① To prepare 500 g of 0.9% physiological saline, 500g×0.9%=4.5g of sodium chloride should be weighed, and the correct operation sequence for completing this experiment is calculation, weighing and dissolution;

② point p represents t 1℃, and the solubility of a and b is equal;

Ii. at t2℃, the solubility of a, b and c is A > B > C；;

Roman numeral 3. Observing the solubility curve, we can see that at t2℃, the solubility of AB is more than 25g, and that of C is less than 25g. Therefore, if 25g of A, B and C are respectively put into 100g of water, it is C that can form a saturated solution. The solubility of C decreases with the increase of temperature, so it can be converted into unsaturated solution by cooling and adding water.

Four. The solubility of AB increases with the increase of temperature, and the solubility of C decreases with the increase of temperature. Therefore, when the saturated solution of A, B and C at t 1℃ is heated to t2℃, AB becomes an unsaturated solution, and the solute mass fraction is the same as that at t 1℃, the solubility of C decreases, crystals are precipitated, and the solute mass fraction decreases.

So, the answer is: ① 4.5g; cba

② Ⅰ, t 1℃, the solubility of A and B is equal;

ⅱ、A > B > C；

ⅲ、C； Cooling down; Add water;

ⅳ、b。