Matter and non-matter

The mass-and-space test. Something qualifies as matter only if it clears two conditions together: it has mass, the quantity of substance it contains, measured in kilograms (kg) or grams (g) on a balance; and it has volume, the space it occupies, measured in cubic metres (m³) or, for liquids and gases, in litres (L) and millilitres (mL). Both conditions must hold — mass alone or volume alone is not enough, though in practice every real object with mass also occupies some space.

Why matter has these properties at all. Matter is not a smooth, continuous stuff. It is built from extremely small constituent particles, with interparticle spaces between them and forces of attraction holding them together. It is this particle structure — countless tiny bits of "stuff," each contributing a little mass and needing a little room — that gives any lump of matter its overall mass and its overall volume. The same particle picture explains why matter shows up in exactly three states (solid, liquid, gas): the difference between the states is only how tightly the particles are packed and how freely they move, not whether particles are present at all.

What falls outside the test. Non-matter covers two kinds of things. First, forms of energy that travel or radiate — heat, light, and sound — which can warm you, illuminate a room, or reach your ears, but cannot be placed on a pan balance or trapped in a fixed volume of their own. Second, abstract or "absence" phenomena, such as a shadow (a region blocked from light) or a thought (an idea with no physical substance) — neither is built of particles, so neither has a mass or a volume to measure.

A quick sorting rule. If you can, at least in principle, weigh it on a balance and say how much space it fills on its own, it is matter. If it is a sensation, a form of energy, or an abstraction, it is non-matter — even when matter is clearly involved in carrying it, such as warm air carrying heat, or air molecules carrying a sound vibration. Once an object passes this test and is confirmed as matter, the chapter's next question takes over: is it a pure substance (an element or compound) or a mixture? That classification only makes sense for things that are matter in the first place — non-matter items like light or sound are never sorted into elements, compounds, or mixtures.

There is no single equation for "what is matter," so the working here is entirely reasoning from the particle picture rather than a formula. The particulate nature of matter is the mechanism behind every part of the mass-and-space test, and understanding it is what stops the test from feeling like an arbitrary rule to memorise.

Step 1. Start from the particle model. Every solid, liquid, or gas you can name is made of enormous numbers of tiny particles — atoms or molecules — each too small to see individually. Each particle, however tiny, has some mass, and each particle needs some room, however small, to exist in.

Step 2. Mass adds up from the particles. When you weigh a glass of water on a balance, you are really weighing the combined mass of trillions of water molecules. Because the particles are real physical bits of "stuff," their masses simply add together to give the object's total mass — this is why mass never disappears just because an object is invisible (like air) or very light (like a puff of smoke).

Step 3. Volume adds up in the same way, but through spacing. The constituent particles are not glued into a single point; interparticle spaces separate them, and the particles jostle, vibrate, or move within those spaces. The total region taken up by all the particles together, spaces included, is what we measure as volume. This is also why the three states differ: particles packed close with little space give a solid its fixed volume, particles a bit farther apart and freer give a liquid its fixed volume but flowing shape, and particles far apart and moving freely give a gas a volume that expands to fill any container.

Step 4. See why non-matter fails both steps. Heat, light, and sound are not collections of particles carrying mass through space in this sense — they are energy being transferred or a vibration passing through a medium. A shadow is not a collection of anything at all; it is simply an absence, a region light did not reach. With no particles to add mass and no particles to occupy space, none of these can pass the test, no matter how strongly you can feel, see, or hear them.

Question: A sealed, deflated balloon is weighed and found to have a mass of 5.0 g. It is then blown up with air and weighed again, showing a mass of 5.3 g. A torch is then shone on the inflated balloon, casting a shadow on the wall, and the balloon warms slightly in the torchlight. Identify which of the following are matter and which are non-matter, giving a reason: the air added to the balloon, the torchlight, the shadow, and the warmth felt by touching the balloon.

Step 1. The air added to the balloon. The balloon's mass increased from 5.0 g to 5.3 g purely because air was added, so that extra 0.3 g is the mass of the air itself, and the air now occupies the balloon's internal volume. It clears the mass-and-space test. Air is matter.

Step 2. The torchlight. Shining light on the balloon does not add any mass to a balance, and light does not sit inside a fixed volume — it simply travels from the torch to the balloon. Torchlight is non-matter, a form of energy.

Step 3. The shadow on the wall. The shadow is only the region where the balloon has blocked the torchlight from reaching the wall; there is nothing there with mass or volume, only an absence of light. The shadow is non-matter.

Step 4. The warmth felt on touching the balloon. The warming is heat energy transferred from the light into the balloon's rubber and the air inside; heat itself has no mass and no volume, even though it is being carried by matter (the rubber and the air). The warmth is non-matter.

Final answer: the added air is matter (it accounts for the measured 0.3 g mass gain and fills the balloon's volume), while the torchlight, the shadow, and the felt warmth are all non-matter, since none of them can be placed on a balance or measured for a volume of their own.

• Thinking that anything sensed — felt, seen, heard, or smelled — must be matter; heat, light, and sound are forms of energy, not matter, even though matter often carries them.
• Assuming invisible substances like air are not matter; the balloon example shows air has a real, measurable mass and fills a real volume, exactly like a visible solid or liquid.
• Treating a shadow as if it were a physical object; it is an absence of light, with no particles, no mass, and no volume of its own.
• Believing that "very small quantity" means "not matter"; a faint smell or a speck of dust is still built of particles with mass, however hard that mass is to measure directly.

Ask the tutor: Not sure whether something tricky like fog, a magnetic field, or static electricity counts as matter — ask the AI tutor to run the mass-and-space test on your own example.