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Q & A: Why does water curve, and what is a meniscus?

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Q:
Define meniscus?
- basil khalousi (age 13)
new westminster.bc .canda
A:
A meniscus is the curved surface at the top of a column of liquid. In a science class, this liquid is usually water or some sort of aqueous solution, and the column is usually a graduated cylinder or a pipet. As you may have noticed, when water is in such a thin glass tube, it does not have a flat surface at the top. Instead, the top is curved inward, making it a little difficult to decide exactly where to read the volume. As pictured below, the volume should be read from the bottom of the meniscus.

Read the volume at the bottom of the meniscus

Why doesn't water lie flat? Well, it has to do with the nature of the water molecules and glass molecules themselves. Water is made up of polar molecules, which have positively and negatively charged ends. Since opposites attract, the positive sides attract the negative sides, and all of the molecules stick to one another. This is why water droplets can form. Glass molecules also happen to be polar. Again, since polar molecules like to stick together, the water in a glass tube will actually tend to stick to the sides of the tube! You can see this at the top of the graduated cylinder, where the water will slightly creep up the sides and form a curve, which is the meniscus.

Barb

(republished on 08/01/06)

Follow-Up #1: partner problems

Q:
To settle an argument between myself and my partner, can you confirm that fact that the surface of water in a full glass can appear as a bubble taller than the surface of the glass is becuase of surface tension and not the gravitational pull of the moon. I may however be wrong... Thanks!
- Rich (age 24)
Sussex, England
A:
The only part of the moonís gravitational pull that has any effect is the difference in its strength over the different parts of the glass. Thatís an extremely small effect. The surface tension is quite a major effect. Thatís what youíre noticing.

Mike W.

(published on 04/17/07)

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