Chemistry (0620) | Experimental design

Welcome to Experimental Design!

Hi future chemists! This chapter is all about understanding how to carry out experiments properly and safely. Even the best theories need to be tested, and knowing your way around the laboratory techniques is essential for success in chemistry, especially for your Practical Assessment (Paper 5 or 6).

We will look at the tools you use, the vocabulary for mixing chemicals, and a vital technique called titration. Let's make sure you know exactly how to design and describe an effective experiment!

12.1 Experimental Design: Apparatus and Terminology

1. The Chemist's Toolkit: Measuring Apparatus

In experimental design, choosing the right tool is the first step. Different apparatus are used depending on how accurate a measurement needs to be.

Key Apparatus and Their Uses (Syllabus 12.1 Core 1)

Here is a quick breakdown of the apparatus you must be familiar with:

• Time: Stop-watches. Used to measure the duration of reactions (e.g., rate of reaction experiments). Measurements are typically taken to an accuracy of 1 second (1 s).
• Temperature: Thermometers. Used to measure heat changes (e.g., in exothermic or endothermic reactions). Usually calibrated with 1 °C graduations.
• Mass: Balances (or electronic top-pan balances). Used to measure mass, usually with a precision of at least 0.1 g.
• Volume (Accurate/Precise Volume Transfer):
  • Burettes: Long glass tubes with a tap (stopcock) at the bottom. Used to dispense *variable* volumes of liquid accurately, usually up to 50 cm³. Essential for titrations.
  • Volumetric Pipettes: Used to measure and transfer a single, fixed volume (e.g., 25.0 cm³) extremely accurately. They measure "volume by transfer."
• Volume (Approximate Volume Measurement):
  • Measuring Cylinders: General purpose apparatus for measuring liquid volumes (e.g., 10 cm³, 25 cm³, 100 cm³). They are quick but less accurate than burettes or pipettes.
  • Gas Syringes: Used to collect and measure the volume of gases produced in a reaction.

2. Advantages and Disadvantages of Methods (Syllabus 12.1 Core 2)

In an exam, you might be asked to compare methods. There's often a trade-off between speed and accuracy.

• Pipette vs. Measuring Cylinder:
  Advantage of Pipette: Much higher accuracy and precision for fixed volumes.
  Disadvantage of Pipette: Slower to use and clean.
• Digital Balance vs. Measuring Mass by Gas Loss:
  Measuring mass loss is often simpler (weigh flask, start reaction, reweigh later).
  Measuring gas collected (using a syringe) gives a more instant rate reading, but gas might escape before the syringe is connected, leading to lower accuracy.

Key Takeaway for Apparatus: Always select the apparatus that gives the highest precision needed for the part of the experiment being conducted.

3. Understanding Solutions (Syllabus 12.1 Core 3)

When you mix chemicals, you often create a solution. It is crucial to use the correct terminology when describing these mixtures.

Definitions Explained

• Solvent: The substance that does the dissolving. It is usually the liquid present in the largest amount.
  (Example: In salty water, the water is the solvent.)
• Solute: The substance that is dissolved. It is typically the solid (or gas/liquid) that is added to the solvent.
  (Example: In salty water, the salt is the solute.)
• Solution: The homogeneous mixture formed when a solute dissolves in a solvent.
  (Example: Salty water is the solution.)

Saturated Solutions – The Maximum Mix

A saturated solution is a solution containing the maximum concentration of solute dissolved in the solvent at a specific temperature.

Analogy: Imagine a small school bus (the solvent). You are trying to load students (the solute). When every seat is filled and no more students can squeeze on, the bus is saturated. If you add more solute to a saturated solution, it just settles at the bottom (it doesn't dissolve).

Results of Separation Processes

When you separate a mixture using techniques like filtration or evaporation, you get specific products:

• Filtrate: The liquid or solution that successfully passes through a filter paper during filtration.
• Residue: The substance that remains behind after a process like filtration (the solid caught on the filter paper) or evaporation (the solid remaining when the solvent boils away).

Key Takeaway: Solutions are mixtures. A saturated solution holds as much solute as possible at that temperature.

12.2 Acid-Base Titrations

1. What is an Acid-Base Titration?

A titration is a precise analytical technique used to determine the exact concentration of a solution (often an acid or a base) by reacting it with a solution of known concentration. This is also called volumetric analysis because it involves precise measurement of volumes.

The goal is to find the point where the acid and base have exactly neutralized each other – this is called the endpoint.

2. Apparatus Used in Titrations (Syllabus 12.2 Core 1)

Titrations require high precision, so specific apparatus must be used:

1. Burette: Holds the solution of known concentration (the titrant). Used to dispense variable volumes accurately, allowing you to control the addition drop by drop.
2. Volumetric Pipette: Used to measure and transfer a fixed, accurate volume of the solution of unknown concentration into the conical flask.
3. Conical Flask: Used to hold the measured unknown volume and the indicator. Its shape allows the contents to be safely swirled without splashing.
4. Suitable Indicator: A chemical added in small amounts that changes colour sharply at the endpoint (when neutralization is complete). (E.g., Phenolphthalein or Methyl Orange).

Step-by-Step Titration Procedure

Don't worry if this seems like a lot of steps! If you follow them carefully, you will get accurate results:

Step 1: Preparation

• Rinse the burette with the known solution (the titrant). Fill it and ensure the tip is filled, then record the initial reading accurately (to 2 decimal places if the scale allows half-scale divisions).
• Rinse the pipette with the unknown solution. Use the pipette to transfer an accurate volume (e.g., 25.0 cm³) of the unknown solution into the clean conical flask.
• Add a few drops of the suitable indicator (e.g., methyl orange) to the flask.

Step 2: The Rough Titration (Trial Run)

• Run the known solution from the burette into the flask, swirling continuously.
• Stop when the indicator changes colour permanently. This is the endpoint.
• This first titration is a "rough" reading to estimate the volume needed.

Step 3: Accurate Titrations (Repeats)

• Repeat the procedure, but this time, slow down the addition of the known solution as you approach the rough volume obtained.
• Near the endpoint, add the solution drop by drop.
• Record the final reading of the burette when the permanent colour change occurs.

Step 4: Calculation

• The volume added is the titre (Final Reading – Initial Reading).
• Repeat the accurate titrations until you get two or three titres that are concordant (within 0.10 cm³ of each other).
• Calculate the average of the concordant titres.

3. Identifying the Endpoint (Syllabus 12.2 Core 2)

The endpoint is where the indicator changes colour, showing that the reaction has reached completion (neutralisation).

• Choosing the Indicator: The indicator must change colour sharply when the reactants are stoichiometric (have reacted in their exact required mole ratio).
• Visualising the Endpoint: Place the conical flask on a white tile. This makes it easier to spot the sudden and permanent colour change as the final drops of titrant are added.
• Example: If titrating a strong acid into a base using methyl orange, the solution changes from yellow (in base) to orange/red (neutral/acidic). You stop at the first persistent orange/red colour.

Key Takeaway for Titrations: Titrations use precise volume measurement (burettes and pipettes) with an indicator to find the exact point of neutralisation, allowing us to calculate unknown concentrations.