3.4 Mixtures can be separated according to their size and mass

3.4

Learning intentions

By the end of this topic, you will be able to: • describe the processes of filtration, sieving and centrifuging • explain how the size and mass of molecules of a substance relate to methods used to separate them from mixtures.

Key ideas

• Large particles (residue) can be separated from liquids (filtrate) by filtering. • Heavy particles can be separated from light particles by using a centrifuge.

Video 3.4 Separation techniques (filter paper)

fi ltering a technique used to separate different-sized particles in a mixture depending on the size of the holes in the fi lter used

sieving a separation technique based on the difference in particle size

Mixtures can be separated according to their size and mass

Filtering and sieving Anyone who has cooked pasta will probably have used a colander to separate the boiling water from the cooked pasta. The holes in the colander are designed to let the water through but not the cooked pasta. Filters have a series of holes in them that lets through small things but traps larger particles. A grate on a stormwater drain is an example of a fi lter. The grate lets the water through while fi ltering out the leaf matter and rubbish. Flyscreens on windows and doors fi lter bugs and some dust from the air, and tea bags fi lter the leaves from the liquid (Figure 1).

Filtering separates solids from liquids (or solutions) as the particles of the solution or Filter paper has holes that are too small liquid are smaller than the size of the holes in to see. Solutions can fl ow through the fi lter the fi lter paper (Figure 2). The fi ltrate passes paper because the particles in the solution are through the fi lter and the residue is left behind small enough to fi t through the holes; however, in the fi lter. most solid particles in suspensions are not.

Sieving , on the other hand, separates Different fi lter papers come with differentsolids according to the size of their particles sized holes. Coffee fi lters and the fi lters found (Figure 3). When you use a sieve, anything in vacuum cleaner bags are both made of paper that is smaller than the hole can pass through fi lters. High-effi ciency particle arrestance and the larger solids are left behind in (HEPA) fi lters are used in vacuum cleaners, the sieve. Sieves can separate solids of air conditioners and face masks to remove even different sizes. tiny dust particles.

Filter paper Glass funnel Solid residue Filtrate Figure 2 Filters are used in science to separate substances. Particles that pass through the filter are called the filtrate. The filter paper traps the residue. DRAFT Figure 3 Sieves can be used to separate solids of different sizes, such as gravel and sand.

Gravel

Sand

Figure 1 Tea bags are a common household filter. Gravel + sand mixture

Sometimes fi lters remove substances using chemicals rather than by physically stopping them. Gas masks often contain a special type of charcoal that attracts and holds onto some poisonous gases (Figure 4).

Centrifuging separates light and heavy particles by spinning a mixture. A centrifuge is a machine that spins very quickly. In a laboratory, small test tubes of mixtures are fi xed to the inside of the bowl of the centrifuge. The spinning motion causes the heavier particles to move to the bottom of the tubes. Centrifuges are used in medical research and at blood banks. When blood is spun in a centrifuge, the red blood cells, which are heavier, sink to the bottom of the test tube, leaving the yellowish liquid part of blood (plasma and platelets) at the top (Figure 5). Medical professionals use different parts of blood depending on the medical need. Centrifuges are used in dairy processing factories to separate cream from milk. Salad spinners and washing machines also use this principle (Figure 6).

Retrieve 1 Recall the missing words and use them to complete the sentences below. Filtering is like using a _______________ . The _______________ lumps are caught in the sieve, and the _______________ goes through the _______________ paper.

The substance caught in the _______________ paper is called the _______________ . The substance that passes through is called the _______________ . 2 Name two places where centrifuges are used. Analyse

3.4 Check your learning Apply 5 Discuss why a forensic scientist, who was investigating a crime, would want to compare a mixture of different types of sand found in a suspect’s car with a similar mixture found at the crime scene. 6 In traditional times, the Gunditjmara peoples of

Southwest Victoria used honeysuckle cones from banksia trees as fi ltration straws. The straws allowed them to separate clean water from muddy pools.

Justify why this is an example of a fi lter. (HINT: compare how a fi lter works with how the honeysuckle cones work.)

centrifuging a technique used to separate light and heavy particles by rapidly spinning the mixture Figure 4 A gas mask uses activated charcoal to filter poisonous gases. Centrifuging Sometimes mixtures do not separate well using sedimentation because the particles are not dense enough. Sometimes things need to be separated using their weight. Some playgrounds have equipment that spins around very fast. When you spin very fast on this equipment, you can feel a force pulling you towards the outside of the spin. Heavy objects feel the pull more than light objects.

Figure 6 A spinning washing machine is a centrifuge, separating water from the clothes. DRAFT Figure 5 When blood (right) is separated by a centrifuge, the red blood cells collect at the bottom of the tube and the less dense liquid, the plasma and platelets, collects at the top (left).

3 Identify the fi lters used around your home and school.

Identify the substances that these fi lters allow to pass through them and the substances they collect. 4 Contrast (the differences between) each of the following pairs. a mixture – pure substance b sedimentation – fl otation c residue – fi ltrate

Quiz me Complete the Quiz me to check how well you’ve mastered the learning intentions and to be assigned a worksheet at your level.