AP Chemistry Practice 2 - Question and Method is the science practice where you design and evaluate experiments. You use it to turn an observation into a testable question, predict what an experiment will show, pick procedures that actually answer that question, read data from lab setups, and identify or explain sources of error.

Think of this practice as the lab-design brain of the course. It shows up in any unit that involves data collection, titrations, chromatography, gas collection, calorimetry, or rate measurements.

This practice carries an exam weighting of about 8 to 12 percent on the multiple-choice section, and it appears on free-response questions too.

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What Practice 2 - Question and Method Means

Practice 2 covers the full arc of experimental thinking: ask, predict, plan, measure, and critique. The grouping description is simple to remember: determine scientific questions and methods.

You are not just recalling facts here. You are deciding whether an experiment is set up correctly and whether the data it produces can support a conclusion.

The six subskills break down like this:

2.A Identify a testable scientific question from an observation, data, or a model.
2.B Formulate a hypothesis or predict the results of an experiment.
2.C Identify experimental procedures that match a scientific question, which may include a sketch of a lab setup.
2.D Make observations or collect data from representations of lab setups or results, attending to precision where appropriate.
2.E Identify or describe potential sources of experimental error.
2.F Explain how modifications to a procedure will alter results.

What This Practice Requires

To use Practice 2 well, you need to connect a goal to a method. Every part of an experiment should point back to the question being asked.

Here is what each subskill asks you to do in practice:

Testable questions (2.A): A good question can be answered by measuring something. "Does increasing temperature increase reaction rate?" is testable. "Why is chemistry hard?" is not.
Predictions and hypotheses (2.B): State what you expect to happen and why, based on a chemical principle. A prediction should be specific, like "the rate will increase" rather than "something will change."
Aligned procedures (2.C): Choose the setup and glassware that produce the data you need. If you want moles of gas, you need a way to measure gas volume precisely.
Observations and precision (2.D): Read values from buret diagrams, chromatograms, graphs, and tables. Record the right number of significant figures for the instrument.
Sources of error (2.E): Identify what could make results inaccurate, like a leaking gas collection setup or an incompletely dried solid.
Modifications (2.F): Explain how a change in the procedure changes the result. If you halve the concentration, what happens to the data?

Skills You Need for This Practice

Tell the difference between accuracy and precision, and choose glassware accordingly. A buret or eudiometer gives precise volume readings; a beaker does not.
Match the measurement tool to the variable. Gas amount needs a gas-collection method. Concentration of a colored species can use a spectrophotometer.
Read significant figures from an instrument. A buret marked to 0.01 mL should be read and recorded that way.
Separate controlled variables from the variable you are testing. Change one thing at a time.
Reason about cause and effect. If you change a quantity, trace how that flows through the calculation to the final result.
Spot error sources that bias results in a specific direction, not just "human error."

How It Shows Up on the AP Exam

On the multiple-choice section, Practice 2 questions often give you a lab scenario and ask which procedure best answers the question, what conclusion the data supports, or how a change affects the outcome.

For example, one question describes a student weighing a piece of Mg(s), placing it in excess 1 M HCl, and wanting the number of moles of gas produced. The best procedure is collecting the gas in a eudiometer tube and measuring its volume. A graduated cylinder or open flask would not capture the gas accurately, which is a 2.C decision about aligning method to question.

Another question shows a paper chromatography result and asks what can be concluded about a dye. The correct read is that the dye has a weaker attraction for the stationary phase than for the mobile phase, since it traveled far up the paper. That is 2.D, collecting an observation from a results representation.

On the free-response section, all six subskills can appear. You may be asked to write a testable question, predict a result, design a procedure, record measurements, or describe how an error would shift the data. Practical tip: when an FRQ asks for error, name a specific error and state the direction it pushes the result.

Examples Across the Course

Practice 2 connects to many units. Here are varied examples so the skill does not feel tied to one topic:

Unit 3, Separation and Chromatography (2.D): Read a chromatogram to compare how far each component traveled, then infer relative attraction to the stationary versus mobile phase.
Unit 4, Titration and Gas Collection (2.C): Choose a eudiometer to measure moles of gas from a metal-acid reaction, or use a buret and indicator to find the equivalence point in an antacid titration. Precise glassware is the key choice.
Unit 5, Kinetics (2.F): Compare reaction of a single chunk of CaCO3 with small pieces of the same mass. Predict that smaller pieces give a larger surface area and a faster reaction, so that curve reaches completion sooner. This is reasoning about how a modification changes results.
Unit 6, Calorimetry (2.E): Identify heat lost to the surroundings as an error source that makes a measured temperature change smaller than the true value, which lowers the calculated heat.
Unit 8, Acid-Base Titrations (2.A and 2.B): Form a testable question about an unknown acid concentration and predict the volume of titrant needed to reach the equivalence point based on stoichiometry.

How to Practice Practice 2 - Question and Method

For each lab you do or read about, write the question it answers in one sentence. If you cannot, the design may be unclear.
Practice choosing glassware. Ask yourself whether the measurement needs precision, like a buret or eudiometer, or just rough volume.
Take any procedure and change one variable on paper. Trace how that change moves through to the final calculated value. This builds 2.F reasoning.
When you see error questions, force yourself to name a specific cause and the direction of its effect, such as "a wet solid adds mass, so the calculated percent is too high."
Read graphs and tables by writing down the values with correct significant figures before answering.
Turn a phenomenon into a hypothesis. State what you expect and tie it to one chemical principle.

Common Mistakes

Writing a question that is not testable, like asking "why" instead of "how does changing X affect Y."
Picking glassware that is too imprecise for the goal, such as measuring exact gas volume in an open flask.
Saying "human error" without naming a specific, direction-based error. Graders want the cause and its effect on the data.
Predicting a vague outcome. "It changes" is weaker than "the rate increases because more collisions occur per second."
Recording data with the wrong number of significant figures for the instrument.
Confusing accuracy and precision. A consistent but biased reading is precise, not accurate.
Forgetting to control other variables when testing one factor.

Quick Review

Practice 2 is about designing and evaluating experiments: ask, predict, plan, measure, and critique.
2.A testable question, 2.B hypothesis or prediction, 2.C aligned procedure, 2.D observation and precision, 2.E error sources, 2.F effect of modifications.
Match the measurement tool to the variable, and choose precise glassware when the question demands it.
For error, name a specific source and state how it shifts the result.
For modifications, trace the change through the calculation to the final value.
This practice spans chromatography, titration, gas collection, kinetics, and calorimetry, so it appears across many units.