Experimental design
Apparatus for Measurement
To design a valid experiment, you must select apparatus that matches the precision required by the data. Precision refers to how closely repeated measurements agree with each other. In Cambridge exams, choosing the wrong apparatus is a common cause of lost marks because it introduces unnecessary uncertainty.
Below is a comparison of standard apparatus. Note that volumetric glassware (pipettes and burettes) is significantly more precise than general-purpose glassware (measuring cylinders).
| Key Characteristic |
|---|
| Used for rate experiments; human reaction time is the main source of error. |
| Read to the nearest half-scale division; ensure it is not touching container walls. |
| Must be tared (zeroed) before use; measures mass, not weight. |
| Measures one specific volume exactly (e.g., 25.0 cm³). Not for variable volumes. |
| Measures variable volumes with high precision. Used in titrations to find exact reacting volumes. |
| Used for approximate volumes or when high precision is not required. Never use for titrations. |
| Collects gas directly; friction of the plunger can cause sticking. |
Why precision matters: If an experiment requires determining the exact point where two solutions react (a titration), using a measuring cylinder is incorrect because its high uncertainty means you cannot determine the 'titre' accurately. Conversely, using a volumetric pipette to add acid drop-by-drop in a rate experiment is impossible because it only delivers one fixed volume.
Solution Components
Understanding the terminology of mixtures is fundamental to chemical analysis. These definitions are often tested in Paper 1 and Paper 3.
| Definition |
|---|
| The substance that dissolves the solute. It is present in the larger amount. Common example: Water. |
| The substance that is dissolved in the solvent. It is usually a solid, gas, or liquid present in the smaller amount. |
| A homogeneous mixture formed when a solute dissolves completely in a solvent. The particles are too small to be seen and do not settle out. |
| A solution that contains the maximum possible amount of solute dissolved in the solvent at a specified temperature. No more solute can dissolve; any additional solid will remain undissolved. |
| The insoluble solid that remains on the filter paper after filtration. |
| The liquid (solution) that passes through the filter paper and is collected in the flask below. |
Building on previous concepts: Recall that 'aqueous' (aq) means dissolved in water. Therefore, a saturated solution of sodium chloride is often written as NaCl(aq) with excess solid NaCl(s) at the bottom.
Selecting Apparatus for Accuracy
Scenario: You need to measure exactly 25.0 \text{ cm}^3 of sodium hydroxide solution to add to a conical flask for a titration.
Correct Choice: Volumetric Pipette.
Reasoning: A volumetric pipette is calibrated to deliver one specific volume with very high precision. It ensures the initial amount of reactant is known exactly, which is critical for calculating concentrations.
Reasoning: A volumetric pipette is calibrated to deliver one specific volume with very high precision. It ensures the initial amount of reactant is known exactly, which is critical for calculating concentrations.
Incorrect Choice: Measuring Cylinder.
Reasoning: While a measuring cylinder can hold 25 \text{ cm}^3, its uncertainty is too high (often \text{±} 0.5 \text{ cm}^3 or more). This large error margin would make the subsequent titration results unreliable.
Reasoning: While a measuring cylinder can hold 25 \text{ cm}^3, its uncertainty is too high (often \text{±} 0.5 \text{ cm}^3 or more). This large error margin would make the subsequent titration results unreliable.
Scenario: You need to add dilute hydrochloric acid drop-by-drop until a reaction is complete, and you do not know how much will be needed beforehand.
Correct Choice: Burette.
Reasoning: A burette allows for the gradual addition of variable volumes. You can read the initial and final levels to calculate the exact volume used (the titre) with high precision (\text{¼alpha} 0.05 \text{ cm}^3).
Reasoning: A burette allows for the gradual addition of variable volumes. You can read the initial and final levels to calculate the exact volume used (the titre) with high precision (\text{¼alpha} 0.05 \text{ cm}^3).
⚠︎ Confusion Between Residue and Filtrate
The Error: Students often swap the terms 'residue' and 'filtrate' or describe them incorrectly.
The Correction:
- Residue = The solid left on the filter paper. Think: 'Residue remains.'
- Filtrate = The liquid that passes through (filtrates) into the flask.
Memory Aid: The filtrate is what you filter out as liquid, while the residue is what is left behind.
⚠︎ Misunderstanding Saturated Solutions
The Error: Thinking a saturated solution means 'concentrated' or that the solute is dissolving quickly.
The Correction:
- Saturation is about capacity, not speed. A saturated solution has reached its maximum limit at that temperature. The rate of dissolution equals the rate of crystallization (dynamic equilibrium).
- Temperature dependence. Solubility changes with temperature. A solution saturated at 20^\text{C} may become unsaturated if heated to 80^\text{C}, allowing more solute to dissolve.
Key Phrase: Always mention 'at a specified temperature' when defining saturation.
Suggesting Advantages and Disadvantages of Apparatus
Context: You are often asked to 'suggest advantages and disadvantages' of a method or apparatus (e.g., using a gas syringe vs. collecting gas over water).
Advantages (Focus on Precision and Control):
- Precision: 'A burette is more accurate than a measuring cylinder because it has smaller scale divisions and lower uncertainty.'
- Control: 'A gas syringe allows for direct measurement of gas volume without water vapor interference.'
- Safety: 'Using a fume hood reduces exposure to toxic gases.'
Disadvantages (Focus on Practical Limitations):
- Mechanical Error: 'Gas syringes may stick due to friction, leading to inaccurate volume readings.'
- Leakage: 'Syringes or rubber tubing connections may leak gas, causing the measured volume to be lower than actual.'
- Solubility: 'Collecting gas over water is unsuitable for gases that are soluble in water (e.g., ammonia or HCl), as they will dissolve rather than collect.'
- Parallax Error: 'Reading a measuring cylinder from above/below the meniscus introduces parallax error; it must be read at eye level.'
Why examiners accept this: Examiners look for specific sources of experimental error (systematic or random) rather than vague statements like 'it is better.' Linking the disadvantage to a physical property (solubility, friction, leakage) demonstrates deep understanding.
Describing Saturated Solutions in Exams
Context: When asked to define or describe a saturated solution (common in Paper 4 and Paper 6).
Required Elements for Full Marks:
- Mention maximum concentration (or 'maximum amount of solute').
- Mention dissolved in the solvent.
- Specify at a given/specified temperature.
Example Answer: 'A saturated solution is one that contains the maximum amount of solute dissolved in the solvent at a specific temperature, so no more solute can dissolve.'
Why examiners accept this: The phrase 'at a specified temperature' is crucial because solubility is temperature-dependent. Omitting it makes the definition scientifically incomplete.
Past Paper Style Questions
Q:
Name one piece of apparatus that can be used to measure exactly 25.0 cm³ of a solution.
A:
Volumetric pipette.
Q:
State the term for the liquid that passes through the filter paper during filtration.
A:
Filtrate.
Q:
Explain why a burette is used instead of a measuring cylinder in a titration experiment.
A:
A burette allows for the accurate measurement of variable volumes (titres) and has higher precision/lower uncertainty than a measuring cylinder.
Q:
Suggest one disadvantage of using a gas syringe to collect carbon dioxide gas.
A:
The plunger may stick due to friction, causing inaccurate volume readings. / The rubber tubing may leak.
Q:
Describe what happens when a solution becomes saturated.
A:
No more solute can dissolve at that temperature; any additional solid remains undissolved (or equilibrium is established).