The average salinity of water in oceans is 35 parts per thousand.
The salinity of seawater varies from less than 10 ppt in the Baltic Sea…
…to as high as 280 ppt in the Dead Sea. This investigation uses precise
measurements of both the mass and volume of different samples of water of different salinities to calculate the density of these water samples.
To obtain precise results you must use a balance that measures to at
least 0.1 g increments. If more precise balances are available, these will give more precise results.
The measuring cylinders must be small enough so that the volume of water samples can be measured to 0.1 cm3 increments.
Suitable alternative equipment may include 10 cm3 syringes or burettes
Label one of the measuring cylinders with ’50 ppt salt water’. Label the other measuring cylinder with ‘distilled water’. This will help you
to avoid any accidental mixing of the salt water and distilled water which may affect your results.
Use the ‘salt water’ measuring cylinder to measure exactly 10 cm3 of 50 ppt salt water . Check the level of the
meniscus carefully to ensure the volume is accurate.
Place the beaker on the balance and zero the balance by pressing the tare or zero button.
Add all the salt water from the measuring cylinder into the
beaker – take care not to spill any as this will affect the results.
Record this mass in a results table.
Repeat this process at least 2 more times to collect 3 sets of data for the 50 ppt salt water.
Finally calculate an overall mean for the mass of the 50 ppt sample by adding the three values for each trial and dividing the total by three.
Then calculate the density of the water by dividing the mean mass of the water sample by the volume, which was 10 cm3.
Now we will use a combination of the 50 ppt salt water and distilled water to produce
samples of different salinities.
Measure 8 cm3 of 50 ppt salt water using the measuring cylinder labelled to measure this in.
Then measure 2 cm3 of distilled water using the other measuring cylinder we labelled earlier.
When added
together this will produce a a more dilute solution of salt, with a concentration of 40 ppt.
Place the beaker on the balance and zero the balance by pressing the tare or zero button.
Add all the salt water
from the first measuring cylinder into the beaker, and all of the distilled water from the second measuring cylinder – taking care not to spill any.
Record this mass in a results table.
Repeat this process
at least 2 more times to collect 3 sets of data for the 40 ppt salt water…
… and calculate an overall mean for the mass of the 40 ppt sample, and calculate the density of the water.
Now we will just use
distilled water to produce a sample of 0 ppt.
Measure 10 cm3 of distilled water using the measuring cylinder labelled to measure this in.
Place the beaker on the balance and zero the balance by pressing
the tare or zero button.
Add all of the distilled water from the measuring cylinder – taking care not to spill any.
Record this mass in a results table.
Repeat this process at least 2 more times
to collect 3 sets of data for the 0 ppt salt water.…
… and calculate an overall mean for the mass of the 0 ppt sample, and calculate the density of the water.
Different densities of water can result in
the formation of layers in seas and oceans. We can show this by colouring samples of 50 ppt water and distilled water and carefully adding them to each other…
…the distilled water is less dense so this floats on
top of the more saline water. Changes in density of water due to salinity or temperature differences can result in the formation of vertical currents where denser more saline water sinks.
These vertical currents
can help to drive the global ocean conveyor of ocean currents around all the oceans.
Salinity and the density of water video transcript
The average salinity of water in oceans is 35 parts per thousand.
