Physical Properties of Potassium
Potassium is represented by the symbol "K" with an atomic number of 19. It is classified as an alkali metal element in Group 1 of the periodic table, Period 4. Its melting point is approximately 63.7°C, and its boiling point is around 759°C. Because potassium has a density of 0.862 g/cm³ and less dense than water, it floats on water surface. The pure potassium is a silver-white, soft, and malleable metal that can be easily cut with a knife. When exposed to air, it rapidly oxidizes to form potassium peroxide (K₂O₂) and turning the surface yellow.
Chemical Properties of Potassium
Potassium has a total of 19 electrons. It has one electron in its outermost shell, which is susceptible to be taken by other elements. Pure potassium is not found naturally as it readily reacts with other substances. Due to the similarity in the number of outermost electrons between potassium and sodium, they share some chemical properties. However, as potassium has a larger atomic radius compared, it is more difficult to hold onto its outermost electron, resulting in higher reactivity than sodium. Potassium reacts with oxygen, chlorine, water, and alcohols similarly to sodium, but with some differences due to its higher reactivity.
Reaction of Potassium and Oxygen
When potassium reacts with oxygen at room temperature, it forms potassium oxide (K₂O) and potassium peroxide (K₂O₂).
4K(s) + O₂(g) → 2K₂O(s)
2K(s) + O₂(g) → K₂O₂(s)
When potassium burns in an abundant supply of oxygen, it produces a purple flame and a significant amount of heat. The products of this chemical reaction are potassium peroxide (K₂O₂) and potassium superoxide (KO₂).
2K(s) + O₂(g) → K₂O₂(s)
K(s) + O₂(g) → KO₂(s)
Reaction of Potassium and Water
When potassium is added to a beaker containing water and phenolphthalein, similar to sodium with water, it floats on the water surface and reacts vigorously to release hydrogen gas. The excess heat ignites the hydrogen gas and potassium, and emit a bluish-purple light (overlaid with yellow light due to impurities of sodium). The resulting product is potassium hydroxide, also known as caustic potash, which gives the phenolphthalein solution a pink color.
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)