1. State or Phases of Matter
One property of a substance is its state of matter. You can tell the state of a material by answering the following questions:
• Can its shape change?
• Can its volume change?
Volume is the amount of space a sample of matter occupies. Three states of matter are solids, liquids, and gases. The table below shows whether shape and volume change for a solid, a liquid, and a gas when moved from one container to another.
Moving Particles
All matter is made of tiny particles. The particles of matter are always moving. Particles in solids move quickly or vibrate back and forth in all directions. They can’t move from place to place. In liquids, particles are farther apart. They can slide past each other. In a gas, particles move freely
rather than staying close together.
Attraction Between Particles
Particles of matter that are close to each other attract, or pull on, each other. The stronger the attraction on each other, the closer together the particles are. Because particles of a solid are close together, they attract each other strongly. Particles of a liquid can flow because the forces between the particles are weaker. Particles of a gas are so far apart that they are not held together by attractive forces.

2. Changes of State Solid to a Liquid Liquid to a Solid
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.
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 (  °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 °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.
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.