Table of Contents
From Idea to Experiment: The Overall Workflow
Designing and carrying out a chemical experiment is more than “mixing stuff in a flask.” It follows a structured cycle:
- Planning and preparation
- Safe and controlled execution
- Recording and evaluating results
- Drawing conclusions and improving the experiment
This chapter focuses on how chemists practically think and work in this cycle, not on the detailed theory behind the phenomena they investigate.
1. Planning and Preparation
1.1 From Question to Experimental Plan
Experiments are driven by a clear goal. Typical starting points are:
- Exploratory questions: “What happens if…?”
e.g. “What gas is formed when metal X reacts with acid Y?” - Testing a hypothesis:
“If temperature increases, then the reaction rate doubles.” - Measuring a quantity:
“What is the concentration of substance Z in this solution?”
From such a question, chemists formulate:
- A precise experimental question
(e.g. “How does doubling the acid concentration affect the rate of gas formation?”) - A hypothesis (a testable prediction)
- A plan for conditions and measurements needed to test it
The plan answers:
- What substances, equipment, and conditions are needed?
- What will be changed (independent variables)?
- What will be measured (dependent variables)?
- What will be kept constant (controlled variables)?
1.2 Choosing and Preparing Chemicals
Important aspects before starting:
- Purity and composition
Is a pure substance, a mixture, or a standard solution required? - Quantities
Are the planned amounts realistic and safe? Will there be enough material to measure reliably? - Storage conditions
Some chemicals require cool, dry, dark, or inert (e.g. nitrogen) storage. - Labelling
Every container must be labelled with at least: - Name (or formula) of the substance
- Concentration (for solutions)
- Hazards (symbols or text)
- Date and initials (in a teaching or research lab)
Preparation often includes:
- Weighing solids with a balance
- Measuring volumes with cylinders, pipettes, burettes, or volumetric flasks
- Preparing solutions of known concentration (details of molar quantities are handled elsewhere)
Accuracy in these steps directly affects the reliability of the experiment.
1.3 Selecting Equipment and Apparatus Setup
The apparatus must match the goal:
- Open vessel for observing gas evolution or simple mixing
- Closed but vented system for controlling gas or preventing contamination
- Heating or cooling equipment to maintain a certain temperature
- Stirring or shaking to ensure homogeneity
- Measuring devices (thermometers, pH meters, timers, balances, sensors)
Key points:
- Choose glassware appropriate to volume and precision (e.g. beakers vs. volumetric flasks).
- Use joints, clamps, and stands correctly to avoid leaks and accidents.
- Before starting, perform a “dry run” (without chemicals) to check:
- Stability of the setup
- Tightness of connections
- Accessibility of valves, switches, and emergency shut-off
1.4 Planning for Safety
Safety is integrated into preparation, not added at the end.
1.4.1 Identifying Hazards
For each chemical and operation, chemists check:
- Chemical hazards
Corrosive, toxic, flammable, explosive, oxidizing, environmentally hazardous, etc. - Physical hazards
Heat, pressure, glass breakage, mechanical risks. - Procedural hazards
Risky steps like adding water to concentrated acid, gas evolution in closed vessels, etc.
Information sources include:
- Labels and pictograms
- Safety Data Sheets (SDS)
- Lab safety manuals
1.4.2 Risk Assessment and Protective Measures
Chemists estimate:
- Probability of something going wrong
- Severity of possible harm
Then they choose protective measures:
- Technical measures
Fume hood, splash guards, proper ventilation, shields. - Organizational measures
Clear instructions, supervision, permission to perform certain tasks, emergency procedures. - Personal protective equipment (PPE)
Lab coat, safety goggles, gloves, closed shoes, sometimes face shield or special gloves.
1.4.3 Planning Waste Disposal
Before starting, one must know:
- Which wastes will be generated (acids, bases, organic solvents, heavy metals, etc.)
- Where each waste type is collected (separate containers)
- Which wastes can be neutralized or diluted safely at the bench (only with permission and guidance)
- Which must be stored and disposed of as hazardous waste
Planning this ahead avoids unsafe improvisation later.
2. Controlled Execution of Experiments
2.1 Following a Written Procedure
Chemists work with:
- Standard Operating Procedures (SOPs) for routine tasks
- Experimental protocols for specific experiments
These include:
- Step-by-step instructions
- Amounts, temperatures, durations
- Safety notes
- How and when to record data
During execution, any deviations must be noted, because even small changes can affect the outcome.
2.2 Handling Chemicals Safely
Some basic working habits:
- Never pipette by mouth; always use pipette aids.
- Add acid to water, not water to acid (to avoid violent splashing).
- Work with volatile, toxic, or smelly substances in a fume hood.
- Keep bottles closed when not in use.
- Never return excess chemicals to the original container (to prevent contamination).
Spills and accidents are handled according to prepared emergency plans (e.g. rinse with water, use spill kits, notify responsible personnel).
2.3 Managing Experimental Conditions
Chemists must keep conditions controlled so results can be interpreted.
Typical controlled parameters:
- Temperature: maintained with hotplates, water baths, ice baths, thermostats
- Pressure: normally atmospheric, but sometimes higher or lower; important for gases
- Concentration: controlled by careful mixing and measuring
- Volume: especially important in titrations and dilution series
- Time: exact timing of reaction start, sampling, and observation
Recording these conditions is as important as recording the observed result.
2.4 Making Observations and Measurements
Two broad types of observations occur:
2.4.1 Qualitative Observations
These describe what happens:
- Change in color
- Formation of precipitate
- Gas evolved (bubbles)
- Change in smell (only if allowed and safe)
- Change in clarity or viscosity
- Light emission (chemiluminescence) or heat generation (exothermic)
Qualitative observations:
- Are recorded immediately in the lab notebook
- Use objective language where possible (e.g. “blue-green precipitate” rather than “pretty color”)
- Include time information (“after 30 seconds, solution turned opaque”)
2.4.2 Quantitative Measurements
These are numerical values taken with an instrument:
- Mass (balance)
- Volume (pipette, burette, cylinder)
- Temperature (thermometer, sensor)
- pH (pH meter)
- Conductivity, absorbance, potential (in electrochemical or spectroscopic setups), etc.
Key practices:
- Always note units (e.g. g, mL, °C, s)
- Understand the precision and limits of instruments (e.g. ±0.01 g)
- Take repeated measurements where needed to estimate consistency
2.5 Keeping a Lab Notebook
The lab notebook is a permanent, chronological record. It typically contains:
- Date, experiment title, and purpose
- Outline of the procedure or reference to a protocol
- Exact quantities of substances and details of equipment used
- All observations and measurements (including “nothing happened”)
- Problems encountered, deviations from the plan, and corrective actions
- Preliminary calculations and thoughts
Important principles:
- Write entries in ink, not pencil.
- Do not erase mistakes—cross them out clearly and write corrections.
- Keep all data, even if it seems “bad” or “useless” at the time.
This record is essential for later evaluation, repetition, and verification by others.
3. Evaluation of Experimental Results
3.1 Organizing and Processing Data
After the experiment, raw notes are turned into an analyzable form:
- Tables for numerical data
- Graphs (plots) to show relationships, such as:
- Quantity vs. time
- Quantity vs. concentration
- Quantity vs. temperature
Basic processing steps can include:
- Calculating averages of repeated measurements
- Determining differences, ratios, or percentages
- Performing simple error estimates or ranges when appropriate
All processing steps must be traceable from raw data to final result.
3.2 Comparing with Expectations
Evaluation always refers back to the original question and hypothesis:
- Did the reaction behave as expected?
- Did the measured trend match the predicted one (e.g. “faster at higher temperature”)?
- Were there unexpected side products or phenomena?
Chemists also compare with:
- Literature values (e.g. known melting points, densities, equilibrium constants)
- Previous experiments (their own or from others)
Discrepancies prompt further questions.
3.3 Considering Errors and Uncertainty
Results are never perfectly exact. Chemists distinguish:
- Random errors: unavoidable small fluctuations (e.g. minor variations in reading a scale)
- Systematic errors: consistent bias (e.g. miscalibrated instrument, wrongly prepared solution)
Evaluation includes:
- Judging how large the uncertainty might be compared to the measured effect
- Asking whether the difference between experiment and theory falls within that uncertainty
- Identifying possible sources of error in:
- Measurement (instrument or reading)
- Execution (imprecise timing, incomplete mixing)
- Preparation (incorrect concentration, contamination)
The goal is not to “blame” someone, but to understand reliability.
3.4 Drawing Conclusions
From evaluation, chemists derive:
- Conclusions about the hypothesis:
- Supported by data
- Not supported
- Inconclusive (data insufficient, errors too large)
- Implications for theory or models (within the limited scope of the experiment)
- Practical conclusions:
- Better procedure for future experiments
- Safer or more efficient process
- Possible applications
Conclusions must be consistent with the data and clearly separated from speculation.
4. Improving and Repeating Experiments
4.1 Refining the Procedure
Often, the first run serves as a trial. Evaluation suggests changes:
- Clarifying vague steps (“stir for 5 minutes at 400 rpm” instead of “stir briefly”)
- Using more precise equipment (e.g. a volumetric pipette instead of a measuring cylinder)
- Adjusting time or temperature to capture more informative data
- Modifying sample size to reduce relative measurement error
4.2 Reproducibility
An experiment is only useful if it is:
- Repeatable by the same person under the same conditions
- Reproducible by others in other labs using the same description
To test this, chemists:
- Repeat experiments on different days or with new batches of reagents
- Compare results across group members or research groups
Good documentation and clear protocols are essential for reproducibility.
4.3 Communication of Results
Once experiments are evaluated, results are communicated in various formats:
- Short lab reports in teaching labs
- Internal reports in industry
- Scientific publications, posters, and presentations in research
These always include:
- Purpose and background (brief)
- Methods (enough detail to reproduce)
- Results (data, tables, figures)
- Discussion (interpretation, errors, comparison with literature)
- Conclusion and outlook (what could be done next)
Thus, the cycle from planning to evaluation ends in a form that allows others to continue the work.
5. The Role of Experiments in Chemistry
Within chemistry as a whole, experiments:
- Test and refine laws, models, and theories (covered elsewhere)
- Provide quantitative data for calculations and predictions
- Serve as a basis for technological applications and process development
- Are essential in education, where learners practice scientific thinking and techniques
The preparation, execution, and evaluation of experiments embody the practical side of chemical thinking: careful planning, controlled action, critical analysis, and continuous improvement.