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A vacuum is a space with no matter in it. There is not 'nothing' in a vacuum – there may be light passing through it, but there is no matter. In fact, the definition above is a perfect vacuum – no vacuum is ever perfect. We describe how close a vacuum approaches to a perfect vacuum by it's 'quality'. Other things equal, lower gas pressure means a higher-quality vacuum. A typical vacuum cleaner produces enough suction to reduce air pressure by around 20% - not a very good vacuum. Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth () of atmospheric pressure and evacuate all but 100. Outer space is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average.

A good quality vacuum may be easily produced in the laboratory with mercury, a (long) test tube and a glass container. The test tube is filled with mercury then inserted upside down into the glass container, the opening below the mercury level as shown. The space at the top of the test tube will be a good quality vacuum, able to support a 760mm column of mercury because of the atmospheric pressure on the mercury surface in the glass container. This is called a torricellian vacuum.

Vacuums – or at least low quality, partial vacuums where the pressure is slightly below atmospheric pressure, are widely used. Every incandescent light bulb contains a partial vacuum, and vacuums can be used to form seals so that food stays fresh, for example.