Deep eutectic solvents are systems formed from a eutectic mixture of Lewis or Brønsted acids and bases which can contain a variety of anionic and/or cationic species. They are classified as types of ionic solvents with special properties. They incorporate one or more compound in a mixture form, to give a eutectic with a melting point much lower than either of the individual components. One of the most significant deep eutectic phenomenon was observed for a mixture of choline chloride and urea in a 1:2 mole ratio. The resulting mixture has a melting point of 12 °C (far less than the melting point of choline chloride, 302 °C and urea, 133 °C), which makes it liquid at room temperature.