Chemistry is Physics. Or maybe not.

Chemistry seems mysterious. How do we know if something is chemistry or physics? What does chemistry really studying? Is there any link between chemistry and physics? And why do we want to know about chemistry at all?

We don't have all the answers. But we do want to share with you our thoughts.

Key difference: if molecules are broken

First, we want to briefly talk about the fundamentals of the world, only to certain extend of course. The world is made up of particles. Protons, neutrons, and electrons make up atoms, atoms form molecules, and billions and trillions of molecules stay together, move together, and be seen by humans together.

We can't see the particles with our naked eyes, as the particles are in microscopic scales. We only see macroscopic things such as a piece of paper.

When we see a piece of paper, we're looking at a tremendous number of molecules within that small piece of paper. While we're drinking a cup of water, unnumbered water molecules flow into our mouthes.

In order for you to have a better understanding, and to gain a visual clue how molecules behave during different processes, we would use sardine fishes as an example.

The sardine fish

Each sardine fish is one molecule, which we won't be able to see without the help of modern instruments. When many molecules or sardine fishes come together, we'll then be able to see the big big shoal of fishes. This is basically what happens to the molecules within any thing you see.

When the shoal of fishes separated into two, the fishes are still alive, and they could recombine to foam the big shoal again. That, from molecules point of view, is physical change. Molecules are separated apart, but they're still the same molecule, and they could come together again.

But if you cut a single sardine fish into two parts, it's dead, and it won't come back alive as a sardine fish again. That is a chemical change when applying to molecules. You have broken the molecule and that molecule is gone forever.

Chemical change

So that is how we define physical and chemical changes, essentially depending on whether the molecule is broken or not.

Chemical change is a process in which molecules break to form new molecules.

Physics largely study physical processes, while chemistry mainly focuses on chemical changes. So physics harvests the sardine fishes, and we, chemists, cook the fishes.

How do we know if molecules are broken?

Again, we use sardine fishes to help us to understand, but this time, we look at only one sardine fish.

Each sardine fish has one head, one tail, two eyes, one heart, and two gills etc. It does not change whether it's swimming alone or with other fishes, neither does it change when it's alive or dead. However, if any sardine fish has no tail, probably it's dead but doesn't matter for our discussion here, we know this fish is broken.

Now keep that in mind while we moving on to molecules.

Each molecule will have its own constituents, just like a sardine fish. For example, one water molecule is made up of two hydrogen atoms and one oxygen atom. If it loses the oxygen atom, remember the poor sardine fish without tail, it's no longer a water molecule, and it's broken. So this is how we know whether one molecule is broken or not, just by examing its constituent atoms.

In other words, by looking at the atoms in the molecules, we're able to tell if the molecules have undergone a physical or chemical change. Thus, we know if we should apply physics or chemistry to study the process.

Let's look at one example now.

Imaging we're boiling a pot of water. We know something is happening because water in the pot turns into steam in the air. Is this a physical or chemical change? We would have to examine the constituent atoms of the molecules in water and steam first.

By using some modern equipments, we've found that all the molecules have two hydrogen atoms and one oxygen atom. No matter how hard we try, by changing temperature and pressure, or by changing the pot and source of heat, we couldn't get any molecule with different constituent atoms. So we concluded that the molecules in water and steam were the same, no molecule was broken during boiling, and it was a physical change. We could apply physics to further study this process.

Basically, the shoal of sardine fishes was just separated into several smaller ones, but all sardine fishes remained the same. No poor sardine fish has lost its tail.

Ok. Example Number 2. We were burning a piece of paper. Before we burned it, we examined the molecules in the paper, and we found that they were all made up of carbon, hydrogen, and oxygen atoms in a certain ratio. After burning, we examined all the molecules again. This time, surprisingly, we found molecules with various constituents. Some were made of carbon only, some were made of carbon and oxygen, and we even found some water molecules. New molecules have been produced, and that was a chemical change. We'll need chemistry knowledge to understand what has happened.

See? It's that simple. Just focus on the molecules.

We hope you could now better understand the difference between physical and chemical changes, as well as physics and chemistry.

In one word, physics studies processes where molecules are intact, and chemistry learns about how molecules break and form new molecules.

So they're totally different?

Hmmmm. Not that different though.

All are artificial

We've always talked about physics, chemistry, and biology etc. These are the subjects that we, humans, created so that we could understand our world easier. They are all artificial.

The reason we created these subjects is that we have limited time and energy to learn about this world. We have to purposely break down the world into different subjects, such that it becomes possible for us to learn.

But our world doesn't work in this way. It is integral and holistic. It has zero knowledge about how we classify the subjects, neither does it care about that. Physics, chemistry, biology, and of course, maths and all other subjects, they all work together at the same time to make things happening in this world.

There is indeed no way to 100% classify anything to any single subject. All the subjects have their own contributions to all happenings in the world.

We could definitely use physics to describe chemical processes, and we'd be surprised if we couldn't. But maybe using physics to study those aspects of the world is just too inefficient. That's why we created another subject chemistry so that we could better understand that particular aspect of the world.

This is very similar to addition and multiplication in mathematics. Multiplication could be considered as just a simplified way of performing addition in certain scenarios. For example, we want to add up 1000 apples, instead of writing \(1+1+1+...+1+1 = 1000\), we simply write \(1000\times1=1000\).

Same thing here, rather than using physics to explain everything, we use chemistry to study certain aspects of the world, which could be a more efficient way.

Meanwhile, people could decide whether they want to focus on physics or chemistry studies, especially at higher levels, because we just can't learn all of the vast knowledge of both subjects.

Thus we say, chemistry and physics, are not much different. They are just different ways of describing the same world from different perspectives.

In fact, there're some more fundamental links between physics and chemistry. Let's move on.

It's just electrostatic charges

Have you ever wondered why atoms make up molecules, and why molecules group together to form macroscopic substances? The answer is electrostatic interactions.

Electrostatic interaction is the interactive force between charged particles. It is classified to be one kind of electromagnetic interaction, which is defined in physics as one of the four fundamental interactions in this world (Wikipedia).

Physicists have been studying electromagnetic interaction for a long time. You probably have heard the story that Benjamin Franklin did the kite experiment to explore lightning and found that lightning is electricity (Wikipedia).

As chemists, we share the same principles of charge, and electrostatic interactions between charge particles etc. When atoms form molecules, the binding forces among atoms are the attraction forces between various charged particles.

For example, in water molecules, \(\ce{H2O}\), hydrogen atoms would share their electrons with the oxygen atom. Electrons get concentrated in between the nuclei of hydrogen and oxygen atoms. There would be an attrative force between the negatively charged electrons and the positively charged protons in the nuclei. Hence, the atoms are now bounded together to form a molecule, and would not separate apart easily.

In another example, in sodium chloride, \(\ce{NaCl}\), or table salt, \(\ce{Na}\) and \(\ce{Cl}\) form cations and anions respectively. These ions show significant attractions to each other and therefore would arrange themselves to form a special structure of sodium chloride, or a lattice structure. The electrostatic attractions between the cations and anions are so strong that the table salt will only melt at over 800\(\pu{^oC}\) whereby the ions start to separate from each other.

Now we know that electrostatic forces are holding atoms together to form molecules. How about among different molecules?

It is the same electrostatic interaction that groups molecules together.

Again, we use water as an example. We've already seen that boiling water is a physical process and water molecules remain intact before and after this process. So what's happening then? The answer is water molecules are separated apart by heating.

Despite being neutral in overall charge, water molecules have uneven distribution of charges. This leads to a slightly negative oxygen atom, and slightly positive hydrogen atoms. As such, the oxygen atom would be attracted to the hydrogen atoms on a neighbouring water molecule. As liquid, water molecules are touching each other because of this intermolecular electrostatic forces.

When we supply energy to water molecules by heating them up, we're promoting movements of water molecules so that they could overcome the intermolecular electrostatic forces and hence could escape from each other. Consequently, water molecules leave the liquid water in the pot, and fly into the air to become steam.

We could now conclude that a chemical process is just the same as a physical process since electrostatic interactions are broken in both cases. The only difference is that chemical processes break electrostatic forces inside the molecules, while physical processes only break electrostatic forces among different molecules.

Can you understand why we said chemistry and physics are the same but different?

It's important to have both

Generally, electrostatic forces among different molecules are weaker than those in the molecules. It's much much easier to boil water as compared to produce hydrogen and oxygen gases from water.

Not all physical processes would be accompanied by chemical processes, but chemical processes usually involves physcial changes. This is because when we supply energy, weaker electrostatic interactions would break before stronger ones.

By purposely creating both physics and chemistry subjects, we would allow physics to explore all physical phenomenons, not only electrostatic interactions, but immense knowledge on other physical processes too. Meanwhile, chemists could focus on a similarly vast knowledge on how to break molecules and create new one.

Chemistry and physics are not isolated, but are complementary to each other. Without physics, chemistry wouldn't understand how atoms and molecules make up substances. On the other hand, chemistry extends the horizon of physics by exploring the possibilities of breaking molecules. Together, chemistry and physics could give us a full picture of this world. They're equally important and are both the foundation of our modern science.