Keith S. Taber
Bill was a participant in the Understanding Science Project. Bill (Y7) was explaining that he had been learning about the states of matter, and introduced the notion of there being particles:
So how do you know if something is a solid, a liquid or a gas?
Well, solids they stay same shape and their particles only move a tiny bit
So what are these particles then?
Erm, they're the bits that make it what it is, I think.
Ah. So are there any solids round here?:
Yeah, this table. [The wooden table Bill was sitting at.]
That's a solid, is it?:
Yeah
Technically the terms solid, liquid and gas refer to samples of substances and not objects. From a chemical perspective a table is not solid. A wooden table (such as those in the school laboratory where I talked to Bill) is made of a complex composite material that includes various different substances such as a lignin and cellulose in its structure.
Wood contains some water, and has air pockets, so technically wood is not a solid to a chemist. However, in everyday life we do thing of objects such as tables as being solid.
Yet if wood is heated, water can be driven off. Timber can be mostly water by weight, and is 'seasoned' to remove much of the water content before being used as a construction material. Under the microscope the complex structure of woods can be seen, including spaces containing air.
Bill also suggested that a living plant should be considered a solid.
I think teaching may be a problem here, as when the states of matter are taught it is often not made clear these distinctions only apply clearly to fairly pure samples of substances. In effect the teaching model is that materials occur as solids, liquids and gases – when a good many materials (emulsions, gels, aerosols, etc.) do not fit this model at all well.
This is actually a very important distinction, not an exotic technical quibble.
Most children are taught 'science' as a series of definitions and formulas, which must be learned to satisfy the authorities, and which have almost no role in explaining and understanding real life.
We need to teach children to think. So, when I am teaching the solids-liquids-gases distinction, I always make a point of saying that almost everything they actually see, is a 'composite' of two or even all three of these things. Thus the solid-liquid-gas distinction refers to a pure, homogeneous substance.
Examples: The air we breathe has water vapor in it, plus lots of tiny solids. The normal 78% Nitrogen, etc pie chart refers to 'pure air'. The example of wood is excellent and I shall add it to my list of examples.
There are many such mis-apprehensions, over-simplifications in science teaching. When something dissolves in something else … is that a 'chemical' or a 'physical' reaction? When salt dissolves in water … has it become a 'liquid'?
I believe teachers would benefit from looking at the concerns of one Alfred Korzybski, concerning the problems with the verb 'to be'. If we approached all teaching with some suspicion of saying that anything 'is' something else … but rather looked at it from the point of view of asking, "how do we use this word? … what are the (possibly several, possibly contradictory) ways in which it is used?"… we would do a better job of getting our students to think, rather than just learn how to emit one set of symbols when they encounter another set of symbols.
In a well-known critique of Sir Arthur Eddington's popular science writings, Susan Stebbing argued that the word 'solid' means having properties such as those of a plank. It makes no sense to talk of a plank without solidity. (S. Stebbing, Philosophy and the Physicists, 1935.)
We perhaps need subscripts: solid1 is what everybody knows that 'solid' means for nearly all purposes in nearly all contexts (like Stebbing's plank*), and solid2 is what scientists strictly mean by 'solid' in technical accounts referring to phase and phase change and related phenomena.
I do not blame the learners for getting confused, or teachers who have to balance making good sense and being technically correct.
* And, of course, the length of the plank is always greater than the Planck length!