Seawater Temperature and Chemistry

Complexity level:	8
Project cost ($):	30
Time required:	1 hour to prepare, 2 days for the science project experiment
Material availability:	Access to basic laboratory equipment required.
Safety concerns:

Hypothesis

Increasing temperature causes the pH and salinity of seawater to increase.

Overview

Seawater

Water that is found in the oceans is known as seawater. Seawater has approximately 35g of dissolved salt in every liter of water (or 3.5% salinity). Most of this salt consists of sodium chloride. The density of seawater is about 1.025g/ml compared to fresh water density of 1.000g/ml. The presence of dissolved salt in seawater will reduce its freezing point to -2 °C.

In fact, salinity of seawater is normally between 3.1% and 3.8%. The level of salinity will not be constant in the oceans all over our planet. At places where fresh water from rivers enter the oceans and in places where ice glaciers are melting, the level of salinity of seawater will be lower because seawater is mixed with fresh water.

In places like the Red Sea where the rate of water evaporation is very high but the amount of rainfall is very low, the level of salinity in seawater is unusually higher. The Red Sea has a density between 1.028 and 1.035g/ml. It is an isolated body of salt water. It has a salinity of 33.7% and a density of 1.24g/ml.

Scientific Terms

Dissolved salt, sodium chloride, density, salinity, evaporating

Materials

- The materials required for this science project:10 beakers of the same size
- 1 pail of seawater
- 1 ruler
- 10 hotplates
- 10 thermometers
- pH paper
- A hygrometer

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Procedure

1. For this experiment, the independent variable is the temperature of the seawater – 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C and 65 °C. The dependent variable is the pH and specific gravity of the seawater. This is determined by using the pH paper and the hygrometer. The constants (control variables) are the size of the beaker, the amount of seawater used and the exposed surface area of the water.

2. Ten beakers are filled with seawater up to 10cm in height. The viscosity and pH levels of the seawater in all 10 beakers is tested using the hygrometer and pH paper. The readings in all 10 beakers should be the same. If different readings are obtained, repeat the test using a fresh pail of seawater.

3. A thermometer is placed inside each of the beakers. Each beaker is then placed on a hot plate. The hot plate is then regulated so that the seawater in each beaker will reach a temperature of 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C and 65 °C respectively. The temperature of the chosen beaker is written outside the beaker. The temperatures are kept constant for 24 hours.

4. The height, pH level and specific gravity of the seawater in the beakers is measured after 24 hours and the results are recorded in the table given below.

Results

The level of water fell as the temperature of the seawater increased indicating a higher rate of evaporation. The increasing temperature also increased the specific gravity and the pH of the seawater after 24 hours.

Measurement	Seawater condition with increasing temperatures measured after 24 hours
20 °C	25 °C	30 °C	35 °C	40 °C	45 °C	50 °C	55 °C	60 °C	65 °C
Water height (cm)	9.9	9.8	9.6	9.3	8.9	8.4	7.8	7.1	6.2	5.1
pH	7.65	7.70	7.75	7.80	7.85	7.95	8.05	8.15	8.30	8.70
Specific gravity (mg/ml)	1.025	1.030	1.040	1.055	1.075	1.090	1.110	1.140	1.160	1.210

The graph below represents the results of our science project experiment:

Salinity and temperature science project

Conclusion

The hypothesis that increasing the temperature will cause the pH and salinity of seawater to increase, is proven to be true.

Drinking a small amount of seawater while we are swimming or playing on the beach is quite harmless to us. However, drinking seawater to quench our thirst can be dangerous because our body will then need to use more water to remove the salt from our body through our urine. This will cause further dehydration in our body. Sea water should be desalinated and converted to fresh water before it can be used as drinking water.

Also consider

The experiment can also be repeated by placing a fan in front of a basin containing the seawater.

Try to repeat the experiment using containers with different surface areas, like pails or large trays.

References

Seawater - http://en.wikipedia.org/wiki/Seawater

Salinity, conductivity - http://www.waterwereld.nu/salinityeng.html

Thirsty – how about a cool, refreshing cup of seawater? - http://ga.water.usgs.gov/edu/drinkseawater.html

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