atmosphere.
Air Pressure & Changing States of Matter
Most solid substances have a melting point, which is the temperature when a solid melts. You can use the melting point of something to help determine what it is. Some solids, however, don't melt but combust--that is, they catch fire--but that's for a later chapter.
Just like melting, substances have a specific freezing point but it's not as practical to scientists as melting. And, let's be honest, it's not as much fun, either.
There are two kinds of vaporization:
#1: Evaporation, which occurs at the surface of a liquid and happens every day, everywhere.
#2: Boiling, which occurs all throughout your liquid (look for the bubbles). Boiling, like melting, is a good way to figure out what an unknown substance is and, like melting, can be pretty fun.
Dry Ice is Carbon Dioxide (the gas you exhale) frozen. CO2 freezes at -78.5C (-109.3F, again for you imperials) and needs a high atmospheric pressure (about 5 times higher than at sea level) to freeze on Earth. For this reason, you have to do it under special conditions, but is commercially available (and pretty cheap, too! Lucky Stores usually sell the stuff, at the front where the service desk is).
If you think about it, you might even realize you've probably already thought about it. For example, we call oxygen a gas because its most common state is gas--but under the right conditions, even oxygen can condense into a liquid or solid. Our atmosphere--and the pressure it exerts--is a huge reason most of the matter on Earth exists in any one of the states.
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