Primary & Middle School Physics · Grades 1–9

14Solids, Liquids, Gases

A stone, a glass of milk, the air in a balloon: three kinds of stuff that behave in three utterly different ways. Last year, water showed us its three states; this year we discover that everything is in one of these three states — and each state has strict rules.

14.1 Three families of stuff

All the stuff in the world can be sorted into three families, called the three states: solid like the stone, liquid like the milk, gas like the air. Ice, water and steam were our first example: one stuff visiting all three families. Now let us learn each family’s rules.

14.2 Solids keep their shape

Definition 14.1 (Solid)

A solid is stuff that keeps its own shape. A key keeps its zigzag in your pocket, in a drawer, or underwater. You can hold a solid between two fingers, and moving it does not change what it looks like. Some solids are hard (stone), some soft (bread), some bendy (a rubber band) — but left alone, each keeps its shape.

Example 14.2 (A solid made of grains)

Sand seems to break the rule: poured from a bucket, it takes the shape of its pile, almost like a liquid. But look closely: sand is a crowd of tiny solid grains, and each grain keeps its shape perfectly. Flour, sugar and rice play the same trick — crowds of little solids that flow together while each member stays solid.

14.3 Liquids flow — but keep their amount

Definition 14.3 (Liquid)

A liquid is stuff that flows: it has no shape of its own and takes the shape of whatever container holds it. But a liquid keeps its amount: pour a full glass of milk into a bowl and it is still the same amount of milk — not a drop more or less, only a new shape.

One glassful of milk poured from container to container: the shape obeys the container, the amount obeys nobody.

Proposition 14.4 (A resting liquid lies level)

The surface of a liquid at rest is always flat and level — parallel to the floor. Tilt the bottle: the water inside does not tilt with it; its surface stays level while the bottle leans.

Tilt the bottle as you like: the water’s surface refuses to tilt. A resting liquid always lies level.

Example 14.5 (The level at work)

Builders use this law every day: their spirit level is a little window of liquid with one bubble; when the bubble sits between its marks, the shelf is as level as the liquid inside. And at breakfast, watch the juice in a tilted carton: however you tip it, the surface inside stays flat and level — until it finds the spout.

14.4 Gases fill everything

Definition 14.6 (Gas)

A gas is stuff that spreads out to fill all the room it is given. Air is a gas: put it in a bottle, it fills the bottle to every corner; open the bottle in a bigger room, and it spreads through that too. A gas has neither its own shape nor its own resting place — offer it more room, and it takes everything.

Example 14.7 (The traveling perfume)

Someone opens a perfume bottle at one end of the room. A moment later, noses at the other end notice it. The perfume gas did not stay politely near its bottle: released, it spread through all the air of the room, corner to corner — exactly what the gas family always does.

A gas on the loose: released in one corner, it spreads until it fills the whole room.

Most gases are invisible — but not all. This sealed glass tube holds bromine, a gas with a real brown-orange color: you can see it filling every corner of its container, right up to the glass. Photo: James St. John, CC BY 2.0 (cropped). — Most gases are invisible — but not all. This sealed glass tube holds bromine, a gas with a real brown-orange color: you can *see* it filling every corner of its container, right up to the glass. *Photo: James St. John, CC BY 2.0 (cropped).*

Remark 14.8 (Sorting the tricky ones)

Some stuff needs a careful eye. Honey flows — slowly, but it flows, takes its container’s shape and keeps its amount: a liquid, just a lazy one. Toothpaste and ketchup flow when squeezed: thick liquids. A sponge is a solid full of air-filled holes. And smoke? Tiny solid specks riding on hot gas. When in doubt, ask the family questions: does it keep its shape? does it flow? does it fill all the room it gets?

14.5 Exercises

Exercise 14.1 ★

Sort into the three families: a coin; orange juice; the air in a tire; a wooden bead; oil; the steam-gas above the clear gap of a boiling pot.

Solution

Solution of Exercise 14.1.

Solids: the coin, the wooden bead. Liquids: orange juice, oil. Gases: the air in the tire, the invisible steam in the clear gap.

Exercise 14.2 ★

What does a solid keep that a liquid does not? What does a liquid keep when poured from glass to bowl?

Solution

Solution of Exercise 14.2.

A solid keeps its own shape; a liquid does not. Poured from glass to bowl, a liquid keeps its amount.

Exercise 14.3 ★

A full bottle of lemonade is poured into a wide salad bowl. Is there now more lemonade, less, or the same amount? What changed?

Solution

Solution of Exercise 14.3.

The same amount — only the shape changed, from bottle-shape to wide-bowl-shape.

Exercise 14.4 ★

Draw a fish tank half full of water: first standing flat, then tilted up on one edge. Draw the water surface in both — which way does it lie?

Solution

Solution of Exercise 14.4.

In both drawings the water surface lies flat and level, parallel to the floor — in the tilted tank the surface does not tilt with the glass; the water gathers at the low side.

Exercise 14.5 ★

How does a builder’s spirit level tell that a shelf is level? Which law of this chapter is it using?

Solution

Solution of Exercise 14.5.

The liquid in its little window always lies level, so the bubble shows where “up” is: bubble between the marks means the shelf matches the level liquid. It uses the law that a resting liquid lies level.

Exercise 14.6 ★

Why does the smell of pancakes reach your bedroom upstairs? Which family is at work, and what is its rule?

Solution

Solution of Exercise 14.6.

The gas family. The pancake smell is a gas, and a gas spreads to fill all the room it is given — kitchen, staircase, bedroom.

Exercise 14.7 ★

Sand pours like a liquid, yet we call it solid. Explain, using Example 14.2.

Solution

Solution of Exercise 14.7.

Sand is a crowd of tiny solid grains. The crowd flows and takes the shape of its pile, but each grain keeps its own shape perfectly — so each grain is a true solid.

Exercise 14.8 ★

Honey takes a whole minute to flow off the spoon. Solid or liquid? Ask the family questions of Remark 14.8 and answer.

Solution

Solution of Exercise 14.8.

It flows (slowly), takes its container’s shape, and keeps its amount: honey answers the questions like a liquid — a lazy one.

Exercise 14.9 ★

Which family: keeps its own shape? takes its container’s shape but keeps its amount? takes all the room it can get? Give one member of each family found in your kitchen.

Solution

Solution of Exercise 14.9.

Keeps its shape: solid (a spoon). Takes the container’s shape but keeps its amount: liquid (milk). Takes all the room: gas (the air in the kitchen).

Exercise 14.10 ★★

An ice cube (solid) melts into water (liquid), which the summer sun dries into invisible flying water (gas). Which family rule breaks at each step: what does the stuff stop keeping when ice melts? when water evaporates?

Solution

Solution of Exercise 14.10.

When ice melts, the stuff stops keeping its own shape (but still keeps its amount). When water evaporates, it stops keeping even its resting place: as a gas it spreads into all the room there is.

Exercise 14.11 ★★

A balloon is blown up and knotted. Mira says the air inside is “balloon-shaped, so a gas does have a shape”. Explain what is really giving the shape, and what the air would do the instant the balloon popped.

Solution

Solution of Exercise 14.11.

The rubber skin gives the shape: it is a stretched solid cage, and the squeezed air fills every corner of whatever shape the cage takes. Pop the balloon, and the air instantly spreads into the whole room — showing it never had a shape of its own at all.

Terms defined in this chapter

See all 393 terms in the glossary