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Chemistry LibreTexts

11.1: Extra-Long Straws

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  • Page ID
    47523
  • A drinking straw is a tube for transferring a beverage from its container to the mouth of the drinker. A thin tube of plastic (such as polypropylene and polystyrene) or other material, straight or with an angle-adjustable bellows segment, it is used by being held with one end in the mouth and another end in the drink. Muscular action (a combination of the muscles of the tongue and cheeks) reduces air pressure in the mouth and above the liquid in the straw, whereupon atmospheric pressure forces the beverage through the straw.

    Eight long drinking straws made from plastic and having flexible heads positioned next to each other in order to look like the color gradation of the rainbow. The are standing against a white background. Image used with permission (CC BY 2.0; Horia Varlan).

    How Straws Work

    With the straw just sitting in the glass, the pressure on the surface of the tea is the same all over, including on the little bit of surface inside the straw. When you suck the air out of the straw, you decrease the pressure inside the straw, allowing the higher pressure on the rest of the surface to push the tea up the straw and into your mouth. Because it is really the atmosphere that is doing the pushing, the atmospheric pressure limits how high water will go up a straw. At sea level, the air pressure is enough to support a column of water about thirty feet high.

    Many people believe that when they drink a liquid they are sucking the liquid up, however the liquid is really being pushed up. A straw works because when you suck the air out of the straw it creates a vacuum. The atmospheric pressure outside the straw is greater then in the inside pressure, therefore the liquid is pushed up into your mouth

    DRINKING STRAW: A drinking straw is used by creating a suction with your mouth. Actually this causes a decrease in air pressure on the inside of the straw. Since the atmospheric pressure is greater on the outside of the straw, liquid is forced into and up the straw.

    SIPHON: With a siphon water can be made to flow "uphill". A siphon can be started by filling the tube with water (perhaps by suction). Once started, atmospheric pressure upon the surface of the upper container forces water up the short tube to replace water flowing out of the long tube.

    Your misconception seems to be coming from the idea of a vacuum and a straw. The vacuum itself is not what causes the sucking. It is the atmospheric pressure that causes sucking.

    How long of a Straw is Possible?

    Straws work because sucking creates a pressure difference between the inside of the straw and the outside. If you formed a perfect vacuum within the straw, the pressure outside of the straw at sea level would be enough to push the orange soda (which is mostly water) to a total height of about 10.3 m. • A 10.3-m column of water exerts the same pressure— 101,325 N/m2 or 14.7 lb/in2 (psi)—as do the gas molecules in our atmosphere. At sea level, the air pressure is enough to support a column of water about thirty feet high. This means that even if you could suck all the air out of a forty-foot straw, the water wouldn’t rise more than thirty feet.

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