Chloroethane \(\left(T_{b p}=-13^{\circ} \mathrm{C}\right)\) has been used as a local anesthetic. When the liquid is sprayed onto the skin, it cools the skin enough to freeze and numb the skin. Explain the cooling effect of this liquid.

Phase transition refers to the change of state of matter among solid, liquid, and gas. These transitions involve the absorption or release of energy, and are typically represented by melting, freezing, evaporation, and condensation. In the context of chloroethane used as a local anesthetic, the key phase transition is evaporation—the substance transforms from liquid to gas.

During evaporation, molecules at the surface of the liquid gain enough energy to overcome their intermolecular attractions and enter into the gas phase. This energy is usually in the form of heat absorbed from the surroundings. When chloroethane evaporates after being sprayed onto the skin, the required heat is drawn from the skin itself, leading to the cooling sensation as the skin loses thermal energy.

Intermolecular forces are the forces that mediate interactions between molecules, including dispersion forces, dipole-dipole interactions, and hydrogen bonding. These attractions keep the molecules together in a liquid or solid state. The strength and type of these forces determine many physical properties of a substance, such as boiling point, vapor pressure, and surface tension.

In case of chloroethane, these intermolecular forces are relatively weak, making it easier for the molecules to separate and transition into a gas during evaporation. When chloroethane is sprayed, it's the breaking of these intermolecular forces that requires the absorption of heat, resulting in the cooling of the skin.

Boiling point is the temperature at which a substance transitions from a liquid to a gas. It is an intrinsic property of a substance and is influenced by ambient pressure and the strength of intermolecular forces within the liquid. Substances with strong intermolecular forces require more energy to separate the molecules, thus have higher boiling points.

Chloroethane has a low boiling point of -13°C, meaning it can easily vaporize at room temperature. When applied to the skin, the ambient temperature is sufficient to cause the chloroethane to reach its boiling point and evaporate—this is why it acts effectively as a cooling agent, taking away heat from the skin as it changes phase.

A local anesthetic works by numbing a specific area of the body, blocking the nerve signals in that particular area. The cooling effect from a substance like chloroethane plays a part in this mechanism. As chloroethane evaporates, it absorbs heat from the skin, causing a decrease in temperature which can lead to a reduction of nerve activity.

The cold environment slows down the nerve's ability to transmit pain signals, essentially 'numbing' the area. Moreover, by reducing the skin temperature to a point of mild freezing, chloroethane creates a physical barrier to sensation. Hence, the evaporative cooling effect of chloroethane not only soothes by lowering temperature but also serves a practical function in medical procedures where a local anesthetic is required.