Changing States of Matter
All matter can move from one state to another. It may require extreme temperatures or extreme pressures, but it can be done. Sometimes a substance doesn't want to change states. You have to use all of your tricks when that happens. To create a solid, you might have to decrease the temperature by a huge amount and then add pressure. Some of you know about liquid nitrogen (N2). It is nitrogen from the atmosphere in a liquid form and it has to be super cold to stay a liquid. What if you wanted to turn it into a solid but couldn't make it cold enough? You could increase the pressure to push those molecules together. The opposite works too. If you have a liquid at room temperature and you wanted a gas you could use a combination of high temperatures and low pressures to solve your problem.
Phase changes happen when certain points are reached. Sometimes a liquid wants to become a solid. Scientists use something called a freezing point to measure the temperature at which a liquid turns into a solid. There are physical effects that can change the freezing point. Pressure is one of those effects. When the pressure surrounding a substance goes up, the freezing point and other special points also go up. That means it's easier to keep things solid at higher pressures. Just remember that there are some exceptions. Water (H2O) is special on many levels. It has more space between its molecules when it is frozen. There's a whole expanding effect when the molecules organize into a solid state. Generally, when temperatures get colder, solids shrink in size. They become more dense.
CHEMISTRY TERM PHASE CHANGE Fusion (melting)
Solid to Liquid
Liquid to Solid
Liquid to Gas
Gas to Liquid
Solid to Gas
Gas to Solid
Solid to Liquid and Back to Solid
Imagine that you are a solid. You're a cube of ice sitting on a counter. You dream of becoming liquid water. You need some energy. The atoms in a liquid have more energy than the atoms in a solid. The easiest energy to find is probably heat. There is a special temperature for every substance called the melting point. When a solid reaches the temperature of its melting point, it can become a liquid. For water, the temperature has to be a little over zero degrees Celsius (0oC). If you were salt, sugar, or wood, your melting point would be higher than that of water. How do you know that? If their melting points were lower, they would be liquids at room temperature. The reverse of the melting process is called freezing. Liquid water freezes and becomes solid ice when the molecules lose a lot of energy.
Solid to Gas and Back to Solid
You are used to solids melting and becoming liquids. Some of you may have also seen a solid become a gas. It's a process called sublimation. The easiest example of sublimation might be dry ice. Dry ice is solid carbon dioxide (CO2). Amazingly, when you leave dry ice out, it just turns into a gas. Have you ever heard of liquid carbon dioxide? It can be made, but not in normal situations. Can you go from a gas to a solid? Sure. It's called deposition when a gas becomes a solid without going through the liquid state of matter. Those of you who live near the equator may not have seen it, but closer to the poles we see frost on winter mornings. Those little frost crystals on plants build up when water vapor becomes a solid.
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