(1) Quality of rock sugar:

m2 = 20g+5g+2g = 27g；

(2) The volume of fine sugar can be obtained from Figure A:

V 1 =30mL=30cm 3，

The total volume of fine sugar and rock sugar can be obtained from Figure B:

V=40mL=40cm 3，

Volume of rock sugar:

v2 = V-V 1 = 40cm 3-30cm 3 = 10cm 3，

(3) Density of fine sugar:

ρ 1 = m 1v 1 = 54g 30cm 3 = 1.8g/cm3 = 1.8× 10 3kg/m3；

Density of rock sugar:

ρ2 = m2 v2 = 27g 10 cm3 = 2.7g/cm3 = 2.7× 10 3kg/m3；

(4) When measuring the volume of rock sugar, there are tiny gaps between tiny sugar particles, and the measured volume is too large; After adding rock candy, the gap is filled with fine sugar, and the measured volume increment (the volume of rock candy) is more accurate, so the density error of fine sugar is relatively large.

So the answer is: (1) 27; (3) 1.8× 10 3 ; 2.7× 10 3 ; (4) fine sugar.