IGCSE Coordinated Science: EvaporationEvaporation Evaporation mostly occurs at the surface of a liquid , and for evaporation to occur, the molecules must have enough kinetic energy. First, let’s define the word Evaporation. Evaporation is basically the change of state of a substance from liquid to gas. If you look back into the Topic “States of Matter”, you will noticethat the particles in a liquid are stuck quite close together, where’s the particles in a gas is scattered all over the place. Evaporation makes the particles in a liquid behave like the particles in a gas. However, energy is needed to enable the molecules to become more energetic and allow some particles to overcome the bonds holding the liquid particles together and transform intoa gas in order to break the relatively strong bonds holding the particles together in a liquid. Because they release more energy than others, some molecules more quickly turn into gases. 2. Demonstrate understanding of how temperature, surface area and air flow over a surface influence evaporation. Flow rate of air: This is in part related to the concentration points above. If fresh air is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. Temperature: If substance has higher temperature, molecules will have a higher average kinetic energy, and hence the rate of evaporation will increase. 3. Relate evaporation to the consequent cooling. Evaporation is one of the four principal ways that heat can be transferred (the other three areradiation, convection, and conduction). Evaporation of sweat from the skin is an endothermicreaction: the reaction essentially “consumes” heat, which cools the body in the process. For a liquid to turn into gas, either by boiling or evaporation, energy is required. This is known as the latent heat of vapourisation. In the case of boiling this is provided by external heat sources but in the case of evaporation it has to come from within the liquid itself, thus cooling it. The energy is required to break the strong bonds between the molecules in the liquid and allow them to break free in gaseous form. Evaporation is an endothermic (heat-absorbing) process because molecules must be supplied with energy to overcome the intermolecular forces. Evaporation occurs at the surface of a liquid, and energy is required to release the molecules from the liquid into the gas. The use of this energy, known as latent heat, causes the temperature of the liquid to fall. Evaporation cools the surface of the thing that the liquid is evaporating from. Reason being evaporation is the departure of the warmest and most active (energetic) particles because
they are the ones with enough energy to ‘escape’ into the air. Which leaves behind the cooler, less energetic particles, which brings down the average temperature of what’s left behind.