Experimental design

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.

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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.

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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:

1. The mixture is poured through filter paper.
2. The residue (sand) stays on the paper.
3. The filtrate (salt solution) passes through.

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Extended Content (Extended Only)

There is no specific "Extended" content for this sub-topic (12.1) according to the current syllabus objectives.

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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): $$\text{Solubility} = \frac{\text{Mass of solute (g)}}{\text{Mass of solvent (g)}} \times 100$$

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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.

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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.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

A student investigates the solubility of potassium nitrate in water. They add potassium nitrate to 50.0 cm$^3$ of water at 20 °C until no more dissolves. The mass of undissolved potassium nitrate remaining is then measured.

(a) Define the term solute. [1]

(b) State two pieces of apparatus the student would need to accurately measure the 50.0 cm$^3$ of water. [2]

(c) Suggest one way the student could ensure that all the potassium nitrate that can dissolve does dissolve, before measuring the undissolved mass. [2]

Worked Solution:

(a)

1. A solute is a substance that dissolves in a solvent.
   The solute is being dissolved, not doing the dissolving.

How to earn full marks:

• State that a solute dissolves, not the other way around

(b)

1. To accurately measure the volume, a burette or pipette should be used.
   These are much more precise than a measuring cylinder.
2. And a second piece of apparatus used with it: a pipette filler (for a pipette) or a stand and clamp to hold a burette vertical.

How to earn full marks:

• Give two distinct pieces of apparatus
• Do not include measuring cylinder

(c)

1. Do not heat the water: heating increases the amount of potassium nitrate that can dissolve, so the result would no longer be for 20 °C.
2. Stirring the mixture will help the potassium nitrate dissolve.
   Stirring increases the rate of dissolving.

How to earn full marks:

• State stirring (the mixture), keeping the water at 20 °C
• Explain that stirring increases the rate of dissolving; heating is not accepted, as it changes the solubility

Common Pitfall: Students often confuse "solute" and "solvent". Remember that the solute is the substance being dissolved, and the solvent is the substance doing the dissolving. Also, be specific when naming apparatus; a beaker alone isn't accurate for measuring volume.

Exam-Style Question 2 — Extended Response [10 marks]

Question:

A student wants to investigate how the rate of cooling of a beaker of hot water changes with the insulation around the beaker. They have access to the following materials:

• A beaker
• Hot water
• A thermometer
• A stopwatch
• Various insulating materials (e.g., cotton wool, bubble wrap, newspaper)

(a) Plan an experiment to investigate how the type of insulation affects the rate of cooling of hot water in a beaker. Your plan should include:

• A description of the procedure, including how you will control variables.
• A list of the measurements you will take.
• How you will process your results to draw a conclusion. [7]

(b) Suggest one safety precaution that should be taken during this experiment. [1]

(c) State one advantage of using a digital thermometer compared to a traditional liquid-in-glass thermometer. [1]

(d) Sketch a graph to show how the temperature of the water changes with time, assuming the surrounding temperature is lower than the initial water temperature. [1]

Worked Solution:

(a)

1. Add a fixed volume of hot water to the beaker. Use the same volume each time to keep the surface area consistent.
   $\boxed{\text{Use a measuring cylinder to add 200 cm}^3 \text{ of hot water to the beaker.}}$
2. $\boxed{\text{Use a thermometer to record the initial temperature.}}$
3. $\boxed{\text{Wrap the beaker with one layer of cotton wool and secure it with tape.}}$
4. $\boxed{\text{Record the temperature every minute for 10 minutes using the thermometer and stopwatch.}}$
5. Repeat steps 1-4 using different insulating materials (e.g., bubble wrap, newspaper).
6. Control variables: use the same volume of water, the same initial temperature, the same type of beaker, and the same ambient temperature.
7. $\boxed{\text{Calculate the rate of cooling by dividing the temperature drop by 10 minutes.}}$
8. Compare the rates of cooling for each insulation material. The material with the lowest rate of cooling is the best insulator.

How to earn full marks:

• Correct procedure to measure the temperature change
• Mention of repeating the experiment with different insulating materials
• Controlling at least two key variables (volume, starting temperature, ambient temperature)
• Calculating the rate of cooling (or temperature change)
• Comparing the results to draw a conclusion about the effectiveness of the insulation

(b)

1. Use caution when handling hot water to avoid burns.
   Safety is paramount.
   $\boxed{\text{Wear safety goggles to protect eyes from splashes.}}$
   Appropriate safety precaution.

How to earn full marks:

• Mention wearing safety goggles or using caution with hot water

(c)

1. A digital thermometer provides a more precise reading.
   It usually reads to 0.1 °C and is easier to read, as there is no liquid level to judge. More precise does not always mean more accurate.

How to earn full marks:

• State that it is more precise (reads to smaller divisions) OR easier to read

(d)

1. Temperature decreases over time.

How to earn full marks:

• Show a decreasing curve that levels off

Common Pitfall: When planning experiments, students often forget to mention controlling variables. Be sure to explicitly state which variables you will keep constant to ensure a fair test. Also, remember to include specific volumes and time intervals in your procedure.

Exam-Style Question 3 — Short Answer [6 marks]

Question:

A student wants to separate a mixture of sand, salt, and iron filings.

(a) Describe the steps the student should take to separate the iron filings from the mixture. [2]

(b) Describe the steps the student should take to separate the sand from the salt. [4]

Worked Solution:

(a)

1. Use a magnet to attract the iron filings.
   Iron is magnetic.
2. The iron filings will stick to the magnet, leaving the sand and salt behind.

How to earn full marks:

• Mention using a magnet
• Mention that the iron filings are attracted to the magnet

(b)

1. Add water to the sand and salt mixture to dissolve the salt.
   Salt is soluble.
2. Stir the mixture to ensure the salt dissolves completely.
3. Filter the mixture. The sand will remain as the residue, and the salt water will pass through as the filtrate.
4. Evaporate the water from the filtrate to obtain the salt.

How to earn full marks:

• Dissolve the salt in water
• Filter the mixture to separate the sand
• Evaporate the water to recover the salt

Common Pitfall: Students sometimes miss the crucial step of dissolving the salt before filtration. Remember that filtration only works if one substance is dissolved and the other is not. Also, be sure to clearly identify the residue and filtrate in your description.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A scientist is studying the solubility of ammonium chloride in water at different temperatures. They measure the mass of ammonium chloride that dissolves in 100 g of water at various temperatures. The data is shown below:

Temperature (°C)	Mass of Ammonium Chloride Dissolved (g)
20	37
40	46
60	55
80	66

(a) Plot a graph of the mass of ammonium chloride dissolved (y-axis) against temperature (x-axis). Choose a suitable scale for each axis. [4]

(b) Use your graph to estimate the solubility of ammonium chloride at 50 °C. [1]

(c) Predict the mass of ammonium chloride that would dissolve in 200 g of water at 30 °C. [2]

(d) Explain why increasing the temperature generally increases the solubility of solid solutes like ammonium chloride in water. [2]

Worked Solution:

(a)

1. Axes are correctly labelled and scaled.
2. Points are plotted accurately.
3. Smooth curve of best fit is drawn.

How to earn full marks:

• Correctly labelled x and y axes with units (°C and g)
• Suitable scales chosen for both axes to use most of the graph paper
• All four points plotted correctly
• Smooth curve of best fit drawn through the points

(b)

1. Find 50 °C on the x-axis and read the corresponding value on the y-axis.
   $\boxed{\text{Approximately 50.5 g}}$

How to earn full marks:

• Correctly read the value from your graph at 50 °C (allow a reasonable range based on plotting accuracy)

(c)

1. From the graph, the solubility at 30 °C is approximately 41.5 g per 100 g of water.
2. Since we have 200 g of water, we double the mass of ammonium chloride that can dissolve.
   $\boxed{\text{83 g}}$

How to earn full marks:

• Correctly read the value from your graph at 30 °C (allow a reasonable range based on plotting accuracy)
• Multiply the value by 2 because the mass of water is doubled

(d)

1. Increasing the temperature increases the kinetic energy of the water molecules.
2. This allows the water molecules to overcome the attractive forces between the solute ions or molecules, allowing more solute to dissolve.

How to earn full marks:

• State that increasing temperature increases the kinetic energy of water molecules
• Explain that this allows the water molecules to overcome the attractive forces between the solute particles, allowing more solute to dissolve

Common Pitfall: When plotting graphs, students often choose scales that are difficult to work with or don't utilize the full graph paper. Make sure your scales are easy to read and allow you to plot the points accurately. Also, remember to extrapolate or interpolate from your graph carefully, showing your working if necessary.