Usually if you want to replicate air movement like in this experiment, you need a Hoover or Dyson. Well, that's kind of what's happening... you're creating a vacuum! When the piece of paper inside of the overturned jar is lit, the air inside the jar is heated, causing it to expand. When the flaming paper is extinguished, the air inside the jar cools and contracts, leaving a void of air within the jar. This is an example of an imperfect vacuum.
When you think of a vacuum, you think of suction, but scientists like to avoid the term "suck," and instead rely on the pushing forces from outside. With the vacuum inside the jar, air from outside really wants to get inside. Were it not for the wet paper towel, the outside air would succeed and the vacuum would be no more. However, the water in the paper towel effectively stops the air from rushing back into the jar.
Water (as well as other substances) has two properties called adhesion and cohesion. Adhesion is water's ability to stick to other materials, and cohesion is, you guessed it, water's ability to stick to itself. When the flaming paper is extinguished and you add pressure onto the jar, the adhesion and cohesion of the water create an air-tight seal between the plate, paper towel, jar, and water. This air-tight seal means that air is constantly trying to push its way up and into the jar without success. The air pushing at the seal creates the phenomenon of the plate sticking to the mouth of the jar.
Usually if you want to replicate air movement like in this experiment, you need a Hoover or Dyson. Well, that's kind of what's happening... you're creating a vacuum! When the piece of paper inside of the overturned jar is lit, the air inside the jar is heated, causing it to expand. When the flaming paper is extinguished, the air inside the jar cools and contracts, leaving a void of air within the jar. This is an example of an imperfect vacuum.
When you think of a vacuum, you think of suction, but scientists like to avoid the term "suck," and instead rely on the pushing forces from outside. With the vacuum inside the jar, air from outside really wants to get inside. Were it not for the wet paper towel, the outside air would succeed and the vacuum would be no more. However, the water in the paper towel effectively stops the air from rushing back into the jar.
Water (as well as other substances) has two properties called adhesion and cohesion. Adhesion is water's ability to stick to other materials, and cohesion is, you guessed it, water's ability to stick to itself. When the flaming paper is extinguished and you add pressure onto the jar, the adhesion and cohesion of the water create an air-tight seal between the plate, paper towel, jar, and water. This air-tight seal means that air is constantly trying to push its way up and into the jar without success. The air pushing at the seal creates the phenomenon of the plate sticking to the mouth of the jar.
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