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Gallery 8

# Tilt a Filter

The size of holes in filters determines how the filter can clean a cup of dirty water.

## How it works

The exhibit contains a range of ball sizes that can pass through one or more different-sized filters, to model how the size of holes in membranes controls water filtration.

## Things to try or ask around the exhibit

• How can we allow the smaller balls to pass through, but not the larger balls to pass through?
• How can we allow the larger balls to pass through, but not the smaller balls?
• Let's try something similar at home using strainers or coffee filters and some dirty water.

## Background

Some filters are used to remove solid impurities from water, such as sand or dirt. Filters have tiny holes that allow water and soluble chemicals such as dissolved salt to pass through, but the holes are not large enough to allow insoluble impurities such as dirt to pass through, so they get trapped in the pores or holes.

A solid material such as sand is slightly soluble, but only 0.1 g of sand will dissolve in one litre of water, so it is considered to be reasonable insoluble, Salt crystals are considered to be soluble, as you can dissolve more than 350 grams of salt in one litre of water. A water mixture containing salt and sand can be filtered if the sand grains are larger than the pores within the filter. If the filter's pores are too large, they will simply allow sand particles to pass through the filter. Therefore, different grades of filter paper containing different sized holes are used to separate mixtures containing different-sized grains.

Once a filter has been used, it can often be cleaned by flushing water backwards through the filter to clear its pores (holes).

## Finding the science in your world

Water filters are used from campsites through to sophisticated water treatment plants, to act as a first stage of water cleansing and removing solid particles from waste water.

Water can be cleaned by passing the water through a filter to remove dirt and other large particles, but not necessarily viruses or tiny microbes which are small enough to pass through the filter's holes.

Filters are often used in the kitchen at home, such as filters in drip coffee percolators, or within water filters fitted to taps.