(1) At a certain temperature, the mass of solute dissolved when a solid substance reaches saturation in 100g solvent is called the solubility of the substance in the solvent, and the unit of solubility is grams;

② In the temperature range of 0℃ ~ 40℃, the solubility of substance B is greater than that of substance A;

③ At 20℃, the solubility of substance A is less than 20g. When 30g of substance A is dissolved in 50g of water, substance A remains, so 30g of substance A is dissolved in 50g of water at 20℃ to obtain a saturated solution.

④ At 60℃, the solubility of substance B is 30g；; At 60℃, the mass fraction of saturated solution of substance B is 30g+100g = 313;

⑤ At 40℃, the solubility of the two substances is equal.

A. the solution quality is not necessarily equal;

B. The densities of solutions are not necessarily equal;

C. The mass fraction of the solution must be equal;

D. if the temperature drops to? At 20℃, the mass fractions of solution A and solution B are not equal.

So, the answer is: ① gram;

②0℃~40℃;

③ Saturation;

④3 13;

⑤C。