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Candle under water presents: How can we show the presence of atmospheric pressure? REQUIREMENTS Water A Glass cup Coloured ink A saucer A candle A kitchen lighter WHAT TO DO Pour some water into the saucer and then add a few drops of colour to it and mix well. Now light up the candle and place it in the middle of the saucer and then cover it up with the cup. WHAT HAPPENS With the flame still burning inside the cup, some of the water starts collecting inside the glass, absorbing it from the outside, and when the flame goes off there is a much bigger gush of water into the glass. WHY THIS HAPPENS The flame uses up the oxygen in the cup creates a low pressure. Atmospheric pressure then tries to force its way in. Because the water at the base of the cup has created an air tight seal, the pressure difference between the inside and outside of the cup causes the water to raise until pressure (inside the cup) is equal to the pressure outside.
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