So what exactly is chemistry? And what does it really study?
Changes
Let's first understand about changes, or processes you may say. Depending on their natures, changes could be categorized into physical changes, or chemical changes.
You probably have seen some examples of physical changes in you daily life. For example, when you're boiling water, you may see some "white smoke" coming out of the kettle. That "smoke" is nothing but still water. This is evident that if you put a piece of glass lid on top of the "smoke", some water droplets could be seen. So, water in the kettle is "moving" to the glass lid, and that's all. Nothing really "changed" and water is still water. That's why we call this process a physical change, where the nature of the substance is not changed. Water in this particular case, is only changing its physical state (as liquid, or gas), but does not become some other thing.
While for a "chemical change", it will be a different story. For example, when you burn a piece of paper, you'll notice some "black smoke", some "black powder" leftover, and maybe you can smell something. But you'll never get that piece of paper back. That's because the paper has gone through some chemical changes to become something else, so the nature of the substance is changed.
In short, we say physical change does not change the material itself, while chemical change will. And chemistry as one of the subjects of modern sciences, studies everything about chemical changes and the associated physical changes as well. It covers chemical changes from microscopic level to macroscopic level, and from lab scale of milligrams to industrial scales of tons. It is also closely related to various disciplines, such as physics, medicine, and biology.
Matter
But fundamentally what are the differences between physical changes and chemical changes? And how does chemistry differ from physics? Well, the story starts with matter.
Matter is anything that has a mass and occupies a volume in space. For example, a rock, some water in a glass cup, and air which you may not see but does exists.
Matter is having mass. When you put a rock on a weighing scale, you can see some numbers, say 1 kg. That is the mass of that rock.
There is a misconception that mass is equal to weight, but that's not true. If you put the same rock onto the Moon, you'll feel it lighter because the weight is changed when the Moon has less attraction on that rock as compared the Earth. However, the mass of that rock is still the same, because mass actually tells us how much material is inside that rock, or the quantity, and that doesn't change with where the rock is. We're not going to talk about very detailed principles behind this, for example, we do not care about the origin of mass because that's way beyond the scope of chemistry. For us, knowing that mass is different from weight and mass does not change is sufficient.
Matter also has volume. Volume tells us how much space matter is occupying. A laptop in a suitcase will stop you from putting in a thick stack of paper, because that laptop is occupying some space. When your cup is half-filled, you can only pour in another half cup of water because half of the space in that cup has already been taken up. Even the air you breath will have volume although you cannot really touch or see the air. The special thing about air is that it will occupy whatever volume is available, say in a box, and you can squeeze in more air even when there is already air in the box.
Nevertheless, all matters have mass and will occupy some volume in space. Mass of a particular matter does not change, while volume does change. We'll learn more about volume very soon.
State
But for now, let's talk a little bit more about air. Remember we said that it is special because it always takes up all the volume, and you can put in more air in the same box which already has air inside? Air is different from water and rock apparently because those are not going to fill up all the space in your room.
So what's different between rock, water, and air in terms of science? Well, they are in different states we say.
For example, water will exist in 3 states under different conditions. During winter when temperature is really low, water exists as ice, which is a solid. Solid is hard, rigid, incompressible, and having fixed shape and volume.
Water could also exist as liquid water which we drink daily. Liquid is soft, incompressible, having fixed volume, but does not have fixed shape as it can flow and adapt to the shape of the container.
When we're boiling water, water will turn into steam, which is a gas. As opposed to solid and liquid, gas is compressible, just think of compressing a balloon which has gas inside. And when you compress the balloon, it could shrink in size, or volume to be more specific. Therefore, you would know that gas has no fixed volume, or shape either. Gas will simple fill up whatever space available and take up the volume and shape of the container.
However, as you may have noticed, ice could melt and turn into liquid water, and liquid water could be boiled to become steam. We say that matters can change in their states. When a solid becomes a liquid, that's called melting or the solid has melted. Liquids can then freeze to become solids again. Liquids can also evaporate or boil into gases while gases will condense into liquids.
Although not very commonly seen, solids could turn into gases directly, which is named as sublimation. This phenomenon finds its application as the "dry ice effect" where dry ice or solid carbon dioxide is put into very hot water. The dry ice will then sublimate into gaseous carbon dioxide and at the same time reduce the air temperature surrounding it. Water in the air will then condense to form tiny water droplets which look like fog. People have been using this method to create fog on stage for years.
On the other hand, gases could also deposit to become solids instead of forming liquid. This is fundamental to modern semiconductor industry where people have been utilizing deposition of gases to produce thin film of materials on chips.
So, what is matter
Next, we'll really zoom into matters to understand what exactly matters are made of.