IB Chemistry HLInquiry 2 — Collecting and Processing DataPaper 3 & IACore skill~13 min read
Interpreting Results
You have a processed table. Now you have to say what it means — and there are two separate jobs hiding in that. Describing the trend is the easy half. Explaining it with chemistry is where the marks are, and it is the half most students rush.
📘 What you need to know
A scientific graph needs a descriptive title, the independent variable on the x-axis, labelled axes with units, and a line or curve of best fit.
A line of best fit is not dot-to-dot. It is a single smooth line with roughly equal numbers of points either side.
Describe the trend using proper terms, then explain it using chemical theory. Both are needed.
The gradient, the intercept and the area under a curve can each carry real meaning.
Error bars show the spread of your repeats. If the best-fit line passes through all of them, the fit is good.
An anomaly gets circled, left out of the line of best fit, and explained — never deleted.
Accuracy, precision, reliability and validity mean four different things. Use them precisely.
Build the graph properly
Marks for graphing are almost free, and they are lost for the same handful of reasons every year: no title, no units, points joined dot-to-dot, or a scale that squashes all the data into one corner.
Notice the best-fit line passes through every error bar. That is your evidence that a straight line is a fair description of this data, rather than just the shape you were hoping for.
Describe, then explain
These are two separate sentences and you need both. Describing says what the graph does. Explaining says why the chemistry makes it do that.
Shape you see
How to describe it
What it often means
Straight line through the origin
Directly proportional
First order in that reactant; doubling it doubles the rate
Straight line not through the origin
Linear, with a positive intercept
Something contributes even at zero — often a background effect
Rising ever more steeply
Positive, non-linear, accelerating
An exponential relationship, such as rate against temperature
Rising then flattening
Rate increases then plateaus
A reactant is running out, or a surface is saturated
Falling curve towards zero
Inversely proportional
Time against concentration, before you convert it to a rate
A quick way to check you have done both jobs: highlight the word “because” in your paragraph. If it is not there, you have described the trend and stopped. The explanation always contains a because.
What the shape of the line tells you
The gradient
A gradient is a real quantity with real units — the units of the y-axis divided by the units of the x-axis. Work it out from two points on the line of best fit, spread as far apart as possible, not from two of your data points.
The intercept
The y-intercept is the value of your dependent variable when the independent variable is zero. Sometimes it should be zero and is not, and that gap is often a systematic error worth discussing.
The area under a curve
On some graphs the area means something — the area under a Maxwell–Boltzmann curve is a number of particles, and the area under a rate–time graph is an amount of product formed.
Error bars
Error bars turn your uncertainties into something visible. Draw them using either the instrument uncertainty or the spread of your repeats, and say which you used.
Long bars mean a lot of random scatter, so your repeats disagreed.
The line passing through every bar means your trend is a fair description of the data.
The line missing several bars means either you chose the wrong shape, or something systematic is going on.
Overlapping bars between neighbouring points mean you cannot claim those two points are really different.
Anomalies: circle, exclude, explain
A good justification ties the odd point to something specific that happened — a late start on the stopwatch, a bung fitted slowly, a splash lost from the flask.
Four words that mean four different things
Word
What it means
Affected by
How you show it
Accuracy
How close your result is to the true value
Systematic errors
Compare with a literature value
Precision
How close your repeats are to each other
Random errors
Small spread, short error bars
Reliability
Whether the same method gives the same answer again
Repeats and consistency
Concordant trials across the whole set
Validity
Whether the method actually answers the question
Controlled variables
Show it was a fair test
You can only comment on accuracy if you have something to compare against. No literature value means no accuracy claim — you can still discuss precision, reliability and validity.
WORKED EXAMPLE
Describing and explaining a trend
A graph of initial rate against HCl concentration gives a straight line passing through the origin, for concentrations from 0.50 to 2.50 mol dm–3 reacting with magnesium ribbon. Interpret it.
Step 1: Describe what the graph does
The initial rate increases linearly with concentration, and the line passes through the origin.
So rate is directly proportional to [HCl].Step 2: Say what that means chemicallyDoubling the concentration doubles the rate → first order in HCl.Step 3: Explain it with theory
Because a higher concentration means more acid particles in the same volume, collisions between H⁺ ions and the magnesium surface happen more often, so more successful collisions occur each second.
Step 4: Check the origin makes sense
At zero concentration there is no acid, so the rate must be zero. The line through the origin is consistent.
Description, order of reaction, mechanism, and a sanity check“the rate went up” on its own is worth almost nothing
WORKED EXAMPLE
Getting a value out of the gradient
On that graph the line of best fit passes through (0.50, 0.31) and (2.50, 1.55), with rate in cm3 s–1 and concentration in mol dm–3. Find the gradient and give its units.
Step 1: Pick two widely spaced points on the line
Not two data points — two points the line actually passes through.
Step 2: Change in y over change in x(1.55 − 0.31) ÷ (2.50 − 0.50) = 1.24 ÷ 2.00= 0.62Step 3: Work out the units
y-axis units divided by x-axis units:
cm³ s⁻¹ ÷ mol dm⁻³ = cm³ s⁻¹ dm³ mol⁻¹gradient = 0.62 cm³ s⁻¹ dm³ mol⁻¹a gradient with no units is only half an answer
WORKED EXAMPLE
Comparing with a literature value, and using an observation to explain the gap
A student measures the enthalpy change of neutralisation as −52.6 kJ mol–1. The accepted value is −57.3 kJ mol–1. They also noted that the outside of the polystyrene cup felt warm. Comment on the accuracy.
Step 1: Find the percentage error|−52.6 − (−57.3)| = 4.7 kJ mol⁻¹4.7 ÷ 57.3 × 100 = 8.2%Step 2: Compare it with your uncertainty
If the experimental uncertainty was only about 3%, the 8.2% gap is too big to be random.
→ something systematic is happening.Step 3: Use the observation as evidence
A warm cup means heat left the system, so the measured temperature rise was too small, so the calculated value is less exothermic than it should be.
Step 4: Check the direction agrees
Heat loss should make the answer less negative — and −52.6 is less negative than −57.3. Consistent.
8.2% out, systematic, and the direction matches heat losschecking the direction is what turns a guess into an argument
💡 Exam tip
Independent variable on the x-axis, every single time.
Use at least half the grid. A scale that squashes your points into one corner loses a mark on its own.
Take gradients from the line of best fit, using points far apart, and always give the units.
Say which uncertainty your error bars represent — instrument or spread of repeats.
Compare with a literature value wherever one exists, and quote a percentage error.
Then compare that percentage error with your total percentage uncertainty. If the error is much bigger, the problem is systematic — say so.
⚠ Common mix-up
Joining the points dot-to-dot. A best-fit line is one smooth line or curve.
Describing without explaining. “The rate increased” says nothing about chemistry.
Reading the gradient off two data points instead of off the line.
Using “accurate” to mean “precise”. Precise repeats can all be equally wrong.
Claiming accuracy with no literature value to compare against.
Calling a point anomalous without giving a cause. The cause is where the mark is.
Forgetting units on the gradient, or on a derived quantity in the processed table.
Up next: Inquiry 3 — Concluding and Evaluating — answering your research question directly, judging how good the answer is, and suggesting improvements that would actually work.
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