measure

Measuring Liquid Volume with a Graduated Cylinder



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okay now we're gonna talk about finding

volume or measuring volume and to find

volume we use graduated cylinders which

are these tall looking containers here

okay you may notice that a beaker has

volume marks on it but it's terribly

inaccurate so to order to find a volume

correctly we need to use a graduated

cylinder okay all right so the really

important thing to know about finding

volume in a graduated cylinder okay when

water is poured into a graduated

cylinder it forms what's called a

meniscus in other words you see how on

this picture here I've shown that water

kind of curves up the edges alright if

you measure at the top of the curve that

is not an accurate measurement you must

measure at the bottom of the curve or

the bottom of the meniscus okay water

kind of sticks to the edges and forms a

little curve and we'll look at that in

just if you're using a glass graduated

cylinder notice it has this yellow piece

of plastic at the top of it okay that's

not to use to measure it actually

protects it if it falls over it keeps it

from breaking okay so that needs to stay

at the top always stay at the top okay

so let's practice measuring now okay so

let's say you have an amount of water

and you pour it into the graduated

cylinder now how much water do we have

okay first thing never look at it from

up here okay if you're going to measure

using a graduated cylinder you always

need to get down i level with the water

or whatever liquid you're measuring okay

and then once I get close to it here you

can see the meniscus you see how that

liquid curves up the sides okay you're

not measuring at the top of the meniscus

you need to measure at the bottom of the

meniscus okay let's see how much water

we have in here okay it's colored water

okay notice right here the big line is

70 milliliters okay it was below there

is 60

there's ten marks in between one two

three four five the five is the longer

one six seven eight nine and ten seventy

okay

so that means each mark is one

milliliter all right so let's count

starting at 70 all right one two three

four five so that's 75 okay the top of

the demand meniscus is actually at 76

but we always read the bottom of the

meniscus so this is 75 milliliters of

water okay if you're from up here you

might get a wrong measurement if you

look from down here again you might get

a wrong measurement because that looks

more like 76 you have to be exactly at

eye level what do I do if I want a

little bit more let's say I want 77

milliliters instead of 75 okay

I'm gonna use a pipette which is a fancy

word for a dropper okay

so I've gotten some more liquid in my

dropper in my pipette and I'm gonna hold

it over here above hold it in the

graduated cylinder and then I need to

look as I'm dropping drops in to see

when it gets up to the appropriate place

all right remember we want to look at

the bottom of the meniscus only not at

the top okay so now I'm gonna add drops

with my dropper with my pipette and I

keep dropping it in okay now I'm up to

notice the bottom of the meniscus now is

at 76 the top is at 77 so I need a

little bit more so I'll keep dropping

until the bottom of my named meniscus

now is at 77 so that's how you add water

okay or add a liquid to bring it up to

the correct measurement all right so if

you need to remove some water or liquid

the way to do it is to dump some of it

out and then fill back up with the

pipette again okay if you try to stick

the pipette down in there to take some

out well when you put this pipe it in it

actually changes the measurement just a

little bit so it's best just to dump it

out and then fill it

got with the pipette