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.
Filtering and sieving
R
sieving a separation technique based on the difference in particle size
D
centrifuging a technique used to separate light and heavy particles by rapidly spinning the mixture
Filter paper
AF T
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
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
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 flow through the fi lter paper because the particles in the solution are small enough to fit 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-efficiency 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.
56
Gravel + sand mixture Figure 2 Sieves can be used to separate solids of different sizes, such as gravel and sand.
OXFORD UNIVERSITY PRESS OXFORD SCIENCE 7: VICTORIAN CURRICULUM No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means.
