C. Structure and Properties of Matter · Structure and Properties of MatterProperties of Liquids: Viscosity, Surface Tension, Capillary Action and Vapour Pressure
C3.4 A1.5 A1.10About 70 minutes
Learning goals- I can explain viscosity, surface tension, capillary action and vapour pressure using intermolecular forces.
- I can distinguish cohesion from adhesion and predict the shape of a meniscus.
- I can relate vapour pressure to boiling point and to atmospheric pressure.
- I can predict and compare the properties of liquids from their structures.
Words to know
viscosity
A liquid's resistance to flow.
surface tension
The energy needed to increase a liquid's surface area; caused by the net inward pull on surface molecules.
cohesion / adhesion
Attraction between like molecules / between unlike substances.
capillary action
The movement of a liquid up a narrow tube or porous material because of adhesion and surface tension.
vapour pressure
The pressure of a vapour in equilibrium with its liquid at a given temperature.
volatile
Evaporates easily; has a high vapour pressure.
What makes maple syrup so slow?
Pour water and maple syrup side by side and the syrup crawls. Watch a water strider skate on a pond, or see water climb up a paper towel. These everyday properties of liquids, viscosity, surface tension, capillary action and vapour pressure, all come from the same cause: the intermolecular forces you studied last lesson, together with the size and shape of the particles.
In this lesson you will explain each property at the particle level, the way chemists, chemical engineers and food scientists do.
Viscosity
Viscosity is a liquid's resistance to flow. Molecules must slide past one another to flow, so viscosity increases when:
- IMFs are stronger (glycerol, with three O–H groups per molecule, is about 1400 times as viscous as water at 20 °C), and
- molecules are long or tangled (motor oil and honey contain large molecules that get in each other's way).
Viscosity decreases as temperature rises, because faster-moving molecules overcome their attractions more easily. That is why maple syrup is warmed before pouring, and why motor oils are rated for both cold and hot temperatures. Maple syrup is mostly sucrose dissolved in water; the large sugar molecules form many hydrogen bonds with each other and with water.
Surface tension
Surface tension is the energy needed to increase a liquid's surface area. A molecule inside the liquid is pulled equally in all directions by its neighbours, but a molecule at the surface is pulled only sideways and inward. The surface behaves like a stretched elastic skin and pulls drops into spheres.
Water's surface tension (about 73 mN/m at 20 °C) is one of the highest of any common liquid because of hydrogen bonding; ethanol's is about 22 mN/m. Surfactants such as soap and detergent lower surface tension: their molecules have a polar "head" that likes water and a long non-polar "tail" that does not, so they crowd at the surface and disrupt the hydrogen bonding there.
Capillary action and the meniscus
Two kinds of attraction compete:
- Cohesion: attraction between molecules of the same substance.
- Adhesion: attraction between the liquid's molecules and a different substance (such as glass).
Glass is covered with polar O–H groups, so water molecules adhere to it strongly. Where adhesion > cohesion, water climbs the walls, forming a concave (U-shaped) meniscus, and in a narrow tube the whole column is pulled up: capillary action. The narrower the tube, the higher the water rises. Capillary action draws water into paper towels and the fibres of a cotton shirt, and it helps move water through the tiny vessels in plant stems.
Mercury atoms attract one another very strongly (metallic bonding) and attract glass only weakly. Where cohesion > adhesion, the liquid pulls away from the walls, forming a convex (domed) meniscus, and the level in a narrow tube is pushed down.
Vapour pressure and boiling point
In a closed container, some molecules at the surface of a liquid have enough energy to escape into the gas phase, and some vapour molecules return. When the rates are equal, the pressure of the vapour is the liquid's vapour pressure.
- Weaker IMFs → higher vapour pressure (the liquid is more volatile).
- Vapour pressure increases with temperature.
- A liquid boils when its vapour pressure equals the pressure of the surroundings. At sea level (about 101.3 kPa) water boils at 100 °C. In mountain towns, where air pressure is lower, water boils at a lower temperature (about 95 °C at an elevation of around 1400 m, roughly the height of Banff, Alberta), so pasta takes longer to cook.
| Liquid (20 °C) | Main IMFs | Viscosity (mPa·s) | Surface tension (mN/m) |
|---|
| water | H-bonding, London | 1.0 | 73 |
| ethanol | H-bonding (one O–H), London | 1.2 | 22 |
| propanone (acetone) | dipole-dipole, London | 0.3 | 24 |
| glycerol | extensive H-bonding (three O–H), London | about 1400 | 63 |
Worked exampleExample 1: Explaining viscosity
Explain why glycerol (C₃H₈O₃) is far more viscous than propan-1-ol (C₃H₈O), even though their molecules are similar in size.
- Both have three-carbon chains with O–H groups.
- Glycerol has three O–H groups per molecule; propan-1-ol has one.
- Each glycerol molecule forms many more hydrogen bonds, linking molecules into a network that must keep breaking and reforming to flow.
Glycerol's much more extensive hydrogen bonding makes it far more viscous.
Worked exampleExample 2: Predicting vapour pressure
Rank hexane (C₆H₁₄), ethanol (C₂H₅OH) and water in order of increasing vapour pressure at 25 °C.
- Water: extensive hydrogen bonding (2 H atoms and 2 lone pairs per molecule): the strongest overall IMFs here for its size.
- Ethanol: one O–H group per molecule, so fewer hydrogen bonds, plus London forces.
- Hexane: London forces only (no dipole, no hydrogen bonding), even though it is the largest molecule.
- Measured boiling points support this: water 100 °C, ethanol 78 °C, hexane 69 °C. A higher boiling point goes with a lower vapour pressure at the same temperature (measured at 25 °C: about 3.2, 7.9 and 20 kPa).
Vapour pressure: water < ethanol < hexane.
Worked exampleExample 3: Boiling at altitude
Water's vapour pressure is 84.5 kPa at 95 °C and 101.3 kPa at 100 °C. A hiker is at a place where the air pressure is 84.5 kPa. At what temperature will water boil there, and why does this matter?
- Water boils when its vapour pressure equals the surrounding pressure.
- At 95 °C the vapour pressure is 84.5 kPa, matching the air pressure.
- So the water boils at 95 °C and cannot get hotter in an open pot, no matter how high the stove is set.
Water boils at about 95 °C, so food cooks more slowly (pressure cookers solve this by raising the pressure).
Just for funA paper clip can float on water even though steel is about eight times denser than water. Surface tension holds it up. Add one drop of dish soap and it sinks immediately, which is one of the quickest magic tricks in chemistry.
Hands-on activitySurface tension and capillary investigations
You need: a shallow bowl of water, a fork, 2 or 3 metal paper clips, ground pepper, liquid dish soap, a toothpick, two clear glasses, a paper towel, and food colouring (optional).
Part A: Floating paper clips. Lay a paper clip on the prongs of a fork and lower it gently onto the water surface so it floats. Look at how the surface dips under it. Touch the water near it with a soapy toothpick and watch.
Part B: Pepper scatter. Sprinkle pepper over clean water. Touch the centre with a toothpick dipped in dish soap.
Part C: Capillary bridge. Fill one glass with coloured water and leave the other empty beside it. Fold a paper towel into a long strip and put one end in each glass. Check after 30 minutes and after a few hours.
What to look for: in A, the clip floats on the "skin" until soap lowers the surface tension and it sinks. In B, the pepper shoots to the edges because soap weakens the surface tension in the centre while the stronger surface tension around the edge pulls outward. In C, water climbs the towel by capillary action (adhesion to the polar cellulose fibres) and slowly moves into the empty glass. Write one sentence for each part explaining the result using cohesion, adhesion or surface tension.
Your turn
1Define viscosity and give two factors that increase it.
Resistance to flow. It increases with stronger IMFs and longer or more tangled molecules (and decreases with higher temperature).
2Why does warming maple syrup make it pour more easily?
At higher temperature, molecules move faster and overcome their intermolecular attractions more easily, so viscosity decreases.
3Explain surface tension at the molecular level.
Surface molecules have no neighbours above, so they feel a net inward pull; the surface acts like a stretched skin.
4Why does water form a concave meniscus in a glass tube?
Adhesion between water and the polar glass surface is stronger than cohesion between water molecules, so water climbs the walls.
5Which has the higher vapour pressure at 20 °C, water or propanone? Explain.
Propanone: it has only dipole-dipole and London forces, which are weaker than water's hydrogen bonds, so its molecules escape more easily.
6What condition must be met for a liquid to boil?
Its vapour pressure must equal the external (surrounding) pressure.
7Why does soap help water clean greasy dishes?
Soap lowers surface tension so water spreads and wets surfaces, and its non-polar tails surround grease while polar heads face water, lifting grease away.
More practice: warm-up
Attraction between molecules of the same substance.
Attraction between molecules of different substances.
3Does vapour pressure increase or decrease with temperature?
4Which liquid has the higher surface tension, water or ethanol?
Water (about 73 vs 22 mN/m).
5What shape is the meniscus of mercury in glass?
More practice: on your own
1Why does water rise higher in a narrow tube than in a wide one?
The adhesive pull acts around the tube's edge (circumference), but the weight of water to lift depends on the cross-sectional area. In a narrow tube, the edge is large compared with the area, so water is pulled higher.
2Ethoxyethane (diethyl ether, bp 35 °C) has a much higher vapour pressure than butan-1-ol (bp 118 °C), although they have the same formula, C₄H₁₀O. Explain.
Butan-1-ol has an O–H group and forms hydrogen bonds; ethoxyethane has no O–H, so its molecules attract each other only by weaker dipole-dipole and London forces and escape more easily.
3Predict which is more viscous at room temperature: octane (C₈H₁₈) or ethane-1,2-diol (ethylene glycol, HOCH₂CH₂OH). Explain.
Ethylene glycol (about 16 mPa·s versus about 0.5 for octane): two O–H groups per molecule give extensive hydrogen bonding, whereas octane has only London forces.
4At a pressure of 70 kPa, water boils at about 90 °C. Would a pressure cooker (about 200 kPa) raise or lower the boiling temperature? Why?
Raise it (to about 120 °C): water must reach a higher temperature for its vapour pressure to equal the higher pressure inside.
5Explain how capillary action helps a paper towel soak up a spill.
Paper is made of cellulose fibres with many O–H groups. Water adheres to them and is drawn into the tiny spaces between fibres, which act like narrow tubes.
Challenge
1The height of capillary rise is h = 2γ/(ρgr) for water in clean glass, where γ = 0.0728 N/m, ρ = 1000 kg/m³ and g = 9.81 m/s². Calculate h for a tube of radius 0.50 mm.
h = 2(0.0728) ÷ (1000 × 9.81 × 0.00050) = 0.1456 ÷ 4.905 = 0.030 m (3.0 cm).
2Using the same formula, how would the rise change if the tube radius were halved? What if ethanol (γ = 0.0223 N/m, ρ = 789 kg/m³) were used in the original tube?
Halving r doubles h, to 6.0 cm. For ethanol: h = 2(0.0223) ÷ (789 × 9.81 × 0.00050) = 0.0446 ÷ 3.870 = 0.0115 m (about 1.2 cm), lower because ethanol's IMFs (and so its surface tension) are weaker.
3Water at 20 °C has a vapour pressure of 2.3 kPa. A closed 10.0 L flask containing some liquid water and no air reaches equilibrium at 20 °C. Use PV = nRT (R = 8.314 kPa·L/(mol·K)) to find the mass of water present as vapour.
n = PV/(RT) = (2.3 × 10.0) ÷ (8.314 × 293) = 23 ÷ 2436 = 0.00944 mol. Mass = 0.00944 × 18.02 = 0.17 g of water vapour.
Quiz
1. Which liquid would you expect to have the highest viscosity at room temperature?
2. What happens to the viscosity of a liquid as it is heated?
3. Why does a liquid with weak intermolecular forces have a high vapour pressure?
4. Water forms a concave meniscus in glass because:
6. Which substance lowers the surface tension of water?
Big idea (C3.4): the physical properties of a liquid depend on the particles present and the forces between them. Stronger IMFs mean higher viscosity, higher surface tension, lower vapour pressure and a higher boiling point.
How to help: all three parts of the surface-tension activity use kitchen materials. Ask your teen to predict each result before trying it, then explain it in terms of forces.
Common difficulties:
- Thinking boiling point is a fixed property independent of pressure.
- Mixing up cohesion and adhesion.
- Saying a liquid with a high vapour pressure has strong IMFs (it is the reverse).