Mixture Separations

Mixture Types – Relative Particle Sizes Solution Colloid Suspension Objectives: Identify separation techniques which are effective for each mixture type Choose the separation technique that will best separate and retain the desired mixture component.

Separating solutions Solutions cannot be filtered. Solute particles are too small to be retained by filter paper. The components of a liquid solution can be separated using heat to evaporate or distill A liquid solution can also be separated using a technique called chromatography, which exploits the different polarities and solubilities of the mixture’s components.

Separating solutions Evaporation is employed when you want to separate solid solutes from the liquid solvent. The solvent is evaporated away into the air.

Separating solutions Distillation is employed when you want to separate liquid solutes from the liquid solvent. The mixture is heated to boiling. The lowest boiling point liquid vaporizes and rises first. That vapor travels through tubing to a condenser, which is cooled by a water jacket. The cooler temperature causes the vapor to condense and drip out into a collection vessel.

Separating solutions Column chromatography is employed when you want to separate and retain all parts of a solution without using heat. The sample (black liquid, a mixture of red, blue and green) is poured onto a column of very fine, very pure sand called silica. A carefully chosen solvent is added which runs through the sand, carrying the mixture down with it. The different components will travel down at different rates, depending on their relative polarities and solubilities in the solvent.

Separating Suspensions Suspensions can be solids suspended in a liquid, two or more immiscible liquids, or a combination of these. The separation method used depends on the type of suspension and what you are trying to collect.

Separating Suspensions Separating two or more immiscible liquids exploits their different densities to get them apart. Although not very efficient, it would be possible to skim or scoop the less dense material off the top. An even better device to use is the separatory funnel which is a common piece of lab equipment. The narrowed lower passage and stopcock allows for more precise separations. Allowing the higher density liquid on the bottom to flow out of an opening in the bottom of the container is a more efficient separation technique.

Separating Suspensions Separating a suspended solid from a liquid is easily done using a funnel and filter paper. Filtration works with suspensions because the particles are large enough to be retained by the filter paper. The smaller liquid particles (molecules and dissolved ions) will pass through the tiny holes in the paper. The liquid in the collection flask or beaker below the funnel is called the filtrate.

The components of a colloid cannot be separated using filtration. Separating Colloids The components of a colloid cannot be separated using filtration. The method most often used to separate the parts of a colloid is centrifuging.

Separating Colloids A centrifuge spins a sample very fast. The centripetal force causes the dispersed droplets to come together with each other to form layers (a suspension). The heavier (more dense) components of the mixture end up at the bottom of the tube because the centrifuge spins at an angle (bottom of tube is on the outside). Blood Samples Suspension After centrifuging Before centrifuging After centrifuging Colloid with dispersed particles

Mixture Separations Homework Questions 1. Why can’t solutions (solute and solvent) be separated using filtration? 2. You have a mixture of sand, salt water and seaweed chunks from the beach. If you pour this mixture through a filter paper, what is the composition of the filtrate in the collection beaker? Explain how you know. 3. You are stranded on a desert island with only seawater. How do you make the water drinkable? 4. You are a food chemist who is tasked with determining the components of McDonald’s secret sauce (it’s a creamy, off-white mixture). What technique will you use to first separate the components so you can then analyze them? 5. You most likely have one or more centrifuges in your home, though they are probably used to separate a liquid from a liquid-solid suspension. This works because the outside wall of the spinning part is pierced with holes to allow liquid to flow out. What are these home-use centrifuges? 6. Why can a colloid, like hand sanitizer, appear clear, yet it exhibits the Tyndall effect and scatters light?