3.4 Mixtures can be separated according to their size and mass

3.4

Mixtures can be separated according to their size and mass

In this topic, you will learn that:

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

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

centrifuging a technique used to separate light and heavy particles by rapidly spinning the mixture

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.

Filtering separates solids from liquids (or solutions) as the particles of the solution or liquid are smaller than the size of the holes in the fi lter paper. The fi ltrate passes through the fi lter and the residue is left behind in the fi lter.

Sieving , on the other hand, separates solids according to the size of their particles. When you use a sieve, anything that is smaller than the hole can pass through and the larger solids are left behind in the sieve. Sieves can separate solids of different sizes.

Glass funnel

Solid residue Filter

paper

Filtrate

Figure 3 Filters are used in science to separate substances. Particles that pass through the filter are called the filtrate. The filter paper traps the residue.

Filter paper has holes that are too small to see. Solutions can fl ow through the fi lter paper because the particles in the solution are small enough to fi t through the holes; however, most solid particles in suspensions are not. Different fi lter papers come with differentsized holes. Coffee fi lters and the fi lters found in vacuum cleaner bags are both made of paper fi lters. High-effi ciency particle arrestance (HEPA) fi lters are used in vacuum cleaners, air conditioners and face masks to remove even tiny dust particles.

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 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.

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. 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 5 A spinning washing machine is a centrifuge, separating water from the clothes. Figure 6 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).

Figure 7 A butterfly net separates the large butterfly. from other butterflies

3.4 Check your learning

Remember and understand

1 Describe 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. 2 Complete the sentences below by fi lling in the missing words. 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 _________. 3 List two places where centrifuges are used. Apply and analyse

4 Contrast (the differences between) each of the following pairs. a mixture – pure substance b sedimentation – fl otation

c residue – fi ltrate

5 Explain 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.

Evaluate and create

6 Explain why a butterfl y net is an example of a fi lter. (Hint: Compare how a fi lter works with how the net works.)