A solution of substance A was evaporated to 20 g of water at a constant temperature to precipitate 2 g of crystal A. (1) At t°C, the solution of substance A evaporates 20 g of water at a constant temperature, precipitates 2 g of crystal A, and evaporates 20 g of water again at a second time at a constant temperature resulting in the precipitation of 8 g of crystal A, and the solubility of substance A at t°C. (2) To obtain crystals from lime water, the () method is usually used because () A solution of substance A was evaporated to 20 g of water at a constant temperature to precipitate 2 g of crystal A. (1) At t°C, the solution of substance A evaporates 20 g of water at a constant temperature, precipitates 2 g of crystal A, and evaporates 20 g of water at a second time at a constant temperature, resulting in the precipitation of 8 g of crystal A, and the solubility of substance A at t°C. (2) To obtain crystals from lime water, the () method is usually used because ()

A solution of substance A was evaporated to 20 g of water at a constant temperature to precipitate 2 g of crystal A. (1) At t°C, the solution of substance A evaporates 20 g of water at a constant temperature, precipitates 2 g of crystal A, and evaporates 20 g of water again at a second time at a constant temperature resulting in the precipitation of 8 g of crystal A, and the solubility of substance A at t°C. (2) To obtain crystals from lime water, the () method is usually used because () A solution of substance A was evaporated to 20 g of water at a constant temperature to precipitate 2 g of crystal A. (1) At t°C, the solution of substance A evaporates 20 g of water at a constant temperature, precipitates 2 g of crystal A, and evaporates 20 g of water at a second time at a constant temperature, resulting in the precipitation of 8 g of crystal A, and the solubility of substance A at t°C. (2) To obtain crystals from lime water, the () method is usually used because ()

(1) First evaporation: unsaturated before evaporation and saturated after evaporation.2g can not be used for calculation.
The second evaporation: saturated before evaporation and saturated after evaporation. It can be calculated by 8g. The formula is:8g/20g=S/100g. The solution is S=40g, i.e. the solubility of substance A at t°C is 40g.
(2) In order to obtain crystals from lime water, the (elevated temperature crystallization) method is usually used because (the solubility of calcium hydroxide decreases with increasing temperature)

At 20°C, a potassium nitrate solution is evaporated at a constant temperature.2 g of crystal is precipitated when 10 g of water is evaporated for the first time,3 g of crystal is precipitated when 10 g of water is evaporated for the second time, and 10 g of water is evaporated for the third time. The mass of crystal is precipitated:() A. Equal to 3g B, greater than 3g C, less than 3g D, unable to determine its mass

Answer: A
Cause:10g is evaporated for the first time, the solution is an unsaturated solution, the second solution is saturated, the mass fraction of the solution is unchanged, so 3g crystal will be evaporated for the third time.

Answer: A
Cause:10g of the first evaporation, the solution is unsaturated solution, the second solution has been saturated, the solution mass fraction remains unchanged, so the third evaporation will also be 3 g of crystal.