there are four states of matter, Solid, liquid, gas, and plasma. all states of matter have atoms which all move. atoms in a solid are closer together and vibrate, atoms in a liquid move more freely, but in a placid the atoms move freely like atoms but are more closer together, and atoms in a gas move without restriction. for a solid to convert into a liquid it has to melt ( like ice to water) for a solid to convert into a gas it must it has to go through the stage of converting into a liquid (except for dry ice) for a liquid to convert into a solid it mu st solidify (like water freezing into ice) for a liquid to convert into a gas it has to evaporate. for a gas to convert into a solid it has to solidify. for a gas to convert into a liquid it must also solidify. same for plasma
I'll give you an example: ice is a solid, when it melts it is a liquid called water. When water is heated, it will turn into a gas called water vapor. You can reverse this by cooling water vapor & it becomes water. Then u can get it cold and turn it into ice. This procedure can be used with almost any matter. Note, these are all physical changes and not chemical changes or u could not do this.