Water Revision Notes

Anhydrous cobalt(II) chloride is a crystalline solid that appears blue in its dry form. It is highly hygroscopic, meaning it readily absorbs water from its surroundings.

Anhydrous copper(II) sulfate is a white powder that also absorbs water to form a hydrated compound, changing its colour.

Both compounds undergo distinct colour changes upon hydration, which forms the basis for their use as chemical tests for water presence.

• Anhydrous cobalt(II) chloride: blue solid, turns pink when hydrated.
• Anhydrous copper(II) sulfate: white powder, turns blue when hydrated.

When anhydrous cobalt(II) chloride absorbs water molecules, it forms hydrated cobalt(II) chloride, which has a pink colour. This is due to changes in the coordination environment of the cobalt ions affecting light absorption.

Similarly, anhydrous copper(II) sulfate reacts with water to form copper(II) sulfate pentahydrate, which is blue. The water molecules coordinate with copper ions, altering the compound's electronic structure and colour.

These reversible hydration reactions are represented as: CoCl2 (blue) + 6H2O → CoCl2·6H2O (pink) and CuSO4 (white) + 5H2O → CuSO4·5H2O (blue).

• Hydration changes the coordination of metal ions, causing colour shifts.
• Colour change indicates presence of water molecules chemically bound to the salt.

Place a small amount of anhydrous cobalt(II) chloride on a watch glass or white tile to observe its blue colour.

Add a few drops of the liquid sample suspected to contain water onto the cobalt(II) chloride.

Observe any colour change carefully. A change from blue to pink indicates the presence of water in the sample due to hydration of the salt.

• Use clean apparatus to avoid contamination.
• Repeat the test to confirm results.
• Avoid excess sample to prevent dissolving the salt instead of just hydrating it.

Place a small amount of anhydrous copper(II) sulfate powder on a watch glass or white tile, noting its white colour.

Add a few drops of the sample to the powder and observe any colour change.

A change from white to blue confirms the presence of water, as the salt forms its hydrated blue pentahydrate form.

• Ensure the copper(II) sulfate is anhydrous before testing.
• Repeat the test for reliability.
• Avoid contamination with other substances that might affect colour.

A clear colour change in either test is a positive indication of water presence.

No colour change suggests absence of water or that the sample is anhydrous.

These tests detect water in liquid or moist samples but may not detect water vapour unless sufficient moisture is present to hydrate the salts visibly.

• Colour changes are qualitative indicators, not quantitative.
• Tests are simple and rapid but can be affected by impurities.
• Handle chemicals with care; cobalt(II) chloride is toxic and should be used with gloves and in a well-ventilated area.

In chemistry, the purity of substances used in experiments is crucial. Impurities can cause unexpected reactions or alter the results, leading to inaccurate conclusions. Water is a common solvent and reagent in many experiments, so its purity directly affects the reliability of the results.

Tap water contains dissolved minerals, salts, and other chemical impurities that vary depending on the source and treatment processes. These impurities can react with chemicals in an experiment or change the concentration of solutions, causing errors. Therefore, using water with minimal impurities is essential for precise and reproducible outcomes.

• Impurities can interfere with chemical reactions.
• Accurate results require pure substances.
• Water purity affects solution concentration and reaction pathways.

Distilled water is produced by boiling water to create steam and then condensing this steam back into liquid form. This process removes most dissolved solids, minerals, and many chemical impurities because they do not vaporize with the water.

As a result, distilled water is much purer than tap water. It contains very low levels of dissolved salts, metals, and organic compounds, making it ideal for use in chemical experiments where contamination must be minimised.

• Distillation removes dissolved solids and many impurities.
• Distilled water is nearly free of minerals and salts.
• It is produced by boiling and condensing water.

Tap water typically contains dissolved calcium and magnesium salts (hardness), chlorine compounds used for disinfection, iron, and sometimes organic matter. These impurities can affect chemical reactions in several ways.

For example, calcium ions can precipitate with certain reagents, altering the expected products. Chlorine can react with sensitive chemicals, causing side reactions. Organic matter may introduce contaminants that interfere with colour changes or titrations.

• Calcium and magnesium salts cause water hardness.
• Chlorine is added for disinfection but can react chemically.
• Organic matter and metals may contaminate experiments.

Using tap water instead of distilled water can lead to inaccurate results. For instance, in preparing standard solutions, impurities add extra ions that change the solution's concentration, affecting titration results.

In precipitation reactions, unwanted ions from tap water may form unexpected precipitates, confusing observations. In electrochemical experiments, dissolved ions can alter conductivity and electrode potentials.

Therefore, tap water's impurities can cause systematic errors and reduce the reliability of experimental data.

• Impurities alter solution concentrations.
• Unexpected precipitates may form.
• Electrochemical measurements can be affected.

Distilled water minimises contamination, ensuring that chemical reactions proceed as expected. It improves the accuracy of quantitative experiments such as titrations and solution preparations.

Using distilled water also reduces variability between experiments, making results more reproducible. This is essential for comparing data and drawing valid conclusions.

In summary, distilled water provides a controlled and consistent medium for chemical work, free from interfering substances found in tap water.

• Ensures accurate and reliable results.
• Reduces experimental variability.
• Prevents interference from impurities.

Sedimentation is the first stage in treating domestic water supplies. Water containing suspended solid particles, such as sand, silt, and organic matter, is held in a large tank or reservoir. Because these particles are denser than water, they gradually settle to the bottom under gravity.

This process reduces the amount of solid material in the water before further treatment. The settled solids form a layer called sludge, which is periodically removed.

For example, water from a river may carry fine soil particles after rainfall. Allowing this water to stand undisturbed in a sedimentation tank for several hours causes these particles to settle out, clarifying the water.

• Sedimentation relies on gravity to separate solids from water.
• It removes larger and heavier suspended particles.
• Sludge formed at the bottom must be removed regularly.
• It is a passive process requiring no chemicals.

After sedimentation, the water still contains smaller suspended particles and some microorganisms. Filtration passes the water through layers of porous materials, such as sand, gravel, and sometimes charcoal, which physically trap and remove these particles.

The filter acts like a sieve, catching particles too small to settle during sedimentation. This improves water clarity and reduces turbidity.

For instance, water passing through a sand filter loses fine dirt and some bacteria attached to particles, making it cleaner and safer for further treatment.

• Filtration removes smaller suspended solids not settled by sedimentation.
• Common filter materials include sand and gravel.
• It improves water clarity and reduces turbidity.
• Filters require regular cleaning or replacement to maintain effectiveness.

Even after removing solids, water can have unpleasant tastes or odours caused by dissolved organic compounds, chlorine residues, or chemicals.

Activated carbon is used to adsorb these substances. It has a large surface area with many pores that trap molecules responsible for bad tastes and smells.

For example, water treated with activated carbon can lose the 'earthy' or 'chemical' taste sometimes present in tap water, making it more acceptable to drink.

• Activated carbon adsorbs dissolved organic compounds causing tastes and odours.
• It improves the sensory quality of water without adding chemicals.
• Carbon filters need periodic replacement as pores become saturated.
• It does not remove dissolved salts or microbes.

The final stage in domestic water treatment is disinfection to kill harmful microorganisms such as bacteria, viruses, and protozoa that cause diseases.

Chlorination involves adding a small, controlled amount of chlorine or chlorine compounds (e.g., sodium hypochlorite) to the water. Chlorine is a strong oxidising agent that destroys microbial cell walls and enzymes.

For example, adding chlorine to treated water ensures pathogens like Escherichia coli are killed, preventing waterborne illnesses such as cholera or typhoid fever.

• Chlorination disinfects water by killing harmful microbes.
• It requires careful dosing to avoid harmful chlorine levels.
• Residual chlorine remains to protect water during distribution.
• Over-chlorination can cause taste and health concerns.