0620 · 10.1Water

Testing for water

0:00River water, tap water and distilled water can look similar, but they are not chemically identical. A sample may contain water without being pure, and clear liquid is not automatically safe to drink. These distinctions connect everything in this topic. We will explore how to identify water, check its purity, understand natural contaminants and explain domestic water treatment. In chemistry, a pure substance contains only one substance rather than a mixture.

0:24Pure water therefore has nothing else mixed into it. Natural supplies usually contain dissolved materials, suspended solids or living organisms. Something dissolved can be invisible, so appearance alone cannot tell us what a sample contains. A useful starting point is to separate three different questions. A chemical test can establish whether water is present in a sample. Measuring melting or boiling temperature helps us assess purity. Water treatment addresses another issue altogether, making a natural supply suitable for domestic use by removing solids and killing microbes.

0:57The first chemical test uses anhydrous cobalt(II) chloride. Anhydrous means without water, so the starting material has no water chemically incorporated into it. When water contacts this substance, its colour changes from blue to pink. That colour change provides evidence for the presence of water, including water within a mixture. For a liquid sample, bring a small amount into contact with the anhydrous cobalt(II) chloride. Describe the observation as blue turning to pink, rather than simply saying that it changes colour.

1:23Both the starting and finishing colours identify the test clearly. A positive result does not reveal whether the liquid contains any other substances. The second chemical test uses anhydrous copper(II) sulfate. This solid starts white and turns blue when water is present. Again, the word anhydrous matters because the test relies on starting with material that has not already incorporated water. If the solid is already blue, that initial condition cannot demonstrate the required change.

1:50In both tests, water joins the solid, producing its hydrated form. Hydrated means that water is chemically incorporated into the substance. The hydrated material has a different colour from the anhydrous starting material. You do not need an equation here, but this explanation shows why the correct starting substance is essential. These tests must be carried out using appropriate laboratory precautions, not improvised at home. Neither tasting a sample nor smelling it closely is a suitable chemical test.

2:17In a practical examination, distinguish the observation from your conclusion. The colour change is what you observe, while the presence of water is what you conclude.

Assessing water purity

2:25To investigate purity, we use physical properties rather than those colour changes. Pure water melts at zero degrees Celsius and boils at one hundred degrees Celsius under normal atmospheric pressure. These temperatures are characteristic of the pure substance under the stated conditions. Dissolved impurities generally lower its freezing or melting temperature and raise its boiling temperature. For a melting test, freeze the sample and measure the temperature as the solid melts.

2:50Compare the result with zero degrees Celsius. Alternatively, heat a liquid sample and measure its temperature while it boils. Under normal atmospheric pressure, compare that result with one hundred degrees Celsius to assess whether it behaves like pure water. Use the temperature during the change of state, not an arbitrary reading taken while the sample is warming. A beaker passing through one hundred degrees Celsius is not, by itself, a complete purity test.

3:14The boiling behaviour is what matters. Pressure also matters, because a different atmospheric pressure shifts the boiling temperature even without impurities. At this level, measurements matching the characteristic temperatures indicate that a sample is pure. A different temperature, or a change of state spread over a range of temperatures, suggests impurities. This does not mean that one temperature reading identifies every possible contaminant. It also does not replace checks for harmful microbes when deciding whether a supply is safe to drink.

3:41Consider a sample containing dissolved salt that turns anhydrous copper(II) sulfate from white to blue. We are asked to explain whether the sample contains water and whether it is pure. First, identify what the colour change establishes. It shows that water is present because the initially anhydrous solid has become hydrated. Next, consider the dissolved salt separately from that test result. The sample contains another substance mixed with its water, so it is not pure.

4:07There is no contradiction between these conclusions. A mixture can give a positive test for water because that test identifies one component, rather than proving that nothing else is present. If the salt had not been mentioned, the colour change alone would leave purity undecided. We would need further evidence, such as melting or boiling behaviour under suitable conditions. This reasoning prevents a common exam mistake. A positive chemical test tells us that water is present, not that the entire sample consists only of it.

Natural water contaminants

4:34Distilled water is used in practical chemistry rather than tap water because it contains fewer chemical impurities. Tap supplies can retain dissolved substances even after treatment makes them suitable for drinking. Those substances may interfere with reactions or introduce unexpected observations. Distilled water reduces that interference, making the intended chemical behaviour easier to investigate. For example, imagine preparing a solution and then testing it for a dissolved substance. If the water already contains that material, the result may not come solely from your intended sample.

5:06Using distilled water reduces this problem. The important explanation is fewer chemical impurities, not that distilled water is guaranteed to contain absolutely none. Natural water can collect substances as it moves through the environment. Contact with air can supply dissolved oxygen, while rocks and soil can contribute metal compounds. Human activities may introduce plastics, sewage and harmful microbes. Nitrates can enter from fertilisers, and phosphates can come from both fertilisers and detergents.

5:34These materials do not all behave in the same way. Dissolved substances are spread throughout the liquid and may be invisible. Pieces of plastic or other suspended solids remain separate from the surrounding water. Microbes are very small organisms that may also be present without producing any obvious change in appearance. Some substances in natural water are beneficial rather than harmful. Dissolved oxygen supports aquatic life because many organisms need it for respiration.

5:59Fish use oxygen dissolved in the surrounding liquid, rather than obtaining it by breaking apart water molecules. A supply can look clear while still lacking enough dissolved oxygen for healthy aquatic life. Some metal compounds provide essential minerals for life. Their presence does not automatically mean that a natural supply is polluted or dangerous. However, other metal compounds are toxic and can harm organisms. The identity of the compound matters, so the statement that all metal compounds are harmful would be incorrect.

6:27Plastics can harm aquatic life if animals swallow pieces or become entangled in them. Sewage is another potential danger because it contains harmful microbes that cause disease. Removing visible waste does not necessarily remove those organisms. This explains why making a supply look cleaner is not enough to establish that it is safe. Nitrates and phosphates can lead to deoxygenation of water and damage aquatic life. Deoxygenation means a reduction in the dissolved oxygen available in the liquid.

6:55Organisms that depend on that oxygen may then struggle to survive. For this topic, you need that connection, but not the detailed sequence of eutrophication. Keep the sources of these substances clear when describing environmental contamination. Nitrates may come from fertilisers, while phosphates may enter from fertilisers and detergents. Their harmful effect here concerns the loss of dissolved oxygen and resulting damage to aquatic life. Do not describe them simply as visible dirt that a filter will always remove.

Domestic water treatment

7:22Domestic water treatment tackles different problems with different stages. Sedimentation and filtration remove solids, carbon removes tastes and odours, and chlorination kills microbes. Each stage has a particular purpose rather than doing everything at once. Understanding those roles makes the treatment sequence much easier to explain than memorising a list of unfamiliar names. During sedimentation, the supply is held so that suspended solids can settle to the bottom. Gravity separates those particles from the liquid above them.

7:51This removes solids that settle under the treatment conditions. It does not remove every dissolved substance, because dissolved material does not simply fall out like sediment. Filtration removes remaining suspended solids as the liquid passes through a suitable filter. The filter material traps particles while allowing the water through. Think of this as separation according to whether material can pass through the filter. Dissolved substances generally travel with the liquid, so ordinary filtration does not make the result chemically pure.

8:18Carbon is used to remove tastes and odours. Substances responsible for these can be held on the carbon surface, reducing their presence in the treated supply. This has a different purpose from trapping suspended solids. Removing an unpleasant smell is not the same as proving that disease-causing microbes have been killed. Chlorination means treating the supply with chlorine to kill microbes. This addresses a hazard that may remain even when the liquid looks clear.

8:41Filtration and chlorination therefore cannot be treated as interchangeable steps. One removes suspended solids, while the other kills microbes that could cause disease. Treated tap water is not the same thing as distilled water. Domestic treatment is intended to produce a suitable household supply, but some dissolved substances can remain. That explains why tap water can be suitable for drinking yet less suitable for sensitive chemistry experiments. Safe and chemically pure describe different properties, rather than competing names for the same thing.

Exam answers and recap

9:11In an examination, the command word tells you how much explanation to provide. If asked to describe a chemical test, name the reagent and give its colour change. For anhydrous copper(II) sulfate, the solid turns from white to blue when water is present. Anhydrous cobalt(II) chloride turns from blue to pink. If asked to explain why distilled water is used, connect its lower impurity content to the practical work. Distilled water contains fewer chemical impurities than tap water.

9:37These impurities could interfere with reactions or affect observations. That explanation answers the chemical reason, rather than giving an unsupported claim that distilled water is always safer to drink. A question asking you to describe domestic treatment needs the purpose of each stage. Sedimentation allows suspended solids to settle, and filtration removes remaining suspended solids. Carbon removes tastes and odours, and chlorination kills microbes. These are separate creditworthy points, so avoid a vague answer stating only that treatment cleans the water.

10:08For a question asking you to state harmful effects, give the particular link being tested. Some metal compounds are toxic, and some plastics harm aquatic life. Sewage contains harmful microbes that cause disease. Nitrates and phosphates lead to deoxygenation of water and damage to aquatic life. The central distinction is between presence, purity and suitability for use. Colour changes establish the presence of water, while melting and boiling temperatures help assess purity.

10:34Natural substances may be beneficial or harmful, depending on what they are. Domestic treatment removes solids, unwanted tastes and odours, and dangerous microbes without necessarily removing every chemical impurity.