12.1 Experimental Design Revision Notes
1. Overview
Experimental design is a fundamental part of chemistry that focuses on selecting the correct tools and techniques to obtain accurate, reliable results. Understanding how to measure physical quantities and defining the components of mixtures is essential for conducting valid laboratory investigations and industrial processes.
Key Definitions
- Solvent: A substance (usually a liquid) that dissolves a solute to form a solution.
- Solute: A substance (solid, liquid, or gas) that is dissolved in a solvent.
- Solution: A mixture formed when one or more solutes are dissolved in a solvent.
- Saturated solution: A solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature. Any further solute added will not dissolve.
- Residue: The solid substance that remains on the filter paper after filtration or stays behind after evaporation or distillation.
- Filtrate: The liquid or solution that passes through the filter paper during the process of filtration.
Core Content
Measurement of Physical Quantities
To design an experiment, you must choose the appropriate apparatus based on the required precision.
| Quantity | Apparatus | Notes on Use |
|---|---|---|
| Time | Stopwatch, Digital Timer | Usually recorded in seconds (s) or minutes (min). |
| Temperature | Thermometer | Liquid-in-glass or digital probes. Measured in Celsius (°C). |
| Mass | Digital Balance | Ensure the balance is "tared" (zeroed) before use. Measured in grams (g). |
| Volume (Liquid) | Measuring Cylinder, Burette, Pipette | Depends on the volume and precision required. |
| Volume (Gas) | Gas Syringe | Used to collect and measure gas evolved during a reaction. |
Advantages and Disadvantages of Apparatus
- Measuring Cylinder:
- Advantage: Quick to use; available in many sizes (10, 25, 50, 100 cm³).
- Disadvantage: Low precision; not suitable for analytical work.
- Burette:
- Advantage: Very accurate (measures to nearest 0.05 cm³); allows for variable volumes.
- Disadvantage: Slow to set up; requires careful reading of the meniscus.
- Pipette:
- Advantage: Extremely accurate for one specific fixed volume (e.g., exactly 25.0 cm³).
- Disadvantage: Can only measure one fixed volume.
- Gas Syringe:
- Advantage: Measures gas volume directly; no risk of gas dissolving in water.
- Disadvantage: Can leak if the plunger is dry; expensive and fragile.
Dissolving and Equations
When a solute like Sodium Chloride dissolves in water, it changes state.
Word Equation: Sodium chloride (solid) + water (liquid) → Sodium chloride solution (aqueous)
Symbol Equation: $$NaCl(s) \xrightarrow{H_2O(l)} NaCl(aq)$$
Separation Processes: Residue and Filtrate
In a filtration experiment, such as separating sand from a salt solution:
- The mixture is poured through filter paper.
- The residue (sand) stays on the paper.
- The filtrate (salt solution) passes through.
Extended Content (Extended Only)
There is no specific "Extended" content for this sub-topic (12.1) according to the current syllabus objectives.
Key Equations
While this topic is qualitative, the following relationship is often used when discussing saturated solutions and concentrations:
Concentration ($g/dm^3$): $$\text{Concentration} = \frac{\text{Mass of solute (g)}}{\text{Volume of solution (}dm^3\text{)}}$$
Solubility (g/100g of solvent): $$\text{Solubility} = \frac{\text{Mass of solute (g)}}{\text{Mass of solvent (g)}} \times 100$$
Common Mistakes to Avoid
- ❌ Wrong: Reading the volume of a liquid from the top of the curve (meniscus).
- ✅ Right: Always read the volume from the bottom of the meniscus at eye level.
- ❌ Wrong: Calling the liquid that goes through the filter "the water."
- ✅ Right: Use the technical term filtrate, as it may be a solution containing dissolved solutes.
- ❌ Wrong: Forgetting to mention temperature when defining a saturated solution.
- ✅ Right: A saturated solution is only "saturated" at a specific temperature, as solubility increases with temperature.
Exam Tips
- Command Words: If an exam question asks you to "Suggest" an apparatus, think about the volume mentioned. If the volume is 25.0 cm³, suggest a pipette or burette. If it is "approximately 50 cm³", a measuring cylinder is sufficient.
- Real-world Context: You may be asked how to prove a solution is saturated.
- Answer: Add a small amount of solute; if it does not dissolve and sinks to the bottom after stirring, the solution is saturated.
- Units: Always check your units. $1\text{ }dm^3 = 1000\text{ }cm^3$. Conversion is often required in experimental design questions involving concentration.
- Gas Collection: If the gas being collected is soluble in water (like ammonia, $NH_3$), you cannot use an inverted measuring cylinder over water; you must use a gas syringe.