Experimental design

"What do you observe?" wants a change detected by the senses, never a product name. "Zinc chloride is formed" or "hydrogen is produced" scores zero where effervescence, the solid dissolves, or the solution turns colourless is required. Reserve product names for "name the product" questions; use "fizzing", not "gas is made".

"A pure substance has a sharp melting point" earns only one mark — you must also give the direction for the impure case. An impure solid melts lower than the pure value AND over a range; an impure liquid boils higher (and rises during boiling). "It melts over a range" alone, without "below the pure value", leaves a mark behind.

A measuring cylinder reads only to about $\pm 1\ \text{cm}^3$, so naming it for a titration or an accurate volumetric step loses the apparatus mark. Precise volumes need a burette ($\pm 0.05\ \text{cm}^3$) or a pipette (fixed volume). A measuring cylinder is fine only for rough volumes where accuracy does not matter.

"Sharp" means melting at a single temperature (start = finish, range = 0 °C), not just a small range. A sample melting over even 1–2 °C is impure. Do not call a narrow-range result pure — the word examiners want is sharp, and a pure sample should also match the literature value.

The two shifts go opposite ways, which trips students up. An impurity lowers the melting point but raises the boiling point — it always moves the value away from the pure one. A memory aid: impurities make a substance harder to freeze and harder to boil. Never write "lowers both" or "raises both".

A burette delivers a variable volume, read to $\pm 0.05\ \text{cm}^3$ from a scale that is zero at the top and rises downward — used to add solution bit by bit in a titration. A pipette delivers one fixed volume (e.g. $25.0\ \text{cm}^3$) to measure out the analyte. Do not swap them.