States of Matter

How many states of matter are there? If you remember your general science class, you are likely to say three; solid, liquid, and gas. But, somewhere along the line, you like in a physics course or physical science course, you might have heard the number four; solid, liquid, gas and plasma. I have this debate with my students all the time, how many states of matter are there – Is it three? Is it four? Or, are their more?

Because, I teach both physics and chemistry, I love this debate. I typically start out by asking, what is gelatin? Is it a liquid or a solid? How about glass? What about slime or Silly PuttyTM? And if you really, what to stretch it – what happens when you cool a gas to almost absolute zero (-273 degrees C)? These things don’t fit into our traditional understanding of solids, liquids, gases and plasmas. They are different. Gelatin and other gels have some solid characteristics as well as some liquid characteristics. Similarly, the organization of the atoms in a clear pane of glass are going to look more like the organization of the atoms in a liquid than they would a solid, yet, glass will hold its shape and seems to fit our common definition of a solid.

Well, we can add another one to the list. On December 6, Jorg Roller, had a paper approved in the Proceedings of the National Academy of Sciences (PNAS) titled “Observation of liquid glass in suspensions of ellipsoidal colloids.” (I love the titles of scientific papers. They convey specific information but for the causal reader, you may or may not have a clue as to what really cool information is being described within.) This paper essentially announces another state of matter. The authors are saying that the components of a colloid will arrange themselves into a liquid glass state.

To get an idea about what this really means, one must first understand what a colloid is. Colloids are mixtures of matter where you have larger particles suspended or dispersed in another substance. Homogenized milk is a good example, the fat molecules are dispersed throughout the watery whey of the milk. In the laboratory, colloidal solutions usually involve the suspension of micron sized particles made of polymers or other complex materials suspended in a liquid. Ferrofluids, electrorheological fluids, and colloidal silver are additional examples. Opals are fossilized colloids. These materials are particularly handy to have around as they can be manipulated by electrical or magnetic fields to produce devices like liquid crystal displays. So, this new state of matter described and identified in the paper, may have implications for the next generation of liquid crystal displays. Or, other potential liquid crystal applications, such as smart windows.

Back to the original question – how many states of matter are there? On a practical level, there are definitely more than four.

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