1. Water (H2O) – colourless, odourless, tasteless liquid that does not burn.
Hydrogen: colourless, odourless, tasteless gas explosive with oxygen.
Oxygen: colourless, odourless, tasteless gas explosive with hydrogen.
1. Hydrogen and Oxygen Mixture Compared to Water
Hydrogen gas and oxygen gas mix perfectly giving a colourless, odourless, tasteless mixture of gases. It is possible, with proper cooling equipment to separate the gases by a physical process. But this mixture is very unstable and potentially explosive. They readily react releasing a tremendous amount of energy very quickly forming water. Water is a colourless, odourless, tasteless liquid that is quite stable. It is possible to separate the hydrogen and oxygen by electrolysis, a chemical process.
General Properties of Water
• Water is a colourless, odourless, tasteless liquid.
• Water is non-toxic liquid and safe for us to take in drinks and food.
• Melting Point: 0°C. Boiling Point: 100°C.
• Density: 1 g/cc at 4°C.
• Densest at 4°C.
• Expands becoming less dense form 4°C to 0°C (this is an unusual property).
• Expands becoming less dense from 4°C to 100°C.
• Expands by almost 10% when it solidifies at 0°C.
• Ice is less dense than water – ice floats in water. (density of ice: 0.92 g/cc).
• An exceedingly good solvent – dissolves more substances than any other solvent – as a result is very difficult to obtain pure water. Water is such a great solvent that it is commonly given the title of ‘universal solvent’.
Test for Water 1. Turns dry blue cobalt chloride a pink colour.
2. Turns dry white copper sulphate blue.
3 Boils at 100C and solidifies at 0C
Investigate the Solubility of a Variety of Substances in Water
1. A variety of solid and liquid substances are supplied.
2. Half fill a test-tube with water and add a small amount of the substance.
3. Stopper the test tube. Shake the test-tube vigorously up and down for 20 seconds.
4. Examine the test tube to see if any of the substance ‘disappeared’ into the water.
5. Decide from the results if the substance is insoluble, slightly soluble or very soluble in water.
Water (H2O) The chemical name for water is hydrogen oxide
Hardness of Water and Water Treatment
Water is so good at dissolving many different substances that it is known as the ‘universal solvent’. Many salts, because they are ionic compounds, dissolve easily in water. Water in nature is rarely pure; it usually contains a variety of dissolved substances including soluble salts.
Hard Water is water which is difficult to form lather when using soap. A lather is a layer sudsy froth at the surface of the water. Hard water contains the soluble ions of calcium and/or magnesium (Ca2+, Mg2+ )
The sources of these ions are certain common soluble salts.
Calcium Sulphate (CaSO4) Magnesium Hydrogen Carbonate (Mg(HCO3)2)
The calcium and magnesium ions react with soap making a curdy scum. A lather will form only when there are no calcium or magnesium ions in the water. Therefore extra soap is needed to form a lather in hard water.
The extra soap is used to remove the calcium and/or magnesium ions. The soap softens the water. Less soap is needed to make a lather with soft water.
Hard Water Advantages Hard Water Disadvantages
Supplies calcium, needed for strong bones. Causes Limescale, wasteing fuel – it lines kettles & boilers.
Good for brewing beer and ale. The scum causes problems when washing clothes.
Good for tanning, making animal skin to leather. Limescale blocks hot water pipes and radiators.
Some prefer the taste of hard water. Wastes soap – extra soap is needed to form lather.
Limescale is the layer of calcium carbonate (limestone rock, CaCO3) that forms on the inside walls when water hard with calcium hydrogen carbonate is heated. Ca(HCO3)2 + Heat » CaCO3 + H2O + CO2
MandPractivity (a) To Detect the Presence of Dissolved Salts in Water Samples.
1. Place an evaporating dish on top of a beaker of water.
2. Pour some of the water sample into the evaporating dish.
3. Heat the water sample by heating the beaker of water with a Bunsen.
4. The steam from the boiling water heats the evaporating dish.
5. Keep heating until the evaporating dish is dry.
6. Note the fine layer of grey residue on the inside of the evaporating dish – this is the soluble solids that were in solution in the water.
7. If a drop of dilute hydrochloric acid is placed in the dish and produces a fizz then a carbonate salt is present.
(b) Test Water for Hardness (Soap Test)
Use identical small flasks and stoppers. Have the same volume of water in each flask. A pipette will deliver an exact volume of the water sample to the flask. Add one drop of soap solution from a small dropper to each flask. Stopper and shake each flask with equal effort for 30 seconds. Check for a permanent lather i.e. a lather that lasts for one minute. Keep adding soap solution, drop by drop, until a permanent lather is formed. A water sample is hard if it needs more soap flakes than the deionised soft water. The more soap flakes needed, the harder the water.
Softening Hard Water by Ion Exchange
Water is hard because it has soluble calcium and/or magnesium ions. To soften water the ions of calcium and magnesium must be removed. Swap the calcium and magnesium ions in the water for sodium ions. Pass the hard water through a column of porous sodium aluminium silicates or over sodium ion exchange resins. Soft water emerges. The sodium ions in the column pass into the water. The calcium and magnesium ions pass from the water into the column. Sodium ions in the water do not cause hardness.
Other Methods of Softening Water
Distillation – the distilled water is soft.
Add washing soda i.e. sodium carbonate – dishwashers and washing machines.
Remove all ions from the water: deionised water is soft.
Water Treatment for Domestic Use Healthy Attractive Taste Smell
1. Screening, The water is passed through wire meshes to remove large suspended debris such as twigs, leaves, plastic bottles, paper cartons etc..
2. Settling or Sedimentation The water flows into a reservoir or large tanks. Small dense suspended material like fine sand and clay sinks to bottom. Clear water flows out at the top at the far end.
3. Filtration. The water is assed through filter beds of fine sand, coarse sand and gravel. Coarse sand and gravel used to keep the layer of fine sand in place. The smallest suspended material is trapped in the layer of fine sand.
The tiny spaces between the grains of fine sand are too small to let even the finest suspended material to pass through.
4. Chlorination. Chlorine gas or a soluble chlorine compound is dissolved in the water. Bacteria are killed – water is now sterile and safe to drink. In some treatment plant ozone gas is used to sterilise the water.
5. Fluoridation A fluoride salt is dissolved in the water to reduce tooth decay.
The Composition of Water (H2O) A molecule of water is composed of two atoms of hydrogen each individually bonded to oxygen by a single covalent bond. Electrolysis is using an electric current to bring about a chemical change in the liquid conducting the electric current. When an electric current passes through water some of the water is decomposed to its constituent elements, hydrogen and oxygen.
Pure water is a bad conductor of electricity so to improve its ability to conduct a little sulphuric acid is often added. The water is then often titled ‘acidulated water’. After electrolysis the same amount of sulphuric acid is present at the end as added at the start – so sulphuric acid is like a catalyst but not one (dept of ed June 2006).
An electrode is a conductor delivering an electric current to or taking electric current from an electrolyte.
Cathode: the negative electrode at which the electric current leaves the electrolyte. Anode: the positive electrode at which the electric current enters the electrolyte. (A for anode, a for addition, addition sign is +)
Decomposition of Water by Electrolysis .
1. Two graphite carbon electrodes are in the acidulated water.
2. Each electrode is covered by a wide test tube filled with water.
3. The electrodes are connected to a battery and the switch closed.
4. An electric current flows in the circuit through the water.
5. Colourless gas bubbles form at and rise from each electrode when the electric current flows.
6. The gas from each electrode is collected in its test tube and its volume measured.
7. The gas collected from the anode relights a glowing splint — it is oxygen.
8. The gas collected from the cathode burns with a ‘pop’ — it is hydrogen.
9. Therefore water is composed of hydrogen and oxygen.
10. Twice as much hydrogen is produced than oxygen.
The volume of water in the apparatus decreased .
Hydrogen is collected at the cathode: H is the symbol for hydrogen and there is a H in catHode. A special item of equipment used to demonstrate electrolysis of water is Hofmann Voltameter.