Jack Huang: 2004
Introduction: background information
Turbidity is a measure of the density of particles (typically
mud and silt) suspended in a solution. These particles block light,
which means that the more turbid the solution, the less visibility
you have through the solution. Because more light is absorbed
in the solution as the turbidity increases, any consequent temperature
increase will be related to the turbidity.
Turbidity is measured using a Secchi
Disk; a disc divided into black and white quarters. The disk
is submerged in the water until the black and white sectors are
no longer distinguishable. The depth at which this occurs is a
measure of the relative turbidity.
Research Question
What is the relationship between turbidity, measured with a Secchi Disc, and the absorption of heat from sunlight?
Hypothesis
The turbidity and the heat absorption from the sun increase proportionally. If this is the case, the visibility depth of a Secchi disc plotted against temperature rise white be a hyperbola.
Explanation
If the turbidity of the solution is doubled, meaning that the
visibility is halved, then light can travel half as far. This
means that the same amount of light is absorbed by the solution
with half the depth. If the depth of the solution remains the
same, the other half of the depth would absorb more light that
escapes from the top half of the solution. If the light were intense
enough to penetrate through the bottom, the bottom half would
be able to trap the same amount of light as the top half, so the
energy absorbed is doubled. If this explanation of heat absorption
in turbid solutions is true, then the turbidity and the heat absorption
would be proportional.
Procedure: Part 1
Solutions were made
by mixing Chinese ink into water. Chinese ink was used because
it is a suspension of fine carbon particles, and will not settle
to the bottom for a long time. A Secchi Disk was put into the
solution until it was barely visible. The depth was recorded.
250 ml of each solution was poured into glass cups that were placed
under a quartz-halogen lamp for 10 minutes. The change in temperature
was recorded.
Quartz-halogen bulbs have a filament temperature of about 3000°C.
They produces some ultraviolet light and have relatively more
infrared radiation than sunlight. Quartz-halogen bulbs were used
as a radiation source
instead of putting the solutions in sunlight because the light
source could be reproduced.
Data and analysis: Part 1
Preliminary measurements are listed in Tables
1a and 1b at right.
The temperature changes are small, and within errors of ± 0.5°C there is a similar temperature change for all turbidities. The data is not sufficiently accurate to draw clear conclusions. Subsequent measurements showed that the temperature change near the surface on the liquid was as much as 5°C. This unevenness leads to large errors. The experimental set up was redesigned and more data was collected.
Procedure: Part 2
To avoid the problem of uneven temperature rise at different depths,
petri dishes are used.
Petri dishes are shallower; therefore the unevenness of temperature
is reduced. 40 ml of the solutions are put in the petri dishes,
and the exposure time was reduced to 5 minutes. The number of
lamps was increased to 2 lamps.
Beakers were placed under the dishes so the warm table surface
will not affect the results.
Data and Analysis: Part 2
Measurements are listed in Tables 2a and 2b at right.
With the revised procedure, the change in temperature is clearly increased. There is a general trend. The more turbid the solution is, the more temperature change there is. The data is insufficiently accurate to determine the relationship between turbidity and heat absorption from light except for the following general comments.
It appears that the addition of the ink particles to the water
makes a small but measurable difference to the temperature rise
in the rate of heat absorption in the top 1 cm of water. The lamps
reradiate relatively more infrared than is present in sunlight.
Pure water appears to be absorbing this invisible infrared radiation.
Evaluation
This experiment confirms the hypothesis that heat absorption increases
as the turbidity of the water increases but the dominant effect
when using quartz-halogen lamps as radiation sources appears to
be the absorption of infrared radiation by the water itself.
The relationship between the turbidity - measured as the visibility depth of a Secchi disc - and the temperature rise is not inverse as suggested.
An interesting phenomenon found in this experiment is the unevenness of temperature. The surface of the solutions shows more temperature change than the bottom. Relative turbidity may play a role in determining the temperature gradient that develops in still water. This could be important for marine and fresh-water ecosystems in tropical countries such as Thailand.
Further measurements could be made to establish the relationship between the exposure time, turbidity and temperature gradients in large volumes of water when exposed to the radiation from quartz-halogen bulbs and to full sunlight. The effect of rising turbidity may be more important in sunlight that contains relatively less infrared radiation.
Editors comments
This is an edited version of the first draft with some minor additions. The suggestions concerning the absorption of infrared radiation by pure water warrant further investigation.