Changes of State It can be tricky to eat a popsicle outside on a hot day. In just a short time it will begin to melt. Slowly the solid popsicle becomes a liquid. As the popsicle melts, it goes through a change of state or phase change. A change of state when a substance changes physical form but not in molecule make up. The molecules do not change into something different they just gain or lose energy and movement. Ice, liquid water, and steam are all the same substance—water or H2O. The molecules of a substance move differently depending on the state of the substance. The molecules have different amounts of energy in each state of matter. So the main difference between a solid, liquid, and a gas how much the molecules are moving. 1. What is a change of state or phase? Solid to a Liquid One change of state happens when you add energy to the substance. This change of state is called melting. By adding energy to the molecules in a solid the molecules begin to move quicker and can break away from the other molecules. This happens slowly as each and every single molecule in the substance has to get enough energy to move quicker. Once the molecules have enough energy they begin to slide past the other molecules, and that is what makes a solid turn into a liquid. The temperature at which a substance goes from a solid to a liquid is it melting point. Ice melts into water at 32o F (0o C), whereas gold melts at 1,947.52 °F (1064.18 °C). That is quite a difference. A soft metal called gallium will melt in your hand because its melting point is 98 °F (30 °C), but the compound salt will not melt in your hand. It’s melting temperature is 1473 °F (801 °C). For a solid to melt, its molecules must overcome some of their attractions to each other. When a solid melts all of the energy goes into breaking the attractions that hold the molecules in place. Because you are adding energy melt is an endothermic change. 2. What is melting? 3. Adding energy or Taking away energy? Liquid to a Solid The change of state from a liquid to a solid is called freezing. Freezing is the reverse process to melting. For a liquid to freeze, the attraction between the particles must overcome the motion of the molecules sliding past each other. Think of it like a friend trying to grab your arm to hold you in place when you are stuck in the mob of students moving down the hallway. Your friend has to be stronger than the flow of traffic to hold you in place. When a liquid turns into a solid the molecules slow down and begin to vibrate in place. To cause freezing energy must be removed. Because you are removing heat to freeze a substance it is called a exothermic change. The point at which this happens is called the freezing point. Freezing points and melting point happen at the same temperature it just depends on whether you are adding or removing heat. 4. What is Freezing? 5. Adding energy or Taking away energy? 6. What is the difference between freezing point and melting points? Liquid to Gas If you’ve ever filled your bathroom full of steam you’ve experienced this next change of state called evaporation. Or how about that glass of cool aid you left out on the counter. You come back a few days later and it has less liquid in it than before….the liquid evaporated. Evaporation occurs at the surface of a liquid. You can see rapid evaporation in the form of boiling. The difference between boiling and evaporation is that boiling causes more of the molecules to move faster and thus they escape their attractions to each other quicker. Evaporation happens on the surface and is a much slower process. By adding energy you cause the molecules to move faster and break away from the others. The temperature that evaporation occurs is called the vaporization point or boiling point. Water boils at 212 °F (100 °C). 7. What is Evaporation? 8. Adding energy or Taking away energy? Gas to a Liquid The change of state from a Gas to a Liquid is called Condensation. If you’ve ever noticed that you mirror gets foggy in the bathroom or that the grass is wet on a spring morning even though it hasn’t rained, then you’ve seen condensation. For gas molecules to become a liquid the attraction for each other has to overcome the extreme movement of the molecules. Again think of the hallway example, but the crowd is moving much quicker. The point at which a substance goes through condensation is called its condensation point. For the quick moving molecules in a gas to slow down they have energy removed. Condensation is evaporation in reverse. 9. What is Condensation? 10. Adding energy or Taking away energy? Remember the molecules do not change chemically in a change of state, they only change in movement. 11‐14 Label each line on the diagram below with the name of the change of state..