DK Science & Technology: Separating Mixtures
How do we extract salt from seawater? Methods for separating a mixture depend on differences in the physical properties of its components.
Differences in size, density, solubility, and magnetic properties separate one solid from another. Adding water separates salt from sand—salt dissolves, but sand does not. FILTRATION separates an insoluble solid (one that does not dissolve) from a liquid.
When a liquid mixture is heated, the liquid with the lower boiling point evaporates (becomes a vapor), leaving behind the liquid with the higher boiling point. Cooling condenses the vapor back to liquid. Fractional distillation separates substances one by one as the temperature rises.
In this laboratory demonstration, the solution to be distilled is boiled by a Bunsen burner. Vapor from the boiling liquid is directed into a water-cooled condenser. The condenser is angled so that gravity causes the condensed liquid (turned from vapor back to liquid by cooling) to run down into the collecting flask.
Air is separated to produce nitrogen, oxygen, and other gases by first cooling it to the liquid state at –321°F (–196°C). The liquid air is fractionally distilled. Gases may also be separated by the rate at which they diffuse through a barrier. Light molecules diffuse more quickly than heavy molecules.
Filtration is a method of separating a solid from a liquid by trapping solid particles in a material that only lets the liquid through. A coffee filter separates solid coffee grounds from liquid coffee. The size of the pores (holes) in the filter paper determines which particles will pass through.
Your kidneys clean your blood. Waste chemicals dissolved in the blood pass through a membrane to be excreted in urine. Blood cells are too big to pass through and are retained in the body. People whose kidneys fail can be treated on a dialysis machine. This uses a synthetic membrane outside the body.
A centrifuge is a spinning machine, a bit like a clothes dryer, that separates materials. A mixture of liquids and solids is spun at high speed in a tube. The larger, denser particles sink and collect at the bottom. Light particles collect at the top. Blood cells are separated from plasma (the liquid part of blood) by this method.
