Evaporation and filtration
Two methods that are often used one after the other, and an activity at the end that uses both on the same mixture.
Evaporation
You may have seen salt being extracted using sea water. What happens there is that solar heat evaporates the water in the sea water. The water evaporates and the salts dissolved in it are deposited.
Evaporation
Supplying heat to a mixture, evaporating the unwanted component and so separating out the wanted one, is what happens in evaporation.
An unusual case — the gold amalgam
Metals dissolve in mercury to form a special solution called an amalgam. When mercury is added to impure gold, a solution in which only the gold has dissolved is obtained; this is called a gold amalgam. When the gold amalgam is heated, the mercury evaporates and pure gold metal is left. The mercury that evaporates is cooled and used again.
Filtration
When food is cooked at home, coconut milk is added to some dishes. It is made by adding water to scraped coconut and squeezing it by hand. Some parts of the coconut flesh do not dissolve in the water but are suspended in it. When this mixture is put into a milk strainer, the milky solution passes through and the other parts stay behind.
Filtration
Filtration can be used to separate from a mixture those components that are suspended in a liquid rather than dissolved in it. A filter has small holes. Particles smaller than those holes can pass through them; particles larger than them cannot.
The residue
The substance left behind on the filter is called the residue.
The filtrate
The solution that has passed through is called the filtrate.
Activity 3.3.1 — both methods on one mixture
Method
Mix about 10 g of dry soil and about 5 g of salt (NaCl) well. Take about 50 ml of water into a beaker, add this mixture to the water and stir. Set the apparatus up as in the figure and filter the mixture. When the filtering is finished, look at the filter paper. Take about 10 ml of the filtrate into an evaporating dish and evaporate it. See whether anything is left in the dish.
Figure to be added
The observation
The large clay particles in the soil sample do not pass through and are held on the filter paper. Water and salt are made of small particles, so they travel through the filter and end up in the filtrate. Evaporating the filtrate leaves the salt behind in the dish.
So the three-component mixture came apart in two moves: filtration took out the soil by particle size, and evaporation took out the water by boiling point, leaving the salt.