9.2 - Recording Results in a Table
What a table question looks like
- Questions often ask you to design a table to record readings, or to complete one that's partly filled in.
- What's judged is the structure of the table, not the chemistry.
Key features looked for
- Draw it as one table with enough columns and rows.
- Head each column with the quantity and its unit in brackets, written as "quantity (unit)".
- Record every reading in the right place, to a consistent precision.
- Include space for any required calculated values, such as a mean.
A typical question
A student is going to measure the volume of gas produced when calcium reacts with water, recording a reading every minute for five minutes. Draw a table the student could use to record the results.
Setting out the table
- A results table gives every reading a place, laid out so the pattern can be read straight down the columns.
- Put the variable you changed in the first column, with its values going down in order.
- Give the variable you measured its own column - or one column per repeat if you're taking more than one reading at each value.
Worked example
A student measured the volume of hydrogen gas produced when calcium reacts with water, taking a reading every minute.
Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)
| Time (min) | Volume of gas produced (cm3) |
|---|---|
| 0 | |
| 1 | |
| 2 | |
| 3 | |
| 4 | |
| 5 |
- Time (the variable changed) goes down the first column in even steps.
- Volume of gas (the variable measured) has its own column, ready to fill in as readings come.
Headings and units
- Write each column heading as the quantity followed by its unit in brackets: Time (min), Volume of gas produced (cm3), Temperature (°C), Mass of material (g).
- The unit goes in the heading once, so the numbers underneath are written as plain numbers.
- Any calculated column (such as concentration) also needs its own heading with its own unit.
Repeats and the mean
Where an experiment is repeated, the table needs a column for each repeat and a final column for the mean.
Worked example - A titration
A student titrated a sodium hydroxide solution against a standard oxalic acid solution three times.
| 1st titre | 2nd titre | 3rd titre | |
|---|---|---|---|
| Volume used (cm3) | 16.4 | 15.5 | 15.7 |
- The first titre is a rough reading to locate the end-point, so it's left out of the mean.
- Titre volumes within 0.2 cm3 of each other are concordant, and only concordant readings are averaged - here the 2nd and 3rd titres.
When not to discard a reading
- A reading is only excluded when it's clearly anomalous, or (as in a titration) when it falls outside the concordant range - not simply because it differs slightly.
- For example, if three temperature changes are recorded as 23 °C, 20 °C, and 20 °C, none is far enough from the others to be ruled anomalous, so all three are used:
Precision
- Keep the precision the same all the way down a column, matching the resolution of the instrument.
- A burette read to the nearest 0.1 cm3 is recorded to one decimal place throughout (e.g. 15.0, not 15).
Common exam questions
Question 1
How should the heading of a column recording temperature in degrees Celsius be written?
As "Temperature (°C)" - the quantity, then the unit in brackets - so the values below can be written as plain numbers.
Question 2
A student writes "0 cm3, 32 cm3, 40 cm3" down a column of gas volumes. Why might this lose credit?
Because the unit should appear once, in the heading, not beside every value; repeating the unit in the body counts as the unit not being in the heading.
Question 3
A titration gives titres of 24.6 cm3, 23.9 cm3, and 24.0 cm3. Explain why only the second and third titres are used to calculate the average.
Because they are concordant, within 0.2 cm3 of each other, while the first titre is a rough result taken to locate the end-point and is far enough from the other two to be left out.
Question 4
A student calculates a column of concentrations from measured masses and volumes but leaves the new column with no unit. Why does this matter?
Because a column you calculated needs a heading with its unit just as a measured one does; calculated values are still readings in the table.
Question 5
An experiment measures the volume of gas produced by a reaction once every minute for five minutes. How many columns does the table need, and what goes in the first one?
Two columns: time in the first, going down in minutes from 0 to 5, and the volume of gas produced in the second.
Question 6
A student's three temperature-change readings are 23 °C, 20 °C, and 20 °C, and none is ruled anomalous. What is the mean, and why are all three used?
21 °C, found from all three added and divided by three. None of the readings is far enough from the others to be excluded, unlike a titration's rough first titre, which is discarded because it falls outside the concordant range.