Solubility is the ability or property of a solute which can be a solid, liquid, or gaseous chemical substance to dissolve in a solvent (solid, liquid or gas) and form a solution. The factors that affect how soluble a substance is depends on the solvent ,temperature and pressure.
Here, Mario trying to dissolve certain substances (solutes) in water (the solvent shows he is trying to find out the solubility of the substances which can be insoluble, sparingly soluble or very soluble.
K+ ions move faster than the Na+ ions through the neuron membranes.
The concentration of Sodium ions outside the cell is very high. The concentration of Potassium ions inside the cells is very high. Leakage channels thus allow sodium ions to move inside the cells and pumps out the Potassium from the cells.