how come a penny can hold more drops of the salt water solution than just plain tap water from your faucet?
Put in in a narrow tube. The narrower it is, the more surface tension it has at the meniscus.
Surface Tension Of Salt Water
Surface film tension is the property of a liquid by which it acts as if its surface is a stretched elastic membrane. This tension allows insects to stand on the water’s surface. Surface film tension is caused by the attraction of water molecules to each other; in addition, the surface molecules are attracted to the underlying molecules. The amount of surface film tension is affected by both temperature and dissolved substances. An increase in temperature lowers the net force of attraction among molecules and so decreases the surface tension. An increase in dissolved organic substances also lowers the surface tension However, an increase in dissolved inorganic salts causes a rise in surface tension - increases the attraction of the surface molecules for the underlying molecules.
