Experimental design- CIE iGCSE Chemistry Notes - New Syllabus
Experimental design for iGCSE Chemistry Notes
Core Syllabus
- Name appropriate apparatus for the measurement of time, temperature, mass and volume, including:
(a) stop-watches
(b) thermometers
(c) balances
(d) burettes
(e) volumetric pipettes
(f) measuring cylinders
(g) gas syringes - Suggest advantages and disadvantages of experimental methods and apparatus
- Describe a:
(a) solvent as a substance that dissolves a solute
(b) solute as a substance that is dissolved in a solvent
(c) solution as a mixture of one or more solutes dissolved in a solvent
(d) saturated solution as a solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature
(e) residue as a substance that remains after evaporation, distillation, filtration or any similar process
(f) filtrate as a liquid or solution that has passed through a filter
Apparatus for measurement of time, temperature, mass and volume
Apparatus for measurement of time, temperature, mass and volume
Time
- Stop-watches are used to measure time intervals in experiments
- Digital stop-watches are more precise than analogue ones, often reading up to 0.01 s
- Commonly used in rate of reaction experiments such as timing precipitate formation or gas production
- Main limitation is human error when starting or stopping
Temperature
- Thermometers are used to measure temperature, usually in °C
- Mercury and alcohol thermometers are accurate but may be hazardous if broken
- Digital thermometers are safer, faster, and can give decimal readings
- Important in monitoring exothermic and endothermic reactions
Mass
- Balances are used to measure the mass of solids and liquids
- Digital balances are highly accurate, often to 0.001 g
- Essential for preparing standard solutions and weighing reactants
- They must be on a flat surface as they are sensitive to vibrations and air currents
Volume (very accurate)
- Burettes are long graduated glass tubes with a tap, used to deliver variable but precise volumes
- Accuracy is usually to 0.1 cm³, ideal for titrations
- They are fragile and require skill to use without parallax error
- Volumetric pipettes measure one fixed volume very accurately (e.g., 25.0 cm³)
- Used in titrations to transfer known volumes of solutions
- A pipette filler must be used for safety
- Limitation is that each pipette only measures a single specific volume
Volume (less accurate)
- Measuring cylinders are glass or plastic graduated cylinders
- They provide approximate volumes with an accuracy of about ±1 cm³
- Quick and versatile, but less precise than pipettes or burettes
Volume of gases
- Gas syringes are used to collect and measure gas volumes accurately in cm³
- They are widely used in rate of reaction experiments, e.g., magnesium with hydrochloric acid producing hydrogen
- Require airtight connections to avoid gas leakage
- Not suitable for gases that dissolve in water if collected by water displacement
Example
A student is asked to investigate how temperature affects the rate of reaction between magnesium ribbon and hydrochloric acid.
Which apparatus would be most suitable for:
- Measuring the reaction time?
- Measuring the temperature change during the reaction?
- Measuring the mass of magnesium ribbon?
- Measuring the volume of hydrogen gas produced?
For each apparatus chosen, state one advantage and one limitation.
▶️Answer/Explanation
(i) Measuring reaction time: Digital stopwatch
Advantage: Precise to 0.01 s
Limitation: Human error when starting/stopping
(ii) Measuring temperature: Thermometer (digital preferred)
Advantage: Gives quick, accurate decimal readings
Limitation: Some thermometers (mercury) are hazardous if broken
(iii) Measuring mass: Digital balance
Advantage: Accurate to 0.001 g
Limitation: Sensitive to vibrations/air currents
(iv) Measuring gas volume: Gas syringe
Advantage: Accurate direct reading in cm³
Limitation: Needs airtight setup, unsuitable if gas dissolves in water
Final Point: Choice of apparatus depends on required precision and practical limitations (cost, fragility, errors).
Advantages and disadvantages of experimental methods and apparatus
Advantages and disadvantages of experimental methods and apparatus
Stop-watches
Advantage:
- Inexpensive and simple to use for measuring time intervals
- Digital stop-watches give higher precision (0.01 s)
Disadvantage:
- Human error in pressing start/stop introduces inaccuracy
- Cannot record very short time changes as accurately as electronic sensors
Thermometers
Advantage:
- Mercury and alcohol thermometers are reliable and inexpensive
- Digital thermometers are quick, safe, and can give decimal values
Disadvantage:
- Mercury thermometers are hazardous if broken
- Digital thermometers may need calibration and depend on battery power
Balances
Advantage:
- Digital balances provide very high accuracy, often to 0.001 g
- Essential for quantitative work such as standard solutions
Disadvantage:
- Expensive and sensitive to vibration or air movement
- Require careful calibration to maintain accuracy
Burettes
Advantage:
- Provide precise and controlled delivery of liquids to 0.1 cm³ accuracy
- Ideal for titrations where small differences in volume matter
Disadvantage:
- Fragile glass apparatus that can easily break
- Requires careful technique to avoid parallax errors and air bubbles
Volumetric pipettes
Advantage:
- Extremely accurate for a fixed volume, such as 25.0 cm³
- Commonly used in titrations for standardised results
Disadvantage:
- Only measures one fixed volume, not versatile
- Requires pipette filler for safe use
Measuring cylinders
Advantage:
- Easy to use, durable, and available in many sizes
- Faster and more convenient than pipettes or burettes
Disadvantage:
- Less accurate, usually only to ±1 cm³
- Not suitable where high precision is required
Gas syringes
Advantage:
- Provide accurate measurement of gas volume directly in cm³
- More reliable than collecting gases over water, especially if the gas is soluble
Disadvantage:
- Requires airtight connections, otherwise gas may escape
- Some gases may react with or damage the syringe material
Example
State and explain the advantages and disadvantages of using a burette in an acid–base titration experiment.
▶️Answer/Explanation
Advantages:
- Can measure liquids very accurately (to 0.1 cm³).
- Allows controlled release of liquid, drop by drop, making it suitable for titrations.
- Can deliver variable volumes, not just one fixed volume.
Disadvantages:
- More expensive and fragile compared to a measuring cylinder.
- Needs to be set up vertically using a stand and clamp.
- Slower to fill and rinse compared to simple apparatus like a pipette.
Final Point: A burette is the most precise tool for titrations but requires careful handling and setup.
Solutions and separation techniques
Solutions and separation techniques
Solvent
- A solvent is a substance that dissolves a solute to form a solution
- The solvent is usually present in the largest amount
- Example: Water is a common solvent in many laboratory and biological processes
Solute
- A solute is the substance that is dissolved in a solvent
- It is usually present in a smaller amount compared to the solvent
- Example: Sodium chloride is the solute when salt dissolves in water
Solution
- A solution is a homogeneous mixture of one or more solutes dissolved in a solvent
- It appears as a single phase with uniform properties throughout
- Example: Salt solution or sugar solution
Saturated solution
- A saturated solution contains the maximum amount of solute that can dissolve in a solvent at a specified temperature
- If more solute is added, it will remain undissolved at the bottom
- Example: A saturated salt solution at room temperature leaves undissolved crystals at the bottom
Residue
- Residue is the substance that remains after processes like evaporation, distillation, or filtration
- It is usually the undissolved solid or the material left behind
- Example: Sand remains as residue after filtering a sand-salt mixture
Filtrate
- Filtrate is the liquid or solution that passes through filter paper during filtration
- It contains the dissolved substances but no solid particles
- Example: Salt solution collected after filtering a sand-salt mixture
Example
A chemistry student is given a mixture of common salt (sodium chloride) dissolved in water, together with sand. The task is to separate all the components and obtain pure dry salt and sand. Suggest a suitable method and explain the steps involved.
▶️Answer/Explanation
Step 1: Dissolving – Add water to the mixture. Salt dissolves in water, while sand remains insoluble.
Step 2: Filtration – Filter the mixture. Sand remains as residue on the filter paper, while the salt solution (filtrate) passes through.
Step 3: Evaporation – Heat the salt solution in an evaporating dish to remove water, leaving pure, dry salt.
Final Answer: The mixture is separated by dissolving, filtration, and evaporation. Sand is collected as residue, while pure sodium chloride crystals remain after evaporation of the filtrate.
Example
A student is given a sample of water taken from the sea. The water contains dissolved salts as well as some insoluble sand particles. The student needs to obtain pure water from the sample.
(a) Describe the steps the student should take to separate the insoluble sand from the solution.
(b) The remaining solution still contains dissolved salts. Suggest a separation method to obtain pure water, and explain why this method is suitable.
▶️Answer/Explanation
(a) Filter the sea water through filter paper. The insoluble sand remains as residue on the filter paper, while the salt solution (filtrate) passes through.
(b) To obtain pure water, use simple distillation. The salt solution is heated until water boils and forms steam. The steam is cooled in a condenser and collected as pure water (distillate). The dissolved salts remain in the flask because they do not evaporate at the boiling point of water.
Final Answer: Sand is separated by filtration, and pure water is obtained from the salt solution by distillation.