Question

I add pure water (at 25 degrees C) and K2CO3 to a flask, then stopper the flask. I fasten this flask onto a stirrer, turn the stirrer on, and watch the flask and stirrer continuously for several hours. At the end of this time, there is still solid material left at the bottom of the flask. This happens even though:

I) the stirrer does not stop or change speed;

II) the flask is not removed from the stirrer;

III) nothing is added to or removed from the flask;

IV) the temperature does not change;

V) and a small amount of K2CO3, added to pure water (at 25 degrees C) in a seperate beaker, dissolves almost immediately.

Why, then, would there still be K2CO3 (s) left in the flask? Explain.

PRELAB QUESTIONS 1. I add pure water (at 25 °C) and K2CO3 to a flask, then stopper the flask. I fasten this flask onto a stirrer, turn the stirrer on, and watch the flask and stirrer continuously for several hours (I'm easily amused). At the end of this time, there is still solid material left at the bottom of the flask. This happens even though: i) the stirrer does not stop or change speed; ii) the flask is not removed from the stirrer; iii) nothing is added to or removed from the flask; iv) the temperature does not change; and v) a small amount of K2COs, added to pure water (at 25 °C) in a separate beaker, dissolves almost immediately Why, then, would there still be K COs (s) left in the flask? Explain.

Answer 1

Potassium carbonate is a white solid which is soluble in water. The soluti​on of K2CO3 and pure water must have reached it's saturation level, that's why, even though it was stirred for several hours, the excess amount of K2CO3 left at the bottom of the flask. But if we add small amount of K2CO3 in pure water, it dissolves almost immediately because the solution is not saturated yet. We can keep adding the salt and it will keep dissolving till it hits it's saturation level. If we still keep adding, the excess amount of salt will be left at the bottom of the flask.

Note: saturation level can be increase with the increase in temperature. Therefore, excess of K2CO3 that is left at the bottom of the flask, can be dissolved with increase in temperature of the solution