A student adds an alkali gradually to an acid. The measured pH values after successive additions are 2, 4, 6, 7, 9 and 11.
.1
Describe the change and identify the neutral reading.
[2 marks]
.2
Explain neutralisation in terms of the reacting ions.
[2 marks]
6 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
A sample contains very long covalent molecules. It softens when heated, does not conduct electricity and becomes solid again on cooling.
Explain these observations using a polymer model.
[6 marks]
5 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
Three repeated demonstrations with equal samples of a group 1 metal collect hydrogen volumes of 47, 49 and 48 cm³. A separate run collects 31 cm³ after the gas tube becomes loose.
.1
Calculate the mean volume from the three valid repeated demonstrations.
[3 marks]
.2
Explain why the low run cannot show that the metal was less reactive.
[2 marks]
6 marksHigher
Spec and skills
Original practice question
Answer and supported method checks
A student gives this explanation: "An exothermic reaction has weak product bonds. Breaking the reactant bonds releases energy, and activation energy is the same as overall energy change."
Evaluate the explanation using bond breaking, bond formation and reaction profiles.
[6 marks]
4 marksFoundation and Higher
Spec and skills
Original practice question
Answer and supported method checks
A student says: "Crude oil is a compound. Fractional distillation breaks its molecules into simpler hydrocarbons. Each fraction contains one pure substance."
