Water for Human Consumption
- Potable water is water that has been processed and is safe for human consumption and daily use.
- The difference between pure water and potable water is that pure water is solely made up of H2O molecules, whereas potable water may contain different substances, usually dissolved minerals and salts.
- Potable water should have the following characteristics:
- Have a pH between 6.5 and 8.5.
- The dissolved substances (e.g. salt) wil be within a normal range i.e not saturated.
- Be free of bacteria or potentially harmful microbes.
- Water is considered fresh when it is relatively free from dissolves substances e.g. rainwater.
- Water can collect in reservoirs, lakes and rivers and is known as surface water.
- In addition, it can collect in aquifers which are rocks that collect water underground.
- This water is called groundwater.
- The origin of fresh water depends on the climate in the region in question.
- In warmer areas, such as in the south-east of England, water primarily dries up before it can be collected so is found underground.
- Despite being relatively low in dissolved substances, there is still a process in place to ensure it is safe and suitable for use. Two important steps in this process are:
- Debris such as pieces of soil and dirt, small pebbles, twigs etc. are removed by a wire mesh screen.
- After this, other debris is filtered through sand beds and gravel.
- Ultraviolet and ozone light is used to sterilise water or alternatively chlorine gas is bubbled through the water.
- This removes any dangerous bacteria or microbes.
- Where aquifers are not present and/or the collection of surface water is limited, the process of desalination must be used to give potable water to the population.
- Desalination involves the treatment of seawater to remove the salt by distillation or reverse osmosis, a process that involves the use of membranes.
- When salt water is put through a certain membrane, only water molecules can pass through it. This happens as the membrane stops larger molecules and ions passing through.
- Desalination is an expensive process as it consumes large amounts of energy and is not ideal when producing large quantities of fresh water.
- This is used in regions with a very hot climate such as Saudi Arabia.
Required Practical 8: Analysis & Purification of Water Samples
Practical 8(a): Analysing and Purifying Water and making it Safe to Drink
To determine the amount of dissolved solid in samples of water.
Analysis of the pH and dissolved solids of water samples can help determine the regions the water comes from.
- water samples A, B, C and D
- universal indicator paper
- mass balance
- evaporating basin
- 25cm3 graduated cylinder
- bunsen burner, tripod & gauze
Don’t overheat during step 4 as you run the risk of thermally decomposing some of the solids, leading to erroneous results.
- Use the universal indicator paper to determine the pH of the water sample.
- Accurately weigh an empty evaporating basin to two decimal places.
- Add 25 cm3of water sample A into the evaporating basin.
- Heat the evaporating basin on a tripod and gauze using a Bunsen burner until the solids start to form and the majority of water has evaporated.
- Weigh the cooled evaporating basin again and calculate the mass of the solids that were
dissolved in the water.
Record your results in a suitable table, eg:
The results could be compared to the national water safety levels and by analysis the regions of each sample could be deducted e.g. region of high acid rain, water from a salt water supply etc.
The amount of dissolved solids in water can be determined and is a useful indicator of water quality.
Required Practical 8(b): To Purify a Water Sample by Distillation
To separate pure clean water from a sample containing water and other substances.
A simple distillation apparatus can be set up separate pure water from a mixture of water and unwanted substances.
- 10 cm3 of water sample A
- bunsen burner
- heatproof mat
- clamp and clamp stand
- conical flask with delivery tube and bung
- boiling tube
- ice bath
Diagram showing the apparatus to set-up for a simple distillation experiment
The delivery tube must sit above the filtrate level to prevent cold water being sucked back up the delivery tube after separation, which would break the hot glass.
- Add the water sample to the conical flask and set up the apparatus for distillation as shown in the diagram.
- Heat the water using the Bunsen burner until boiling occurs.
- Reduce the heat so that the water boils gently for some time.
- The distilled water will collect in the cooled test tube.
- Collect about 2 cm depth of water in this way, then stop heating.
- Analyse the water you have distilled by determining its boiling point.
Distillate of pure clean water.
The pH of the water can be tested as well as its boiling point.
Simple distillation can be used to produce pure water from a sample of impure or contaminated water.
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Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.