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• Anand Zambare

# Solids, Liquids, and Gases: The only states of matter?

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One of the easiest topics when it comes to science in our school days was ‘The states of Matter’. We all have enjoyed the ease of classifying the matter and state the definitions of states using different criteria. In this article, we are going to revisit the fundamental topic of states of matter and I hope I can put a different perspective of looking at the matter in front of you.

“We all know that matter exists in three forms: Solid, Liquid and Gases” are such the types of quotes you will easily find in any physics textbook from the high school level. But is it fundamentally correct? Well if all have bothered for thinking about this for some time I am sure we all would realize the story of states of matter. Let’s ask a simple question to ourselves, If matter exists in such a discrete way then why will it exist in three types only? Is it something to do with our three-dimensional world? Why are there only three generations of quarks? (If you are not familiar with this then don’t worry, it’s just another fundamental question related to three).

So the simple answer for me here is ‘The States of Matter’ is just a simple classification scheme.

You can classify using a lot of different schemes. For example, we can classify matter on its electrical properties, its thermal properties, its magnetic properties, and so on. It’s like we divide the given line into small pieces. We can make as many pieces as we want and continue the division and also we can go on combining the things and make a line segment once again. So no. of states of matter should not be a thing to think about. It can be as much as we need.

Before we go ahead with this, I want the readers to be clear enough that this discussion is not about words. We are not talking about words we are thinking about the Ideas behind all such words. When I clarify this I would like to clarify that, the word ‘States’ and ‘Phase’ of matter are also most of the time used as an alternative. But they represent completely different things. For example, a Magnetic material when heated losses its power of magnetization. So we can say that the phase changes from magnetic to nonmagnetic but the state of the matter remains the same.

Now let’s look at the classification scheme. How one should decide which classification scheme is good to have? Well I have few points to share a) Classification scheme should give us moderate no. of classes. It should not create very high as well as low no. of classes. b) For every class there should be a clear and unambiguous definition. c) Ease of classifying. For one it should not become so much difficult to classify things. d) It should be Natural/Useful. One can’t say that the matter with a density greater than 0.2346 is solid otherwise anything else. It should be natural.

With all this background, let’s dive into the important question. The distinguishing point between solids and liquids. It is something that is developed in our senses only, it depends on how we handle them. So in a scientific way if I want to define I can say that we classify depending on the kind of response it gives to the forces which will try to bend it, compress it, and so on. So we can say that the basis of classification is ‘Mechanical Properties of matter’.

Let’s look at the phase diagram below. It has Temperature along X-axis and Pressure along the Y-axis. It is very clear from the diagram that, the phase diagram is of water.

From the phase diagram, we can see that for a particular temperature and pressure the matter exists in a particular state which can be named or classified as Solid, Liquid, and Gas broadly. For any general case, if we fix a pressure value on Y-Axis and increase the temperature slowly, initially we start with a solid which melts to a liquid, and if continue heating will get transformed into a gas. However, it is found experimentally that above a particular value of pressure if we heat the liquid, there is no sharp boiling point. The material just keeps getting hot, so we can say that for critical values of pressure Liquids and Gases are indistinguishable. In other words, we should think like Liquids and Gases are in the same state. And together they will be called the fluid state.

Whenever we have discussed this topic with our high school teacher we may have heard about the ‘Plasma State’ which most of the people called the fourth state of matter. What is Plasma? So if we take gas and heat it, the atoms inside the gas will get ionized. As the temperature increases, the ionization increases, and the heavily ionized material is called as ‘Plasma state’. The important difference between plasma and the gaseous state is that they both react differently to electric fields otherwise there is no distinction between the plasma and gaseous state. They are the same basic states i.e. fluids (now I hope I have been specific enough).

When we started our discussion, I pointed out that we can’t keep on thinking about the states of matter as only Solids, Liquids, and Gases. There are things that behave partly as solids and partly as liquids. One of such things that we will discuss here is a powder that too in brief.

Powder as a different state of matter. Let’s think about the powder and try to classify it as a solid or a fluid. If we bring some powder and try to put that in different vessels (of different shapes) then the tendency of powder is to get fit in that shape just like liquids. But if we start pouring the powder on a table surface the powder will start to form a shape of the pyramid as shown in the figure below. The apex of the pyramid will form an angle called as ‘angle of repose’. This angle of repose will be zero for fluids as they tend to flow when we start pouring them. So by this argument, one can say they behave like a solid.

The flow of powders and liquids is for sure different. Their hydrodynamic, as well as hydrostatic behavior, is completely different. The figure below explains the variation of pressure at the bottom of the cylinder when we pour the water as well as sand to a particular height (h) in the cylinder.

We can clearly see that water is behaving perfectly with the laws of hydrostatics. But sand doesn’t behave in a similar way. On the other hand, after some threshold value when we continue to pour the sand we do not get any substantial increase in the pressure. This is very interesting behavior. I think this is good enough to make a point that the behavior is kind of ‘Solid like’ of the powder in few cases and it is ‘Liquid like’ in a few cases. So one can always argue that this is a new state of matter or one can say that it likes in between the spectrum of states of matter. So below is the figure which explains what I mean by a spectrum of states of matter.

So with this, we have come to a point where we can sense the presence of different states of matter than just solids or fluids. The powder is just one example out of so many. Foams, gels, colloids are such available materials. Before we conclude with this brief discussion here is one more mysterious state of matter which is being explored by a lot of astrophysicists called ‘Dark Matter’. No one can see it, its presence is still tested. If its presence is confirmed then the question of classification will come soon in front of the science community.

Summary:

• There are as many no. of states of matter as we can classify.

• There is a spectrum of states of matter and the extremum of this spectrum are called solids and fluids. Most of the materials that we will encounter in day-to-day life will fall in between the spectrum. They either are ‘Solid like’ or ‘Fluid like’ but again they depend on the situation like what we saw for powder.

• We should understand that it is really difficult to figure things out if they are exactly solids, liquids, or gases. The classification needs corrections depending on the length and time scales we are dealing with.

This blog is inspired by the article of Sir Deepak Dhar who is a theoretical physicist at TIFR Mumbai. He has published an article on 'States of Matter'. For more details, you can refer to his article.