Which of the following materials fall in the category of a "pure substance"?

Which of the following is a homogeneous mixture?

$(a)$ Define an element.
$(b)$ Identify non-metals from the following elements: Carbon,Potassium,Silicon,Chlorine,Mercury.

Compare physical and chemical changes.

$(a)$ Give any one point of difference between true solution,colloidal solution,and suspension.
$(b)$ $20 \, g$ of sodium chloride is dissolved in $100 \, mL$ of water. How will you test whether the given solution is saturated or unsaturated at the given temperature?
$(c)$ Suggest any one method by which we can increase the solubility of saturated solutions.

Observe the apparatus shown and answer the following questions:
$(a)$ Name the apparatus.
$(b)$ State one use of the apparatus.
$(c)$ State the principle involved in this process.