Air Pressure Curriculum

Section 3—Lesson 7

Picture of Practice

How Do Air Pressure Differentials Act as Causes?: An Eighth Grade Science Lesson

The following picture of practice describes a lesson in which students explore what causes liquid to rise in a straw when you drink from it. One group of students is discussing their ideas with Mrs. B. about why the liquid is so hard to draw up in the stoppered flask. This lesson addresses the idea that pressure is dynamic and changes in air pressure result in many things, such as differentials that cause liquid to rise in a straw!

Mike, Emma, and Devon motion for Mrs. B. to come over to their work area.

• Mrs. B.: Do you have a question for me?
• Emma: Yeah, we're trying to figure out why the liquid is so hard to suck up in this flask.
• Mrs. B.: What ideas have you come up with so far?
• Devon: Well, we know that when you first suck on the straw, you're taking the air out of the straw, so that's making the air pressure less... but we thought that would make the liquid go up the straw.
• Mike: That was our model for the first flask. You take the air out of the straw, the air pressure gets less, and the higher outside air pressure pushes the liquid up the straw. But we don't get this one.
• Mrs. B.: Why don't you try it again?

Emma tries once again to drink from the flask and is able to draw up a small amount of liquid, but quickly begins to have problems. In frustration, she blows into the straw. Liquid immediately shoots out of the straw and onto her shirt! Mike and Devon laugh, but Emma does not look amused.

• Mrs. B.: Sorry about that Emma. I thought you said the liquid didn't come up?
• Emma: It didn't when I did it before.
• Mike: Yeah, but this time you blew into the straw!
• Devon: Hmmm. How come that worked? I wonder why.
• Emma: Well, blowing into the straw instead of sucking the air out of it added more air to the flask...
• Mike: I know what happened! Before, the air pressure wasn't high enough to push, but when Emma blew into it, it was.
• Devon: Yeah, but why wasn't it high enough before?
• Mrs. B: Emma, when you first tried drinking, were you able to get any liquid out?
• Emma: Just a little.
• Mrs. B: Okay, think about the effect of removing that liquid from the flask, which is a closed system once you put your lips on the straw. What happens?

Mrs. B. lets Emma, Devon, and Mike puzzle this out as she circulates to other groups. Together they begin to focus on how the molecules that make up the air must spread out, and the subsequent lower air pressure that results.