Summary of Evaporation and Boiling: Separation Techniques

## Introduction Evaporation and boiling are two ways a liquid can turn into a gas. You see evaporation every day when puddles dry or wet clothes become dry. Boiling happens when a liquid gets very hot and bubbles form through the whole liquid. > **Definition:** Evaporation is when a liquid turns into a gas from the surface. Boiling is when a liquid turns into a gas throughout the whole liquid at its boiling point. ## How evaporation works - Evaporation happens at the surface of a liquid. Molecules at the surface gain enough energy to escape into the air as gas. - Evaporation can happen at any temperature, but it happens faster when it is warmer. ### Real-world examples - Puddles drying after rain - Wet clothes drying on a line - Stalactites and stalagmites forming in caves when water drips and slowly evaporates, leaving tiny amounts of solid minerals behind > **Definition:** When water contains dissolved solids (like salt), evaporation removes the water and leaves the solids behind. Fun fact: Sea salt is often made by leaving sea water in shallow ponds until the water evaporates and the salt crystals remain ## How boiling works - Boiling happens when the whole liquid reaches a certain temperature called the boiling point. - Bubbles of gas form throughout the liquid and rise to the surface. - For pure water at normal air pressure, the boiling point is $100^{\circ}\mathrm{C}$. > **Definition:** The boiling point is the temperature at which a liquid turns to gas throughout, forming bubbles. ### Example: Geysers - Underground water can be heated to its boiling point by hot rocks. When it boils, the pressure pushes water up and out of the ground in a tall spray. ## Evaporation vs Boiling (comparison table) | Feature | Evaporation | Boiling | |---|---:|---:| | Where gas forms | Only at the surface | Throughout the liquid | | Requires specific temperature? | No (any temperature) | Yes (boiling point) | | Visible bubbles? | No | Yes | | Rate depends on | Temperature, surface area, wind | Temperature reaches boiling point | ## Making table salt (two ways) 1. Evaporation from sea water in shallow ponds: - Sea water is collected in shallow ponds and left in the sun. - Over days or weeks the water slowly evaporates and salt crystals form and are collected. 2. Mining rock salt and making brine: - Rock salt is dug up or water is pumped into underground salt layers to dissolve the salt (making brine). - The brine is brought to the surface and heated to evaporate the water, leaving table salt. Draw simple flow charts (you can sketch these): - Sea salt: Sea water -> shallow pond -> sun evaporation -> salt crystals - Rock salt to brine: Rock salt underground -> pump in water -> brine -> heat to evaporate -> salt ## Practical lab use: Recovering solids by heating to dryness - In the lab, if a solid is dissolved in a liquid, we can heat the liquid until all the liquid is gone. This is called "heating to dryness." The solid that was dissolved will remain in the container. > **Definition:** Heating to dryness means heating a solution until all the liquid has evaporated or boiled away, leaving the solid behind. ## Questions answered from examples - What happens to the sea water in shallow ponds over a week? The water level drops as water evaporates and salt crystals form on the pond floor. - Why is salt left behind in the ponds? Salt does not turn into gas when water evaporates, so it stays and becomes crystals. - Would evaporation be faster in the cold cave or warm ponds? Evaporation would be faster in the warm salt ponds because higher temperature gives liquid molecules more energy to become gas. Did you know that mined rock salt is often spread on icy roads in winter because salt lowers the freezing point of water and the pieces of rock give vehicle tyres extra grip? ## Safety and notes for young scientists - Never boil water without adult supervision. - When heating liquids in a lab, use proper equipment and safety g