Density

"How heavy something is for its size", "how compact", or "how tightly packed the atoms are" earn nothing. The mark scheme needs the ratio "mass per unit volume" PLUS the unit kg/m³. "How much mass is in an object" drops the volume and fails the ratio test. Correct: density is the mass per unit volume, $\rho = m/V$, unit kg/m³.

Substituting mass in grams (e.g. 450 g) instead of kilograms (0.450 kg) into $\rho = m/V$ makes the answer 1000× too large and loses the accuracy mark. Do the unit check first: $\rho$ is in kg/m³, so $m$ must be in kg and $V$ in m³. Convert, THEN substitute.

Writing "weight per unit volume", or putting weight in newtons into $\rho = m/V$, is wrong — density uses mass (kg, the amount of matter), not weight ($W = mg$, in N). A balance reads mass; a newton-meter reads weight. "Floats because it is lighter than water" is imprecise: it floats because its density is below water's.

g/cm³ and kg/m³ differ by 1000. Giving water's density as "1.0" when the question asks for kg/m³ loses the mark — the value there is 1000 kg/m³. Check the required unit and multiply g/cm³ by 1000 before writing the final answer.

Fill the eureka can to the overflow spout BEFORE lowering the solid, or the volume is wrong. Lower it gently on a thread — dropping it splashes water out and inflates the volume. Read the overflow in the measuring cylinder (not in the can) at eye level, at the bottom of the meniscus.

An object floats if its density is less than the liquid's, not because it is "light". A steel ship floats because its overall density (steel plus the air inside) is below water's 1000 kg/m³, while a small dense pebble sinks. Compare densities, never masses, to predict floating.