Apparatus, separation and purification — GCSE Chemistry practice questions

Question 1

In a test on a cola drink, a student has to run exactly 17.35 cm³ of aqueous sodium hydroxide into a conical flask. The volume she needs changes from one sample to the next.

Which piece of apparatus should she use to measure this volume?

Accepted Answer

B: burette. A burette delivers any volume up to 50 cm³ and is read to 0.05 cm³, so it can measure 17.35 cm³. A volumetric pipette delivers one fixed volume only, and a measuring cylinder or a beaker is far less precise.

Question 2

On a space station, waste water is cleaned and re-used. A sample of the cleaned water still contains a small amount of dissolved salts.

Which row describes the boiling point and the melting point of this sample at normal atmospheric pressure?

Accepted Answer

B: above 100 °C · below 0 °C. Dissolved impurities raise the boiling point of water above 100 °C and lower its melting point below 0 °C, and both then happen over a range. Only pure water boils at exactly 100 °C and melts at exactly 0 °C.

Question 3

A protein from egg white is broken down into a mixture of four different amino acids. Amino acids are colourless. A student runs a chromatogram of the mixture, dries it and sprays it with a locating agent. Only three spots are seen.

Which statement is the most likely explanation?

Accepted Answer

D: Two of the amino acids have the same Rf value in this solvent.. The locating agent makes every amino acid visible, and an insoluble one would still show as a spot on the baseline. Three spots for four amino acids therefore means two of them travelled the same distance (same Rf) in this solvent, so their spots overlap.

	boiling point	melting point
A	above 100 °C	above 0 °C
B	above 100 °C	below 0 °C
C	below 100 °C	below 0 °C
D	exactly 100 °C	exactly 0 °C