What level
of salt can people detect?
Students can make a solution of salt water
and successively dilute it, say by a factor of two each time,
checking after each dilution whether the salt can still be
tasted. The dilution does not need to be done with great precision.
The amount of salt usually used for cooking is typically of
the order of 5 g dm-3 but this may vary considerably between
different cooks. Students may wish to start with a more concentrated
solution than this. One way of making the taste test more
reliable is to give the taster three samples, one of salted
and two of tap water and ask if they can detect the odd one
out.
There are possibilities for much discussion
about the details of the experiment – what concentration
of salt to start with, what dilution factor to use etc.
Apparatus and
equipment
Each group of students will need:
| • |
a measuring
jug in which to do the dilution. It is better to use
a ‘kitchen-type’ jug from the Food Technology
Department rather than a ‘chemistry-type’
measuring cylinder |
| • |
access to a top pan
balance |
| • |
disposable spoons
for the tasting. |
Chemicals
Each group of students will need:
| • |
Salt –
about 20 g. Buy table salt rather than ‘laboratory’
sodium chloride. |
Questions for
students
| These questions
are in the student’s sheet and may be used to
stimulate discussion.
| 1. |
Do different testers get the same
result? |
| 2. |
Would a test in which the original sample is
successively diluted by a factor of ten (rather
than two as above) give a more or less precise
answer? |
|
Safety
| • |
Wear eye
protection. |
| • |
Make sure that students
follow proper hygiene precautions (see the section
Experiments with food in How
to use this material). |
| • |
Your employer’s
risk assessment should be consulted before carrying
out this activity. This activity is covered by model
(general) risk assessments widely adopted for use
in UK schools such as those provided by CLEAPSS, SSERC,
ASE and DfES. Bear in mind, however, that these may
need some modification to suit local conditions. |
Further information
Adding salt (or any other solute) does raise the boiling
point of water. Boiling point elevation is a colligative
property, that is it depends on the number of particles
added, not their nature. Addition of 1 mole of any solute
to 1 kg of water raises the boiling point by 0.52 °C.
Typically a cook might add about 5 g to about 1 dm3 of water
– the equivalent of approximately 0.1 mole per kg
(the molar mass of sodium chloride is 58.5 g). Bearing in
mind that sodium chloride dissociates fully into sodium
and chloride ions and therefore 1 mole yields 2 moles of
particles, this would increase the boiling point by only
0.1 °C – not enough to affect the cooking time
noticeably.
Trials suggest that salt can be tasted down
to levels of roughly 0.5 g dm--3 but this may vary considerably
between tasters. If students start with a concentration
of salt of 50 g dm-3, about six or seven dilutions by a
factor of two will be required before the threshold of tasting
is reached.
Extensions
Students could also try to devise investigations to see
whether salt affects the colour or texture of cooked beans.
These could be carried out if time is available or used
as planning exercises.
Back
Page 1 - 2
- 3
