Have you ever heard of liquid carbon dioxide? It can be made, but not in normal situations. Amazingly, when you leave dry ice out in a room, it just turns into a gas. The easiest example of sublimation might be dry ice. Some of you may have also seen a solid become a gas. You know about solids melting and becoming liquids. Liquid water freezes and becomes solid ice when the molecules lose energy. The reverse of the melting process is called freezing. How do you know that fact? If their melting points were lower, they would also be liquids when the temperature is above zero degrees Celsius. If you were salt, sugar, or rock, your melting point is higher than that of water. For water, the temperature needs to be a little over zero degrees Celsius (0 oC) for you to melt. When a solid reaches the temperature of its melting point, it can become a liquid. There is a special temperature for every substance called the melting point. The atoms in a liquid have more energy than the atoms in a solid. Heat is probably the easiest energy you can use to change your physical state. You're a cube of ice sitting on a counter.
There are many other types of molecular organizations in solid water than we can talk about here. Because the same number of molecules take up more space, solid water is less dense than liquid water. The molecules organize in a specific arrangement that takes up more space than when they are all loosey-goosey in the liquid state. It has more space between its molecules when it is frozen. The freezing process compacts the molecules into a smaller space. Generally, solids are more dense than liquids because their molecules are closer together. It is easier to keep things solid when they are under greater pressure. When the pressure surrounding a substance increases, the freezing point and other special points also go up. There are physical effects that can change the melting point. Scientists use something called a freezing point or melting point to measure the temperature at which a liquid turns into a solid. Sometimes a liquid wants to become a solid. Points of Change Phase changes happen when you reach certain special points. If you have liquid water (H 2O) at room temperature and you wanted water vapor ( gas), you could use a combination of high temperatures or low pressures to solve your problem. Eventually you would reach a point where the liquid became a solid. What if you wanted to turn it into a solid but couldn't make it cold enough to solidify? You could increase the pressure in a sealed chamber. It is nitrogen from the atmosphere in a liquid form and it has to be super cold to stay a liquid. Some of you know about liquid nitrogen (N 2). However, it will freeze at warmer temperatures when the pressure is increased. For example, oxygen (O 2) will solidify at -361.8 degrees Fahrenheit (-218.8 degrees Celsius) at standard pressure. To create a solid, you might have to decrease the temperature by a huge amount and then add pressure. You have to use all of your tricks when that happens. Sometimes a substance doesn't want to change states. It may require extreme temperatures or extreme pressures, but it can be done. \).All matter can move from one state to another.
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