why the reading of the measuring cylinder temains constant in measure the percentage of oxygen in air
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➡️Object of the Experiment
We will try to make a measurement of the approximate percentage concentration of oxygen in air. Air is a mixture of several invisible gases, so we first need to be able to separate just the oxygen from the other gases in an air sample. As that is being done, there needs to be some kind of a visible change that you can see and measure.
➡️Conducting the Experiment
Basically a moist piece of steel wool is stuck into a glass 100 mL graduated cylinder. The cylinder is turned upside down and the open end is immersed in a cup of water. The air in the graduated cylinder is sealed off from the rest of the atmosphere. The oxygen reacts with the steel wool to form rust and is removed from the air sample (it turns from a gas and becomes part of the rust, a solid). As oxygen is removed from the sample, water will rise up into the cylinder and its level can be read on the cylinder scale.
If you simply try to immerse the open end of the cylinder in a cup of water you would find that the water doesn't enter the cylinder. Air pressure keeps the water out. You want the water to enter partway into the cylinder so that the water level can be read on the cylinder scale.
➡️Note that it isn't that the cylinder is full of air that keeps the water out (as shown above at left), there's actually a lot of empty space in the cylinder. Rather it is the fact that the air molecules are moving around inside the cylinder at 100s of miles per hour and they strike the water molecules with enough force that the water can't move into the cylinder (the more accurate drawing above at right). The pressure exerted by the air in the cylinder is what keeps water from rising up into the cylinder
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