To better understand the concept of atom and molecule, imagine that you have to split a piece of something, for example water. And then it goes on dividing, dividing and dividing… until it reaches a portion that it can no longer divide. This portion we call molecule.
Molecule - smaller portion of a substance containing the same composition as that substance.
These molecules then define the substance water:
- tasteless (has no taste)
- odorless (no smell)
- colorless (no color)
The molecules are formed by other smaller particles that we call atoms, which in Greek means not divisible.
Atom - is that which cannot be divided.
The physical state of a substance depends on the organization of its atoms and molecules. This organization will depend on the conditions of this substance.
In the solid state, the molecules are tightly joined together. They vibrate around virtually fixed positions.
In the liquid state, the molecules are slightly farther apart than the molecules in the solid state. They vibrate more intensely and move more freely.
In the gaseous state, there is a great distance between the molecules. The movement between them is disordered and even collide.
Some factors cause the physical state of substances to change. Among these factors, we have the temperature and the pressure.
When we want to boil water, that is, turn liquid water into steam, we provide heat. Then there is a rise in temperature until the water boils. No longer providing heat, the water temperature decreases to room temperature. If we refrigerate this water, it will lose heat, so the temperature will decrease. When you put it in the freezer, the water will lose even more heat until it freezes.
So when the water:
Absorbs heat - molecules become more agitated and temperature increases.
Lose heat - molecules become less agitated and the temperature decreases.
Another factor that influences the change in physical state is the pressure. When we increase the pressure of a gas, for example, the molecules get closer to each other, making them more organized, thus moving to the liquid state. Decreasing this pressure makes the molecules farther apart into gas.
In the figure above we can observe the changes in physical states and what happens when we increase or decrease the temperature or pressure of a substance.