--- title: "Solids, Liquids, Gases" book: "Primary & Middle School Physics" subject: physics language: en chapter: 14 exercises: 11 source: https://one-course.com/books/physics/1/en/chapter/14-solids-liquids-gases --- # Chapter 14 — Solids, Liquids, Gases A stone, a glass of milk, the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-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](#def-g3-solids-liquids-gases-solid) like the stone, [liquid](#def-g3-solids-liquids-gases-liquid) like the milk, [gas](#def-g3-solids-liquids-gases-gas) like the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-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](#def-g3-solids-liquids-gases-liquid). But look closely: sand is a crowd of tiny [solid](#def-g3-solids-liquids-gases-solid) grains, and *each grain* keeps its shape perfectly. Flour, sugar and rice play the same trick — crowds of little [solids](#def-g3-solids-liquids-gases-solid) that flow together while each member stays [solid](#def-g3-solids-liquids-gases-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.](https://one-course.com/images/onecourse/chapters/physics-1/g3-solids-liquids-gases/fig-6cd8fe6335b3.svg) *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](#def-g3-solids-liquids-gases-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.](https://one-course.com/images/onecourse/chapters/physics-1/g3-solids-liquids-gases/fig-4b3163fa115c.svg) *Tilt the bottle as you like: the water’s surface refuses to tilt. A resting [liquid](#def-g3-solids-liquids-gases-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](#def-g3-solids-liquids-gases-liquid) with one bubble; when the bubble sits between its marks, the shelf is as level as the [liquid](#def-g3-solids-liquids-gases-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](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-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](#def-g3-solids-liquids-gases-gas) did not stay politely near its bottle: released, it spread through all the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) of the room, corner to corner — exactly what the [gas](#def-g3-solids-liquids-gases-gas) family always does. ![A gas on the loose: released in one corner, it spreads until it fills the whole room.](https://one-course.com/images/onecourse/chapters/physics-1/g3-solids-liquids-gases/fig-8337cde87d4a.svg) *A [gas](#def-g3-solids-liquids-gases-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).](https://one-course.com/images/onecourse/chapters/physics-1/g3-solids-liquids-gases/img-e1578863ccef.jpg) *Most gases are invisible — but not all. This sealed glass tube holds bromine, a [gas](#def-g3-solids-liquids-gases-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](#def-g3-solids-liquids-gases-liquid), just a lazy one. Toothpaste and ketchup flow when squeezed: thick [liquids](#def-g3-solids-liquids-gases-liquid). A sponge is a [solid](#def-g3-solids-liquids-gases-solid) full of air-filled holes. And smoke? Tiny [solid](#def-g3-solids-liquids-gases-solid) specks riding on hot [gas](#def-g3-solids-liquids-gases-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](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) in a tire; a wooden bead; oil; the [steam-gas](#def-g3-solids-liquids-gases-gas) above the clear gap of a boiling pot. **Solution of Exercise 14.1.** [Solids](#def-g3-solids-liquids-gases-solid): the coin, the wooden bead. [Liquids](#def-g3-solids-liquids-gases-liquid): orange juice, oil. Gases: the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) in the tire, the invisible steam in the clear gap. **Exercise 14.2 ★.** What does a [solid](#def-g3-solids-liquids-gases-solid) keep that a [liquid](#def-g3-solids-liquids-gases-liquid) does not? What does a [liquid](#def-g3-solids-liquids-gases-liquid) keep when poured from glass to bowl? **Solution of Exercise 14.2.** A [solid](#def-g3-solids-liquids-gases-solid) keeps its own shape; a [liquid](#def-g3-solids-liquids-gases-liquid) does not. Poured from glass to bowl, a [liquid](#def-g3-solids-liquids-gases-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 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 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 of Exercise 14.5.** The [liquid](#def-g3-solids-liquids-gases-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](#def-g3-solids-liquids-gases-liquid). It uses the law that a resting [liquid](#def-g3-solids-liquids-gases-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 of Exercise 14.6.** The [gas](#def-g3-solids-liquids-gases-gas) family. The pancake smell is a [gas](#def-g3-solids-liquids-gases-gas), and a [gas](#def-g3-solids-liquids-gases-gas) spreads to fill all the room it is given — kitchen, staircase, bedroom. **Exercise 14.7 ★.** Sand pours like a [liquid](#def-g3-solids-liquids-gases-liquid), yet we call it [solid](#def-g3-solids-liquids-gases-solid). Explain, using [Example 14.2](#ex-g3-solids-liquids-gases-sand). **Solution of Exercise 14.7.** Sand is a crowd of tiny [solid](#def-g3-solids-liquids-gases-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](#def-g3-solids-liquids-gases-solid). **Exercise 14.8 ★.** Honey takes a whole minute to flow off the spoon. [Solid](#def-g3-solids-liquids-gases-solid) or [liquid](#def-g3-solids-liquids-gases-liquid)? Ask the family questions of [Remark 14.8](#rem-g3-solids-liquids-gases-tricky) and answer. **Solution of Exercise 14.8.** It flows (slowly), takes its container’s shape, and keeps its amount: honey answers the questions like a [liquid](#def-g3-solids-liquids-gases-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 of Exercise 14.9.** Keeps its shape: [solid](#def-g3-solids-liquids-gases-solid) (a spoon). Takes the container’s shape but keeps its amount: [liquid](#def-g3-solids-liquids-gases-liquid) (milk). Takes all the room: [gas](#def-g3-solids-liquids-gases-gas) (the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) in the kitchen). **Exercise 14.10 ★★.** An ice cube ([solid](#def-g3-solids-liquids-gases-solid)) [melts](https://one-course.com/books/physics/1/en/chapter/9-ice-water-steam#def-g2-ice-water-steam-meltfreeze) into water ([liquid](#def-g3-solids-liquids-gases-liquid)), which the summer sun dries into invisible flying water ([gas](#def-g3-solids-liquids-gases-gas)). Which family rule breaks at each step: what does the stuff stop keeping when ice [melts](https://one-course.com/books/physics/1/en/chapter/9-ice-water-steam#def-g2-ice-water-steam-meltfreeze)? when water [evaporates](https://one-course.com/books/physics/1/en/chapter/9-ice-water-steam#def-g2-ice-water-steam-evapcond)? **Solution of Exercise 14.10.** When ice [melts](https://one-course.com/books/physics/1/en/chapter/9-ice-water-steam#def-g2-ice-water-steam-meltfreeze), the stuff stops keeping its own *shape* (but still keeps its amount). When water [evaporates](https://one-course.com/books/physics/1/en/chapter/9-ice-water-steam#def-g2-ice-water-steam-evapcond), it stops keeping even its *resting place*: as a [gas](#def-g3-solids-liquids-gases-gas) it spreads into all the room there is. **Exercise 14.11 ★★.** A balloon is blown up and knotted. Mira says the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) inside is “balloon-shaped, so a [gas](#def-g3-solids-liquids-gases-gas) does have a shape”. Explain what is really giving the shape, and what the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) would do the instant the balloon popped. **Solution of Exercise 14.11.** The rubber skin gives the shape: it is a stretched [solid](#def-g3-solids-liquids-gases-solid) cage, and the squeezed [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) fills every corner of whatever shape the cage takes. Pop the balloon, and the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) instantly spreads into the whole room — showing it never had a shape of its own at all.