A Guide to IGCSE Alternative to Practical Chemistry
This is a complete guide to IGCSE alternative to practical chemistry and I have broken it down into several sections just to help us understand what is required in this section.
Please remember in IGCSE Olevel chemistry, there are three sections. We have the essay(theory), objectives, and alternative to practical or Practical.
The mark distribution is 50% for theory, 25% for objectives, and 25% for practical or alternative to practical.
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1. METHODS OF PREPARING GASES
2. METHODS OF COLLECTING GASES
3. METHODS OF DRYING GASES
4. SEPARATION METHODS USED IN CHEMISTRY
5. COLOURS OF COMMON SALTS IN IGCSE CHEMISTRY
6. SOLUBILITY RULE
7. COMMON PIECES OF APPARATUS USED IN CHEMISTRY
8. TEST FOR IONS
9. SALT ANALYSIS AND PLANNING INVESTIGATION
10. SKILLS NEEDED IN INVESTIGATING EXPERIMENTS
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Basic Facts in IGCSE Chemistry Practicals
The modules are arranged in hints as we discuss some salient key points needed to excel in IGCSE chemistry.
Hint 1:Methods of Preparing Gases in IGCSE Alternative to Practical Chemistry
The following gases are commonly prepared in the laboratory:
Carbon dioxide, chlorine, hydrogen, and oxygen
| Gas | How to prepare gases | Equation |
| CO2 | Add dilute HCl to marble chips | CaCO3 + HCl CaCl2 + H |
| Cl2 | Add conc. HCl to Manganese iv oxide | 4HCl + MnO2à MnCl2 + 2HCl +O2 |
| H2 | Add dilute HCl to zinc granules | Zn + HCl à ZnCl2 + H2 |
| O2 | Add hydrogen peroxide to manganese IV oxide as catalyst | 2H2O2 à H2O2 + O2 |
Hint 2: Methods of Collecting Gases
There are two factors considered in collecting gases, solubility, and density.
- There are four ways to collect gases produced in a chemical process.
- Each method is different and depends on the nature of the gas and experiment.
- Downward delivery is used when the gas being collected is denser than air(Note: the mass of air is 28.8g, any gas heavier than this is collected by downward delivery of the gas)
- The opposite method is called upward delivery and is used when the gas is less dense than air.
- A gas syringe can be used to collect and measure the volume of a gas.
- Collection over water is used when the gas being collected is insoluble in water.
Hint 3: Methods of drying gases
- In many cases the gas we are collecting needs to be dried.
- This can be done by passing the gas through a drying agent which removes any water present.
- Depending on the gas being collected, a different drying agent may be used.
- Three common drying agents are:
- Concentrated sulfuric acid which can dry all gases except ammonia as neutralisation occurs.
- Anhydrous calcium chloride can dry all gases except for ammonia as it forms calcium chloride complex
- For ammonia and neutral gases, calcium oxide is the preferred drying agent
Hint 4: Methods of Separation and Purification
Mixtures are usually separated using various separation techniques through IGCSE chemistry focuses on a few of these methods of purification.
The few methods of purification discussed fully in IGCSE Chemistry are Filtration, Decantation, Centrifugation, Crystallization, Solvent Extraction, and Distillation.
Though I have written a thorough post on these methods of separation where I explained all the various separation methods with examples, I will still talk a little about them before I move on.
Filtration; An undissolved solid can be separated from solution or liquid by simply passing through a piece of paper in a filter funnel. The process is called filtration and the liquid that passed through the filter paper called filtrate while the solid that that stays on the filter paper is called the residue.
Decanting: This is a simple method of purification that is suitable for solids that have very heavy particles e.g sand and water.
Centrifugation: Centrifugation involves using a machine called a centrifuge to spin test tubes round and round at very high speeds.
Crystallization: Crystallization is a method of purification used to obtain solids from a solution. The solution is gently heated in an evaporating basin to concentrate it. Heat is applied to the solution until the crystallization point is reached.
If two dissolved substances have different solubilities at different temperatures, fractional crystallization can be used to separate them.
Solvent Extraction: Solvent extraction can be used to separate two solutes dissolved in a solvent. This works perfectly well if one of the solutes is volatile. A second solvent is used to extract one of the solids from the first solvent.
The second solvent must not be able to mix with the second i.e must not be miscible.
Simple Distillation: Simple distillation is used to obtain a solvent from a solution. When a solution of salt in water is heated, the water boils and escapes as steam. Simple distillation can be used to separate liquids of different boiling points and also recover water from salt.
Fractional Distillation: Fractional distillation is used to separate a mixture of liquids with close different boiling points that are miscible.
This method is used to separate even petroleum fractions, the more volatile liquid evaporates and moves very fast while the less volatile liquid does not move as fast as the other one.
Hint 5: Preparation of salts
There are five methods employed in preparing salts
| Preparation of soluble salts | Preparation of insoluble salts |
| 1. Action of metals with acids Use MAZIT =Mg,Al,Zn,Fe,Sn Zn + HCl à ZnCl2 + H2 2. 2. Action of carbonate with acids ZnCO3 +2HCl à ZnCl2 + H2O + CO2 3. Action of insoluble base with acids Use CuO or PbO CuO +2HCl à CuCl2 +H2O PbO + 2HNO3 à Pb(NO3)2 + H2O PROCEDURES –steps taken to prepare the soluble salts 1. Add the (metal, carbonate ,insoluble base) to the acid in the flask till it is in excess 2. Warm the flask gently to complete the reaction 3.filter off the excess metal, carbonate, insoluble base 4. Put the filtrate in the evaporating dish and heat to crystallization point 5. Filter off the crystals and allow to cool 6. Dry the crystals on a filter paper | 1. Precipitation or double decomposition soluble salt + soluble salt à insoluble salt + soluble salt AgNO3+NaCl à AgCl + NaNO3 PROCEDURES 1. Mix the two soluble salts 2. Filter off the precipitate formed 3. Rinse the precipitate with little water 4. Dry the residue |
| 4. Action of soluble base(Alkali) and acid PROCEDURES 1. First rinse the pipette with water and use it to pipette a known volume of the alkali onto the conical flask 2.Add a few drops of the indicator to the alkali in the flask 3. Rinse the burette with little acid and then fill the burette with acid 4. open the burette tap and let the acid onto the alkali until you notice the first permanent color change 5. Record the burette reading and repeat the experiment up to 3 times 6. Take the average of the titre values |
Hint 6: Colours of common salts in IGCSE Chemistry
| Metals | Generally, the salts of metals like Na, Ca, Mg, Al, k are white Example KCl, MgCl2, CaCl2 etc. Transition metals form coloured compounds/ions except for zinc and scandium | |||
| Iron (Fe) | FeO black Fe2O3 reddish-brown | Fe(OH)2 green Fe(OH)3 reddish-brown | Generally, iron ii compounds are green while iron ii compounds are red-brown | |
| Copper (Cu) | CuO-black, Cu2O-red | CuCl2, CuCO3 Blue-green | CuSO4, white CuSO4.5H2O Blue | |
| Silver (Ag) | AgCl –white | AgBr –creamy | Agi-yellow |
Hint 7: Solubility rule
| Salts | Soluble rule | Exceptions |
| Nitrates | All nitrates are soluble | Nil |
| Sulfates | All sulfates are soluble | BaSO4, PbSO4, CaSO4 |
| Chlorides | All chlorides are soluble | AgCl, PbCl2 |
| Iodides | All iodides are soluble | AgI, PbI |
| Bromides | All bromides are soluble | AgBr,PbBr |
| Carbonates | All carbonates are insoluble | K2CO3,Na2CO3, (NH4)2CO3 |
| Hydroxides | All hydroxides are insoluble | KOH, NaOH, (NH4OH |
| *All salts of potassium,sodium, and ammonium are soluble |
Hint 5: Common Pieces of Apparatus used in Chemistry
- I will update the post with diagrams of the pieces of apparatus
Uses of pieces of apparatus
1. Pipette: A volumetric pipette is used to measure out or transfer very accurately single fixed volumes of liquids like 5 cm3, 10cm3 or 25cm3.
2.Burette: A burette is used to accurately measure the volume of liquids up to 50 cm3. It an also be used for measuring the volume of gases but when doing so, it is always turned upside down.
3.Measuring cylinder: Measuring is only used to estimate the volume of liquids and not for accurate measurements because it is only calibrated to 1 or 2cm3
4. Gas syringe: The gas syringe is used to accurately measure volume of gases liberated during reactions.
Hint 6: Test for ions and gases
*Test for gases
| Gases | Test | Result |
| Oxygen | Glowing splint test | Relight the glowing splint |
| Hydrogen | Lighted splint test | Burns with a (squeaky) pop sound |
| Carbon iv oxide | Lime water test | Turns lime water milky |
| Ammonia | Litmus paper test | Turns damp red litmus paper blue |
| Chlorine | Litmus paper test | Bleaches damp litmus paper |
| Sulphur iv oxide | Potassium dichromate paper test Aqueous potassium manganate VI test | Turns paper green Decolourises it |
*Test for Cations
| Cations | Test 1 Add a few drops of sodium hydroxide(NaOH) in drops then in excess | Test 2 Add a few drops of aqueous ammonia (NH4OH) in drops then in excess |
| Aluminium ion (Al3+) | White precipitate dissolves in excess NaOH forming a colourless solution | White precipitate insoluble in excess NH4OH |
| Zinc ion (Zn2+) | White precipitate dissolves in excess NaOH forming a colourless solution | The white precipitate dissolves in excess NH4OH |
| Calcium ion (Ca2+) | White precipitate insoluble in excess NaOH | No precipitate or very slight(faint) precipitate |
| Copper ion (Cu2+) | Light blue or pale blue precipitate formed insoluble in excess NaOH | The light blue precipitate dissolves in excess NH4OH to form a deep blue solution |
| Iron II ion (fe2+) | Green precipitate insoluble in excess NaOH | Green precipitate insoluble in excess NH4OH |
| Iron iii ion(Fe3+) | Red-brown precipitate insoluble on excess NaOH | Red-brown precipitate insoluble on excess NH4OH |
*Test for Anions
| Anions | Test | Result |
| Carbonate ion (CO32-) | Add dilute hydrochloric acid | Effervescence, carbon dioxide given off turns lime water milky |
| Chloride ion (Cl–) | Add excess dilute nitric acid and then aqueous silver nitrate | White precipitate |
| Bromide ion(Br–) | Add excess dilute nitric acid and then aqueous silver nitrate | Cream precipitate |
| Iodide ion (I–) | Add excess dilute nitric acid and then aqueous silver nitrate | Yellow precipitate |
| Nitrate ion (NO3–) | Warm with aqueous sodium hydroxide and aluminums foil | Ammonia gas is given off, pungent-smelling gas turns red litmus paper blue |
| Sulphate ion (SO42-) | Add excess hydrochloric acid followed by barium chloride | White precipitate |
*Flame test
*the ions in bold are commonly asked so commit to memory
| Metallic group | Metal | Flame colour |
| Group 1 | Lithium Sodium Potassium Rubidium Caesium | Crimson Golden yellow Lilac Red Blue |
| Group 2 | Calcium Strontium Barium | Brick red Crimson Apple green |
| Others | Lead Copper | Blue –white green |
Hint 7: Salt Analysis
Hint 8: Skills needed in investigating experiments
Reading the thermometer
Reading the stop clock
Reading the volume of liquids in measuring cylinders
Hint 9: Planning an investigation
I have solved many papers on IGCSE broken down in an easy way to understand. I will update the post once i have permission from Cambridge
WHYS AND WHATS IN CHEMISTRY
1. Why is the burette rinsed with water any acid-base reaction? To remove traces of acid / clean / remove impurities
2. Why is recommended before titration to rinse again acid after rinsing with water? To remove traces of water
3. When doing experiments especially on rates of reactions, what would be the advantage of taking the temperature readings every 15 seconds?
To get more readings and to get a better graph
4. If the pipette is very accurate in measuring out or transferring volumes/, why can’t it be used for any volume like 13.5cm3, 14.8cm3, etc?
Pipette is not calibrated and thus only measures specific volumes
5. Why would measuring the volume of dilute sulfuric acid with a burette rather than a measuring cylinder be an improvement?
More accurate
6. Why is it not recommended to heat the round bottom flask containing organic solvents directly?
Organic solvents are flammable
I could go on and on but there are several planning investigation examples on IGCSE chemistry and we just need to practice past papers on IGCSE to be able
