1.3 Water (WJEC GCSE Chemistry): Flashcards

Distributing water involves taking water from where it has been obtained and transporting it through pipes to areas that need it.

False.

Dams and reservoirs can flood land and destroy habitats, so they do have a significant negative environmental impact.

Ways that individuals can reduce water consumption include:

1. Taking showers instead of baths

2. Turning off taps when not in use

3. Using a water meter to monitor water use

4. Using dishwashers when full

5. Watering plants with waste water

6. Insulating pipes

True.

Climate change is predicted to cause water shortages globally, including in the UK, which would increase the cost of water and make sustainable supply more important.

Methods of abstracting water include:

1. Collecting rainwater

2. Building dams and reservoirs

3. Desalination

4. Collecting from rivers and lakes

5. Pumping water from underground

A sustainable water supply is needed to reduce the environmental impact of abstracting, distributing and treating water.

Sedimentation is the process in water treatment where water is left to stand so that mud, sand and other particles sink to the bottom of the tank due to gravity.

The three main stages of water treatment are sedimentation, filtration and chlorination.

False.

Filtration removes smaller solid particles by passing water through sand and gravel, but bacteria are too small to be trapped by filters. Chlorination is used to kill bacteria.

During chlorination, chlorine is added to water to kill bacteria and other unwanted microorganisms.

Filtration is the process of passing water through layers of sand and gravel to remove smaller solid particles that were not removed during sedimentation.

Two diseases caused by consuming untreated water are:

1. Cholera

2. Typhoid

True.

Untreated water contains insoluble impurities (e.g. soil and plant matter) and soluble impurities (e.g. dissolved calcium compounds and inorganic pollutants). Both types must be removed during treatment.

Chlorine is added because bacteria and other microorganisms are too small to be trapped by sand and gravel filters, so chlorination is needed to kill them.

Fluoridation is the process of adding fluoride ions to drinking water, primarily to help prevent tooth decay.

Fluoride prevents tooth decay partly by increasing the rate of remineralisation and by reducing the ability of bacteria to form acid.

Demineralisation is the process by which acid reacts with and dissolves the enamel on the surface of teeth, causing tooth decay.

True.

Fluoride helps prevent tooth decay by increasing the rate of remineralisation, reducing bacteria's ability to produce acid, and making enamel in young children more resistant to acid attack.

Fluorosis is a condition caused by excessive fluoride intake, observed as white patches forming on the enamel of the teeth.

Two disadvantages are:

1. High fluoride levels can be toxic and have been linked to health problems such as stomach and bone cancers and infertility

2. Fluoride is also seen as a form of mass medication as people have not given consent to receiving it

False.

This comparison cannot produce valid conclusions because other social and economic factors could also affect tooth decay rates, making it impossible to isolate fluoride as the cause.

Mass medication means giving a medication or treatment to an entire population regardless of whether individuals need it. Critics argue that adding fluoride to drinking water is unethical because people have not given consent.

Desalination is the removal of salt from seawater to produce fresh water suitable for drinking or agricultural use.

Two methods used to desalinate seawater are:\n\n1. Distillation\n2. Reverse osmosis

Reverse osmosis is a desalination process in which salt water is pushed at high pressure through a semi-permeable membrane so that only water molecules pass through, leaving the salt behind.

False.

Desalination is an expensive process that consumes large amounts of energy. This means it is not available to poorer countries and produces more greenhouse gases than traditional water treatment.

In reverse osmosis, salt water is pushed through a semi-permeable membrane at high pressure, allowing only water molecules to pass through.

Desalination is useful in countries like Saudi Arabia because they have limited freshwater sources such as rivers and streams and little rainfall, but have a coastline giving access to seawater.

False.

The concentrated salty water (brine) left after desalination is a pollutant and must be disposed of carefully to avoid harming the environment.

Two environmental disadvantages of desalination are:\n\n1. It requires large amounts of energy, producing more greenhouse gases than traditional water treatment\n2. It also produces concentrated salty waste water that is a pollutant requiring careful disposal

Distillation separates a soluble solid from a solution. The solution is heated so the water boils, the vapour rises and passes through a condenser where it cools and condenses into pure water, leaving the salt behind.

Miscible liquids are liquids that are fully mixed together and do not separate into layers.

During distillation of salt water, water boils producing vapour, which rises and passes through the condenser, where it cools and condenses into pure water.

They must have different boiling points so that one liquid evaporates before the other when the mixture is heated. The vapour from the more volatile liquid is condensed and collected separately.

False.

Distillation requires a large amount of energy, making it an expensive method of producing pure water, similar to reverse osmosis.

Ethanol evaporates first because it has a lower boiling point than water. The ethanol vapour is collected in the condenser before the temperature rises high enough to boil the water.

False.

It is not possible to obtain completely pure ethanol by distillation because some water evaporates at temperatures below its boiling point, so small amounts of water will always be present in the distillate.

Distillation can separate two miscible liquids provided they have different boiling points.

Solubility is a measurement of how much of a substance will dissolve in a given volume of a liquid, usually expressed in grams per 100 g of solvent.

A saturated solution is one in which no more solute can dissolve at a given temperature.

A solubility curve has solubility (in g per 100 g of water) on the y-axis and temperature (in degrees Celsius) on the x-axis.

True.

For most solid solutes, solubility increases with temperature. For example, potassium nitrate shows a large increase in solubility with temperature.

In an experiment to find solubility, solute is added to a known volume of water until no more will dissolve. The solution is then filtered and heated to leave the dissolved solute, which is then weighed.

The point where two solubility curves cross represents the temperature at which those two substances have equal solubility (the same mass dissolves in 100 g of water).

False.

The solubility of sodium chloride barely increases with temperature, unlike potassium nitrate whose solubility increases dramatically.

In a solubility experiment, the temperature at which crystals first appear when the solution is cooled is recorded. A higher mass of solute dissolved means the substance is more soluble.

Hard water is water that contains dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions, which prevent soap from easily forming a lather.

Calcium and magnesium ions dissolve into water as it flows over rocks such as limestone that contain calcium compounds, for example when rainwater runs over rocks on its way to a reservoir.

True.

Hard water contains calcium and magnesium ions that react with soap to form scum, making it harder to form a lather. Soft water has only low concentrations of these ions.

Temporary hard water is hard water whose hardness can be removed by boiling. It is caused by dissolved calcium hydrogencarbonate, Ca(HCO₃)₂, which decomposes when heated.

Permanent hard water is hard water whose hardness cannot be removed by boiling. It is caused by dissolved calcium sulfate, CaSO₄, which does not decompose on heating.

Boiling temporary hard water causes calcium hydrogencarbonate to decompose, forming insoluble calcium carbonate, which leads to scale forming on heating elements.

False.

Calcium sulfate is the cause of permanent hardness. It does not decompose when boiled, so boiling does not remove this type of hardness.

When temporary hard water is boiled, the dissolved calcium hydrogencarbonate decomposes to form insoluble calcium carbonate. This insoluble calcium carbonate deposits on the heating element as scale.

The three methods used to soften hard water are:\n\n1. Boiling\n2. Adding washing soda (sodium carbonate)\n3. Using an ion-exchange column

Washing soda is sodium carbonate (Na₂CO₃), a substance that softens both temporary and permanent hard water by reacting with calcium and magnesium ions to form insoluble carbonates.

True.

An ion-exchange column removes hardness from both types by exchanging the calcium and magnesium ions in the water for sodium ions from the resin.

In an ion-exchange column, sodium ions in the resin are swapped for calcium and magnesium ions in the hard water, removing the hardness.

The column is recharged by flushing it with concentrated sodium chloride solution, which washes out the trapped calcium and magnesium ions and replaces them with sodium ions in the resin.

One advantage is that washing soda is inexpensive and softens both temporary and permanent hard water. One disadvantage is that it leads to the formation of limescale, which can block narrow pipes.

False.

Boiling only removes temporary hardness by decomposing calcium hydrogencarbonate. It has no effect on permanent hardness caused by dissolved calcium sulfate.

When washing soda is added to hard water, carbonate ions react with Ca²⁺ ions to form calcium carbonate (CaCO₃), removing them from solution.

Two benefits of hard water for human health are:\n\n1. The dissolved calcium compounds help strengthen teeth and bones\n2. They also help prevent heart and cardiovascular disease

True.

The calcium and magnesium ions in hard water react with soap to form an insoluble scum, which also makes it harder to produce a lather.

Scale forming in a kettle from hard water slows down the rate of heat transfer, making the kettle less efficient and more expensive to run.

Two non-health related advantages some people associate with hard water are:\n\n1. Some people prefer the taste of hard water\n2. Some brewers also prefer hard water for brewing beer

False.

Hard water requires more soap than soft water to produce a lather because the calcium and magnesium ions react with soap to form scum, wasting the soap before a lather can form.

Scale can build up over time until the pipes become completely blocked, preventing water flow. This is costly to repair and can reduce the efficiency of heating systems.

The calcium ions in hard water help strengthen teeth and bones and may help reduce the risk of cardiovascular disease.

The technique involves adding soap solution 1 cm³ at a time to 50 cm³ of each water sample in a conical flask, shaking vigorously after each addition, and recording the total volume of soap solution needed to produce a lather lasting 30 seconds.

True.

The calcium and magnesium ions in hard water react with soap to form an insoluble scum, which also makes it harder to produce a lather.

In the hardness practical, each water sample is tested before and after boiling to distinguish between temporary and permanent hardness.

The sample has temporary hardness only. The large reduction in soap needed after boiling shows that boiling has removed all the hardness, which is characteristic of temporary hard water.

True.

Permanent hardness is caused by calcium sulfate, which does not decompose on boiling. So if boiling has no effect on the amount of soap needed, all the hardness is permanent.

Soft water requires only a small volume of soap solution to form a permanent lather, and this volume does not change after boiling, because soft water contains very low concentrations of calcium and magnesium ions.

During the hardness practical, the conical flask is shaken vigorously and the lather must last for 30 seconds before the result is recorded.

A bar chart is used. The water sample labels go on the x-axis and the volume of soap solution required (in cm³) goes on the y-axis.