Investigating desalination of water

In a nutshell

Desalination is the process in which salt is removed from water. It can be carried out on a large, industrial scale but also in the lab. The purpose of this practical is to analyse and purify a sample of water and making it safe to drink.

Equations

This equation will be used as part of the analysis of your data:

$mass~of~solids=mass~of~solids~and ~evaporting~basin-mass ~of~evaporating~basin$

Definitions

Keyword	Definition
Saline solution	Salt dissolved in water
Desalination	A water purification process which removes salt from water
Distillation	Purification of a liquid by evaporation and condensation

Equipment list

The following equipment are required to complete the water purification investigation.

Equipment	Use
Universal indicator	To check the pH of water
Water sample $1$	Water sample $1$ will be tested for pH and mass in experiment $1$ and will be purified in experiment $2$
Water samples $2$ and $3$	Water sample $2$ and $3$ will be tested for pH and mass in experiment $1$
Bunsen burner	To heat the water
Tripod	To hold the conical flask and evaporating basin
Gauze	To keep the conical flask/evaporating basin in place
Heatproof mat	To protect the surface
Evaporating basin	To hold the water in experiment $1$
Weighing balance	To measure the mass of solids
Conical flask	To hold the water in experiment $2$
Ice bath	To condense the water

Experiment 1: Analysing water samples

Experimental variables

Every experiment will have an independent, dependent and constant variable. The independent variable is the variable that you change. The dependent variable is the variable which depends on the independent variable, therefore it is the one you measure. Control variables are the variables that are kept the same during the experiments.

Independent variable	$Water\ sample\ type$
Dependent variable	$pH\ and\ mass\ of\ solid$
Control variables	$Volume\ of\ water\ samples\ (10~cm^3), type\ of\ pH\ indicator\ (universal\ indicator)\ and\ volume\ of\ universal\ indicator\ added\ to\ the\ sample\ (three\ drops).$

Safety precautions

Follow these safety precautions to mitigate the risk of any harm.

Hazard	Risk	SAFETY measure
Bunsen burner	The Bunsen burner could cause a fire and burning of the skin / eyes.	Stand away from the naked flame and tie up long hair any loose bits of clothing.
Water and ice	The water and ice may spill, which could cause someone to slip and injure themselves.	Keep the ice and water away from the edge of the working space and wipe away any spills immediately.
Glassware	The glassware could break, which could cause cuts in skin / eyes.	Keep all glassware away from the edge of the working space. If glass breaks, inform an adult immediately so that they can remove all the broken glass.

Method

Follow these instructions to complete the experiment.

Chemistry; Chemistry practicals; KS4 Year 10; Investigating desalination of water

1.	Water
2.	Evaporating basin
3.	Bunsen burner under tripod and gauze

1.	Add three drops of universal indicator to water sample $1$ and record the colour observed.
2.	Weigh out an empty evaporating basin and record the value to two decimal places.
3.	Pour $10~ cm^3$ of water sample 1 into the evaporating basin.
4.	Place the evaporating basin on top of the gauze and tripod.
5.	Place the Bunsen burner under the tripod.
6.	Heat until the water evaporates, leaving solids behind.
7.	Measure the mass of the evaporating basin until the mass is constant and record to two decimal places.
8.	Repeat steps 1-7 for each water sample.

Analysis

Process and analyse your results in the following way.

Check the pH of each water sample by looking at a pH chart and compare to the colour recorded in step 1.

Chemistry; Chemistry practicals; KS4 Year 10; Investigating desalination of water

Calculate the mass of solids by subtracting step 2 from step 7:

$mass~of~solid=mass~of~solids~and ~evaporting~basin-mass ~of~evaporating~basin$

Conclusion

Identify patterns between the pH and mass of solids in each water sample.

Evaluation

In your evaluation, assess whether or not there is a relationship between pH and dry mass of each sample. You can also evaluate how pure the sample is by comparing the pH of each sample to the pH of pure water (pH = $7$ ).

Experiment 2: Purifying a sample of water by distillation

The purpose of this experiment is to remove salt from water (desalination) by distillation. In this experiment the water sample is a saline solution of sodium chloride ( $NaCl$ ).

Experimental variables

Independent variable	$Water\ sample\ type$
Dependent variable	$Boiling\ point\ of\ distilled\ water$
Control variables	$Initial\ volume\ of\ water\ sample\ and\ using\ the\ same\ equipment\ in\ any\ repeat\ experiments$

Method

Follow these instructions to complete the experiment.

Chemistry; Chemistry practicals; KS4 Year 10; Investigating desalination of water

1.	Water in a conical flask.
2.	Clamp stand for a test tube.
3.	Ice bath.

1.	Set up the distillation apparatus.
2.	Add $10~cm^3$ of water sample 1 to the conical flask.
3.	Slowly boil the water with the Bunsen burner.
4.	Collect the water to a depth of $1~cm$ in the cooled test tube.
5.	Analyse the boiling point of the distilled water (distillate).

Analysis

To determine the purity of the distilled water, you must analyse the boiling point and pH. The boiling point of pure water is $100~\degree C$ and the pH of pure water is $7$ . Follow these steps to test the purity of the distilled water from the cooled test tube:

1.	Add three drops of universal indicator to the distillate and record the colour that is observed.
2.	You may carry out flame tests for sodium and chloride ions.
3.	Immerse a thermometer in the water.
4.	Slowly heat the water.
5.	Carefully watch the water boil.
6.	Record the temperature at which the water boils.

Conclusion

Assess the purity of your sample by:

Comparing the boiling point of your distillate with the boiling point of pure water.
Comparing the pH of your distillate with the pH of pure water
Checking the results of your flame tests. The test results should be negative if the water is pure.

Evaluation

Evaluate the purity of your water sample by considering the following points:

How close is the boiling point value of your sample compared to the boiling point of pure water?
Were there any limits to the equipment and method that you used?
What changes could you make to improve your method?