When a molecule is going through any physical change, solid to liquid or liquid to gas, as it changes, the temperature remains constant. it does not increase or decrease, it stays at that temperature/.
For example, water evaporates/ boils at 100 C. if you are heating water to the point of boiling, it will remain 100 C until all the water evaporates. Once it becomes a gas, the temperature will continue to increase
During the evaporation of a liquid the temperature remains constant.
Starting with a liquid, if you heat it, its temperature increase, when it reaches the boiling point, the temperature remains constant while the liquid transform into vapor, if the heating continues, the temperature of the vapor will increase. All phase changes of pure substances are done at constant temperature.
Matter is any <u>substance</u> that has mass and takes up space. Any physical object consists of matter. It doesn't matter whether it's an atom, element, compound, or mixture. It's all matter. (Not a pun, i promise ツ)
However, unlike some believe, not everything you encounter in the world is matter. Matter can be converted into energy, which has neither mass nor volume. So, light, sound, and heat <u>are not</u> matter. Most objects have both matter and some form of energy, so the distinction can be tricky. For example, <u>a candle flame</u> certainly emits energy (<u>light </u><u>and </u><u>heat</u>), but it also contains <u>gasses </u>and <u>soot</u>, so it is still matter. The building blocks of matter are called "atoms" and "molecules." They, too, are matter. The matter we can detect normally is called "baryonic" matter. However, there is another type of matter out there, which can't be directly detected. But its influence can. It's called dark matter.
Attached below is a picture of examples of matter.