Physical Properties of Solids, Liquids and Gases – Temperature Measurement and Physical Properties of Matter (Senior 1 Physics) | Ganzaa

Temperature Measurement and Physical Properties of Matter

Lessons

Senior 1 · Physics

Temperature Measurement and Physical Properties of Matter

5 lessons

Lesson 5 of 5

Physical Properties of Solids, Liquids and Gases

25 min

Why these materials?

Roofs in Kigali are made of thin iron or aluminium sheets, not glass. Electric cables are copper wire inside plastic. Honey pours slowly but water splashes. A bicycle tyre full of air is soft enough to ride on, but a tyre full of water would be hard and heavy. Beans take longer to cook high on the slopes of the Virunga volcanoes than beside Lake Kivu. Each of these facts comes from a physical property of a solid, a liquid or a gas, and each property comes from the way the particles are arranged and held together (Lesson 4).

Definition

A physical property is a property that can be observed or measured without changing the substance into a new substance. Examples: hardness, melting point, viscosity, colour, density.

The three states compared

Property	Solid	Liquid	Gas
Shape	fixed	takes the shape of its container	fills the whole container
Volume	fixed	fixed	not fixed: spreads to fill any space
Can it flow?	no	yes	yes
Compressibility	almost none	almost none	easily compressed
Density	usually high	usually high	very low
Expansion when heated	small	larger	very large

Properties of solids

Property and its meaning	Examples
Hardness: resistance to being scratched or dented	diamond and quartz are very hard; chalk and lead are soft
Malleability: can be hammered or rolled into thin sheets without breaking	iron and aluminium roofing sheets; aluminium cooking pots
Ductility: can be drawn out into a thin wire	copper and aluminium electric cables
Conductivity: lets heat or electricity pass through easily	metals are good conductors; wood, plastic and glass are poor conductors (insulators)
Elasticity: returns to its original shape when a stretching or squashing force is removed	rubber bands, springs, a bicycle's steel spokes
Brittleness: breaks suddenly without bending	glass, clay pots, cast iron
Melting point: the temperature at which the solid turns to liquid	ice 0 °C; tin 232 °C; lead 327 °C; aluminium 660 °C; copper 1085 °C; iron 1538 °C

Metals are malleable and ductile because their particles are arranged in layers that can slide over each other without breaking apart. Rwanda is a major producer of tin ore (cassiterite); tin's low melting point, 232 °C, is why it is used in solder, which joins electrical wires when melted with a hot iron.

Definition

A body is elastic if it returns to its original shape and size when the force deforming it is removed. The elastic limit is the greatest force it can take and still return to its original shape; beyond it, the body is permanently stretched or bent. Example: a spring loaded gently springs back, but a spring pulled too hard stays longer.

Worked example 1: stretching a spring

A spring stretches 2.0 cm for every 1.0 N hung on it, up to its elastic limit of 5.0 N. (a) How far does it stretch with a 3.5 N load? (b) What happens with a 7.0 N load?

\text{(a)}\quad \text{extension} = 3.5\,\mathrm{N} \times \frac{2.0\,\mathrm{cm}}{1.0\,\mathrm{N}} = 7.0\,\mathrm{cm}

(b) 7.0 N is beyond the elastic limit, so the spring no longer stretches in proportion to the load, and it stays permanently longer when the load is removed.

Properties of liquids

Experiment: how thick is a liquid? Fill three tall tubes with water, cooking oil and honey. Put two rubber bands round each tube, 20 cm apart. Drop a small steel ball into each and time how long it takes to fall from the upper band to the lower one.

Figure 5.1: The ball falls most slowly through honey, because honey has the greatest viscosity.

Definition

Viscosity is a liquid's resistance to flowing: its “thickness”. Honey and engine oil are very viscous; water and petrol have a low viscosity. Viscosity is caused by forces of attraction between the particles, which resist their sliding past each other. Most liquids become less viscous when warmed: honey pours more easily on a hot day.

The boiling point of a liquid is the temperature at which it boils, forming bubbles of vapour throughout. The freezing point is the temperature at which it turns to a solid; for a pure substance it is the same as the melting point.

Worked example 2: comparing viscosities

Use the times in Figure 5.1 to find the average speed of the ball in each liquid.

\text{water: } \frac{0.20\,\mathrm{m}}{0.25\,\mathrm{s}} = 0.80\,\mathrm{m\,s^{-1}}

\text{oil: } \frac{0.20\,\mathrm{m}}{1.6\,\mathrm{s}} = 0.125\,\mathrm{m\,s^{-1}}

\text{honey: } \frac{0.20\,\mathrm{m}}{8.0\,\mathrm{s}} = 0.025\,\mathrm{m\,s^{-1}}

The slower the ball, the more viscous the liquid: honey $>$ oil $>$ water.

Boiling points in the land of a thousand hills

Pure water boils at 100 °C only at sea level. Higher up, the air pressure is lower, so water boils at a lower temperature: roughly 1 °C lower for every 300 m of height.

Place	Height above sea level	Water boils at about
Sea level (Mombasa, Dar es Salaam)	0 m	100 °C
Kigali	about 1560 m	95 °C
Musanze	about 1850 m	94 °C
Top of Mount Karisimbi	4507 m	85 °C

Dissolved impurities have the opposite effect: salty water boils above 100 °C.

Worked example 3: cooking beans on a mountain

Using a fall of 1 °C for every 300 m, estimate the boiling point of water on the summit of Mount Karisimbi (4507 m), and explain why beans cook more slowly there.

\text{fall} = \frac{4507\,\mathrm{m}}{300\,\mathrm{m\ per}\,^\circ\mathrm{C}} \approx 15\,^\circ\mathrm{C}, \qquad \text{boiling point} \approx 100 - 15 = 85\,^\circ\mathrm{C}

Boiling water on the mountain is only about 85 °C, so the beans are heated to a lower temperature and take much longer to soften. A pressure cooker raises the pressure, and so the boiling point, above 100 °C, which cooks food faster.

Properties of gases: compressibility

Experiment: pull the plunger of a plastic syringe back to 60 $\mathrm{cm^3}$ of air, seal the nozzle with your finger, and push. Repeat with the syringe full of water.

Figure 5.2: The air can be squashed into a third of its volume; the water cannot be noticeably squashed at all.

Gas particles are far apart with empty space between them, so pushing moves them closer together: gases are compressible. In liquids and solids the particles already touch, so they can hardly be squashed.

Definition

Compressibility is how easily the volume of a substance can be reduced by squeezing it. Gases are highly compressible; liquids and solids are almost incompressible. Example: 60 $\mathrm{cm^3}$ of air in a sealed syringe can be pushed into 20 $\mathrm{cm^3}$ , a fall of $\frac{40}{60}$ , about 67%.

Important

Safety. Never point a syringe at anyone, and do not use a needle. Do not heat a sealed container: the gas inside expands and can burst it. Keep honey and oil off the floor; they are slippery.

Using the properties

Compressed gases: cooking gas (LPG) is sold in cylinders, squeezed until it becomes a liquid; hospitals store oxygen in high-pressure cylinders; air in tyres and in the shock absorbers of motos cushions the ride.
Incompressible liquids: a car's brakes use a liquid, brake fluid, in thin pipes: a push on the pedal is passed through the fluid to all four wheels, because the fluid cannot be squashed.
Choosing solids: cooking pots are aluminium (a good conductor) with wooden or plastic handles (insulators); roofs are malleable metal sheets; electric wires are ductile copper.

Warning

Do not confuse these pairs. Hardness is not strength: glass is hard but breaks easily. Viscosity is not density: cooking oil is more viscous than water but less dense, so it floats. Boiling (throughout the liquid, at one temperature) is not evaporation (only at the surface, at any temperature).

Tip

Link every property to the particles. Fixed shape: particles held in fixed places. Flows: particles slide past each other. Compressible: big gaps between particles. Viscous: strong attractions between particles.

Solid, liquid, gas and … plasma? (TED-Ed, Michael Murillo). Watch for how the motion and spacing of particles explain the properties of each state.

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Practice

Give two differences between a liquid and a gas.
Define (a) malleability, (b) ductility, and give a use of each.
Why are cooking pots made of metal but given wooden or plastic handles?
A ball falls 30 cm through engine oil in 2.0 s and through water in 0.30 s. Find each speed. Which liquid is more viscous?
A spring stretches 1.5 cm for each 1.0 N, within its elastic limit. How much load gives an extension of 6.0 cm?
Estimate the boiling point of water in a town 2400 m above sea level, using a fall of 1 °C per 300 m.
A sealed syringe holds 50 $\mathrm{cm^3}$ of air. It is pushed to 20 $\mathrm{cm^3}$ . By what fraction has the volume decreased? Why could you not do this with water?
Explain, using particles, why gases are easily compressed but solids are not.

Show answersHide answers

Any two: a liquid has a fixed volume, a gas does not; a gas is easily compressed, a liquid is not; a gas fills its whole container, a liquid only takes its shape; a liquid is much denser.
(a) Can be hammered into thin sheets: roofing sheets. (b) Can be drawn into wires: copper electric cables.
Metal conducts heat well to cook the food; wood and plastic are poor conductors (insulators), so the handle stays cool enough to hold.
Oil: $\frac{0.30}{2.0} = 0.15\,\mathrm{m\,s^{-1}}$ ; water: $\frac{0.30}{0.30} = 1.0\,\mathrm{m\,s^{-1}}$ . The oil is more viscous.
$\frac{6.0}{1.5} \times 1.0\,\mathrm{N} = 4.0\,\mathrm{N}$
$\frac{2400}{300} = 8\,^\circ\mathrm{C}$ lower: about 92 °C.
$\frac{30}{50} = \frac{3}{5}$ , a 60% decrease. Water particles are already touching, so water is almost incompressible.
Gas particles are far apart with large empty spaces, so they can be pushed closer together. Solid particles are already packed closely together, touching, so there is no space to push them into.

Key points

Solids: fixed shape and volume. Properties include hardness, malleability, ductility, conductivity, elasticity and melting point.
Liquids: fixed volume, flow, take the container's shape. Viscosity is resistance to flow; boiling point falls with height.
Gases: no fixed shape or volume, and easily compressed, because their particles are far apart.
Every property can be explained by how the particles are spaced, how they move and how strongly they attract.

1Why these materials?

2The three states compared

3Properties of solids

4Properties of liquids

5Boiling points in the land of a thousand hills

6Properties of gases: compressibility

7Using the properties

Why these materials?

The three states compared

Properties of solids

Properties of liquids

Boiling points in the land of a thousand hills

Properties of gases: compressibility

Using the properties