Cut the bottom third off each end of the water bottles.
Using the rubber band, tie a piece of cut stocking or kitchen wipe onto the opening of the water bottles. Make this fairly strong as it will need to hold the weight of the sand and gravel.
In one water bottle add sand first and then gravel, in the other bottle put the gravel in first and then the sand. The idea is to see if the order of filtering materials makes a difference to how well the water is filtered... all about variable testing!
Balance each of the two bottle upside-down in the remaining cups. Pour the dirty water into each water bottle and see how well the dirty water gets filtered!
How to teach science, the way primary kids love
Why Does This Happen?
You have created a simple water filter! The sand and gravel particles act as a sieve, trapping larger material from filtering down with the water as it travels downwards. Filtering such as this occurs in the natural environment as well. There are many communities who rely on underground aquifers which have accumulated over millions of years as water has filtered through the soil to the bedrock.
A classic example of an underground aquifier is Australia's Great Artesian Basin which covers much of Queensland and South Australia with extensions into the Northern Territory and NSW.
Australia's Great Artesian Basin
You can create a simple model of soil horizons that produce these natural water filters by alternating different sands and gravel in a clear container.
Soil horizons model using sand and gravel
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