About Water
Technical & Teaching Notes
Introduction and context
Water covers 70% of Earth’s surface. Without it there would be no life on our planet. In plants,
water:

dissolves nutrients from the soil and carries them around the plant

is one of the reactants in photosynthesis (the other is carbon dioxide)

keeps stems, leaves and shoots rigid (plants wilt if they have insufficient water)

evaporates from leaves, which also cools the leaves
Possible barriers to learning
The following are misconceptions that some students may have and which may be a hindrance to
further learning:

Sugar disappears when it dissolves

When ice is heated its particles melt

Particles in a liquid are smaller than in a solid

Sugar melts in hot coffee

Particles expand on heating
Links to National curriculum for science in England at key stage 3
In biology pupils should be taught about nutrition and digestion which includes:

plants making carbohydrates in their leaves by photosynthesis and gaining mineral nutrients
and water from the soil via their roots
In chemistry pupils should be taught about the particulate nature of matter, and pure and impure
substances. This includes:

the properties of the different states of matter (solid, liquid and gas) in terms of the particle
model

changes of state in terms of the particle model

the concept of a pure substance

mixtures, including dissolving

diffusion in terms of the particle model

simple techniques for separating mixtures: filtration, evaporation, distillation and
chromatography

the identification of pure substances
Safety Notes
Materials and equipment used are not hazardous.
Science & Plants for Schools: www.saps.org.uk
Water and plant structure: p. 1
Apparatus
Activity 2: Changes of state
Each student or pair will need:

sheet of paper towel

water

digital thermometer (other temperature probes could be used)

fan or hair dryer

stopwatch

(If available, temperature changes over time could be monitored continuously using a
datalogger or a probe interfaced with a suitable computer programme)
Activity 3: Solvent properties
Each student or pair will need:

some earth (make sure that it does not contain anything sharp or otherwise hazardous; seed
compost could be used as an alternative)

sodium hydrogen carbonate (baking soda)

blue food colouring (other colours could be used but blue is easy to see in the absorbent
paper)

1 mol dm-3 hydrochloric acid (LOW HAZARD)

2 x 250 cm3 beakers (glasses of similar volumes could be used)

wood block, or similar, to support one of the beakers

sheet of paper towel (this can usually be rolled up tightly, but it may be handy to have some
small elastic bands available if it starts to uncurl)
Teaching Notes
Activity 1: Structure of water
Students should be familiar with the simple particle model of matter. Spend some time reminding
them.
Answers
1. (a) hydrogen and oxygen
(b) 2H and 1O
2. (a) ρ = m/V; V = m/ρ
So 1.000 g of ice has volume 1.000/0.917 = 1.091 cm 3 and 1.000 g of water has volume
1.000/1.000 = 1.000 cm 3.
Therefore, ice has the largest volume.
It may not be necessary to go through the calculations as some students may recognise
immediately that for the same mass the material with lowest density will occupy the greater
volume. However, this does give practice in changing the subject of an equation
(b) water molecules in ice are not packed together quite as closely as they are in water – this is
somewhat unusual as most solids increase very slightly in volume when they melt.
Science & Plants for Schools: www.saps.org.uk
Water and plant structure: p. 2
3.
Density /
g/cm3
Temperature / oC
(a) The graph shows a dramatic increase from 0 oC to 1 oC and then decreases; the rate of
decrease gets gradually greater.
(b) The distance between water molecules in liquid water gradually increases (emphasis that it is
a very small but discernible change)
(c) As the temperature rises the water molecules have greater kinetic energy and, therefore, are
moving faster. This means they collide with one another more frequently. The result is that they
are not quite as close together.
4. (a) 1 cm3 of water has mass 1 g and, therefore, produces 1 g of water vapour. Density of water
vapour = 0.0006 g/cm 3
Using V = m/ρ (see answer to 2.)
V = 1/0.0006 = 1700 cm 3 (to 2 significant figures)
(b) Using d = 3√(volume of water vapour / volume of water)
d = 3√1700/1 = 11.9 = 12 (to 2 significant figures)
So water molecules are 12 time further apart in water vapour than in water.
5. Diagrams and descriptions similar to the ones they use for the simple particle model, but with a
refinement that shows the relative distances.
Activity 2: Changes of state
Water evaporates very slowly at room temperature, making temperature changes difficult to
measure (as soon as energy is transferred from the immediate surroundings they warm up again
by transfer of energy from the surroundings a little further away (by conduction, convection or
radiation). A fan is used to speed up evaporation. It is also helpful if the water is about 30-35 oC
initially as it evaporates faster at higher temperatures. However, students may think that the
temperature drop is due simply to the water cooling. A second experiment not using a fan will
show that this does not produce the same magnitude of change as when a fan is used.
Answers
1. Students should observe a drop in temperature, consist with an endothermic reaction (energy
transferred from the surroundings).
2. The description should include the idea that particles are constantly moving around and that at
the surface of the liquid, those with sufficient kinetic energy can escape from the pull of other
particles in the liquid. Key to this is the idea that not all particles have the same kinetic energy,
some have less and others have more than the average.
Science & Plants for Schools: www.saps.org.uk
Water and plant structure: p. 3
3. Transpiration produces water in plant cells. This diffuses out of the cells and collects near the
surface of a leaf. It evaporates to form water vapour which can escape through stomata. It is this
evaporation that helps to cool the leaves.
Activity 3: Solvent properties
Students should be familiar with the idea of solutes, solvents and solutions. Remind them if
necessary. You might also want to discuss briefly the idea of ‘model’ experiments in science –
ones that mimic real processes but allow greater control of variables.
In the experiment student should observe:

Water slowly rises up the absorbent paper. The addition of food colouring makes this easier
to see, but if you think that the presence of the food colouring might be confusing students
can still see the movement of colourless water in they look closely.

After a while (usually a few minutes) liquid begins dripping into the lower beaker. Liquid is
clear blue.

Adding dilute hydrochloric acid to the liquid makes it effervesce, showing the presence of
sodium hydrogen carbonate.
Note: You may want to demonstrate the reaction using a solution of sodium hydrogen
carbonate. Other soluble salts could be used, e.g. sodium chloride (using silver nitrate
solution to identify it) or sodium sulfate (using barium chloride solution to identify it).

From these observations students should be able to conclude that substances which are
soluble in water, such as sodium hydrogen carbonate and blue food colouring, dissolve in
water and the solution is drawn up into the absorbent paper. Insoluble materials are not taken
up.
Science & Plants for Schools: www.saps.org.uk
Water and plant structure: p. 4