Reasoned Explanations

A reasoned explanation combines three things: the relevant results, scientific knowledge that explains them, and a conclusion about how far they support the hypothesis. This is more than describing a trend. A description tells us what happens; an explanation tells us why that pattern makes scientific sense.

Work out what the hypothesis predicts

Consider the hypothesis: liquids containing longer or larger molecules are more viscous. Viscosity is resistance to flowing, so a more viscous liquid flows less freely.

If this hypothesis is correct, alcohols with longer carbon chains should flow more slowly under comparable conditions. Notice the distinction: the hypothesis predicts an increase in viscosity, but an investigation measuring flow rate should find a decrease in flow rate.

Use results as evidence

In an investigation comparing alcohols, ethanol, with two carbon atoms per molecule, had a flow rate of 42.3 mm/s. Octanol, with eight carbon atoms per molecule, had a flow rate of 3.8 mm/s.

Octanol flowed much more slowly than ethanol. This comparison is consistent with the prediction that the longer-chain alcohol is more viscous. Quoting these values makes the link to the hypothesis clear: it shows which substances were compared, what was measured and how the measurements differed.

A reasoned conclusion should also consider the overall pattern, rather than selecting only two results that fit. If some results do not match the prediction, they must be acknowledged when judging how well the hypothesis accounts for the data.

Explain the pattern using science

There are attractive intermolecular forces between alcohol molecules. As the carbon chain becomes longer, the overall attractions between the chains become greater. These attractions make it harder for molecules to move past one another, so the liquid resists flowing more strongly.

The reasoning therefore forms a connected chain: longer carbon chains → greater intermolecular attractions → greater resistance to flow → lower flow rate.

This gives a scientific reason for the observed difference. Simply saying “octanol flows more slowly because it is more viscous” identifies the property, but does not explain the molecular reason for it.

Match the conclusion to the evidence

The alcohol investigation supports the hypothesis for the alcohols tested. It does not establish that the same relationship applies to every liquid or to every possible molecular size. Testing alcohols with two to eight carbon atoms gives evidence within that range, not beyond it.

A suitably bounded conclusion would be: “The results support the hypothesis for the alcohols tested, because longer-chain alcohols flowed more slowly, consistent with greater intermolecular attractions and higher viscosity.”

If a broad hypothesis accounts for only part of the evidence, it may need to be narrowed or modified. Restricting the claim to the materials and conditions supported by the results is more justified than declaring it universally true. Scientific understanding develops through this cycle of proposing explanations, testing them and reviewing them in the light of evidence.