A good comparison says what was observed, what that shows, and which element ranks higher. Each statement should point to a row of the data.
This lesson is part of the periodic table section. Pair it with explaining trends using atomic structure, which gives the reasons behind the order.
How do you write a conclusion from data?
Use three parts: claim, evidence, reason. The claim is the ranking, the evidence is the observation, and the reason links the observation to reactivity.
Keep each part to one sentence. A conclusion without evidence is an opinion, and evidence without a claim leaves the reader to guess.
Worked example: three group 1 metals in water
A student places small pieces of three metals in water. The table is an original dataset.
| Metal | Observation |
|---|---|
| P | Floats, fizzes slowly, no flame |
| Q | Melts into a ball, moves quickly, hydrogen burns with a flame |
| R | Floats, fizzes at a moderate speed, no flame |
The order of reactivity is Q, then R, then P. Q gives a flame and melts, which shows the most heat released. P fizzes slowest, so it is the least reactive.
The conclusion would read: “Q is the most reactive because it melts and the hydrogen ignites, showing that it reacts fastest and releases the most heat.”
Worked example: halogen displacement
A student adds each halogen solution to solutions of the potassium halides. A tick means a colour change occurred, and n/a means the same halogen. This is an original dataset.
| KCl | KBr | KI | |
|---|---|---|---|
| Cl₂ | n/a | ✓ | ✓ |
| Br₂ | no change | n/a | ✓ |
| I₂ | no change | no change | n/a |
Chlorine displaces bromide and iodide, bromine displaces only iodide, and iodine displaces neither. So the order is chlorine, then bromine, then iodine, from most to least reactive. One example equation is Cl₂ + 2KBr → 2KCl + Br₂.
The mistake that costs marks
The slip is to write the conclusion from memory and ignore the table, or to reverse the order for the halogens. Reactivity rises down group 1 but falls down group 17, and the two directions are easy to swap.
| Wrong approach | Why it loses marks | Better approach |
|---|---|---|
| “Reactivity increases down group 17” | Reverses the evidence | Read the displacement results |
| “Q is more reactive” with no observation | No evidence | Cite melting and the flame |
| “Iodine is most reactive, it gives the darkest colour” | Confuses colour with reactivity | Use what is displaced |
The claim evidence revision desk lets you practise rewriting a weak conclusion.
Check yourself
Bromine water is added to potassium chloride, and no change occurs. Chlorine water is added to potassium bromide, and the solution turns orange-brown. What do the two results tell you about chlorine and bromine?
Answer
Chlorine displaced bromine from potassium bromide, while bromine did not displace chlorine from potassium chloride.
Chlorine is more reactive than bromine. The orange-brown colour is evidence that bromine was released: Cl₂ + 2KBr → 2KCl + Br₂.
What to study next
Move on to predicting simple reactions from periodic position, or mix all four lessons in the periodic table practice set.
If you want a teacher to check your conclusions against the data, see online one-to-one Chemistry tuition.