Introduction
"It is well-known that matter exists in three forms: solid, liquid and gas."
I picked this quotation from an NCERT textbook for grade XI, but I could have picked a different textbook, and found a similar statement there. It seems clear and uncontroversial enough, and has been repeated many times. What I would like to do in this lecture is to re-examine critically this commonly-accepted wisdom. In the process, we will learn something about the different states of matter.
However, telling you more about properties of solids, or liquids, or gases is not my main aim. What I really want to do is to impress upon you not to accept any such statement without thought. In fact, if you bother to think for 5 minutes, all of you can realize that it is not fully correct. The student should be able to think for himself/herself, and ask if the matter being taught is correct or incorrect. And if it is correct, to what extent? So that is my real aim. But we will do it by example, by discussing states of matter.
Let us start by asking why is it that there are three states of matter. It is a question like: Why do we live in three-dimensional space? Why there are three generations of quarks? and so on. And with a bit of thought, you will realize that the correct answer in this case is that states of matter is a classification scheme, like filing cabinets. There are a lot of materials, and we choose the grouping scheme that is most convenient. Other possible classification schemes could be alphabetical (e.g. in a dictionary), or based on some common properties (zoological classification of different animal species) or some mixture of these (e.g. books in a library). Materials can be classified based on color, or electrical properties, or whether they are organic or inorganic, or conductors or insulators, etc. All these classification schemes are useful, and are used when convenient.
Once we recognize the fact that different states of matter are like filing cabinets, the number of cabinets is purely a matter of convenience. One can always divide a class into smaller classes, or merge smaller classes into a bigger class. So, the number of different states of matter is not a deep question at all: it is whatever we want it to be.
Sometimes this sort of discussion ends up being a discussion about words, i.e. what is the dictionary definition of solid, liquid and gas? We are not discussing words. We are discussing the ideas behind the words. You may say, "Oh, this is a colloquial word. It doesn't have a very precise meaning". But lots of colloquial words have been adapted with precise meanings in science. Words like force, work, pressure, in ordinary language, can mean a lot of different things. For example, you can speak forcefully, or apply political pressure. However, in science, the meaning has been restricted to something quite specific, and you can quantify it as so many Newtons etc. So we would expect that even a common word like 'solid' can be given a specific meaning in science. Can we do that? And when we do that, what does it mean?
Let me clarify at the outset that 'states' of matter is not the same as 'phases' of matter. So you can have magnets, for example, and if you heat them, they become non-magnetic. There is a phase transition from magnetized phase to non-magnetized phase, but it remains a solid throughout. So you have a change of phase but not a change of state. So the 'state' of matter is a more general notion than 'phases' of matter and we are not going to discuss the latter. Also, we will restrict our discussion to simple materials. Things like salads are complicated and not the same everywhere: different parts are different. We are going to discuss only the simpler homogeneous matter.
The rest of the lecture is organized as follows. First we will discuss classification schemes. Then I will argue why liquid and gases should be treated as one state of matter. Then, we will look at differences between solids and fluids, and discuss different possible definitions of solids. I shall then discuss materials which are different from both solids and fluids, and are better treated as a separate state of matter. For lack of time, I will discuss only one of these, namely powders, and mention some of their unusual properties. And finally summarize our discussion.