PH LAB (neutralization of Acids and bases) SETUP and DEBRIEF Tahoma Jr. High 8 th Grade Science Maple Valley, WA.

pH LAB(neutralization of Acids and

bases)SETUP and DEBRIEF

Tahoma Jr. High8th Grade ScienceMaple Valley, WA

pH - which stands for “power of Hydrogen” (as it’s the hydrogens in an acid that do all

the work) is a very important tool for identifying chemicals and their strengths.

pH is one chemical property that can be measured accurately.

pH paper (AKA “Litmus paper”) works OK, but is not very accurate.

Liquid Universal Indicator is also OK, but not very accurate either.

Electronic pH meters can give accurate decimal measurements.

?

Before we start, we need to learn to be very careful about “cross-contamination” of chemicals. Many things can go wrong if you do not keep your equipment TOTALLY

clean – and keep the chemicals from mixing when you don’t want them to.

1. Make sure your tray is clean and dry.

2. Have ONLY 1 cap off the bottles at a time (liquid inside cap can

contaminate others)3. Drop your drops from a separated height (do NOT dip the nozzle in the

chemicals ! )4. When using the eyedropper to stir, make sure you rinse in a cup of water

before using in another cup.

GETTING READY

Clear your table work area and remove bags from underfoot (they are safety hazards).

Get your equipment:tray with 3 bottlesbeaker with pipette/eyedropper

Fill your beaker with roughly 2/3 of tap water

Rinse and dry your tray to be sure it is totally clean

We’re going to set one up first to verify the chemicals are working properly.

1 4 5 6 9

First we need to put 2 drops of Universal Indicator solution in each of the cups. This chemical will react with a wide

range of chemicals (“Universal”) and shows us (“Indicates”) what the basic pH is: acid, neutral or base.

Universal Indicator is a dark greenish blue.

1 4 5 6 9

They should now look like this.

First we need to check for neutral in Cup 5.Add 4 drops of H2O (tap water is OK from your

beaker because it has not been used yet).What color should it turn?

1 4 5 6 9

Good…

green is neutral

Now add 4 drops of acid (let’s use Acid A) to Cup 4.

What color should it turn?

1 4 5 6 9

Great!

It should turn red

if it’s an acid.

Now to check for base, in Cup 6 put 4 drops of Base.

What color should it turn?

1 4 5 6 9

That’s right…

blue for base

Now that we’ve verified that the chemicals all work correctly, take a look at Cups 4, 5 and 6.

What do they remind you of ?

4 5 6

it’s the pH scale !!(complete Answer A on HandOut)

Now we’re going to see how many drops of Base will be needed to neutralize the Acid.

Add 4 drops of Acid in Cup 1 (should turn red)

1

Now very carefullyAdd 1 drop of Base – stir – observe color.Add another drop of Base – stir – observe color.

Keep doing this until you see green (neutral) or blue (past neutral and now a Base).

Record how many drops to get green or blue on HO.

color ? color ?...

(complete Answer B on HandOut)

Now do the same in Cup 9 to find how much Acid is need to neutralize the Base.

Add 4 drops of Base in Cup 9 (should turn blue)

Now very carefullyAdd 1 drop of Acid – stir – observe color.Add another drop of Acid – stir – observe color.

Keep doing this until you see green (neutral) or red (past neutral and now a Acid).

Record how many drops to get green or red on HO.

color ?color ?...9

(complete Answer C on HandOut)

CLEAN EQUIPMENT and PUT BACK

(Answer D on HO)

DEBRIEF OF

LAB FOLLOWS

When neutralizing acids or bases,it’s like a see-saw.

You are trying to balance the number of Acid particles with the same number of Base particles.

ACID BASENEUTRAL

Remember, what do you get when you mix an Acid and a Base?

H+ + OH- = H2O !!(and you’ll get a type of salt too with the leftovers)

neutral pH

see next slide

What does this formula say?

get it… “base under assault”… ; )

a base under a salt

One final word about pH – many personal products are “pH balanced” so you know it won’t be too acidic or too basic in your eyes (eyedrops) or on your skin

(makeup), in shampoo (scalp and hair)…

end show

To make sure these chemical are safe for your use, they often are tested on animals first – and although it’s better to have a bunny rabbit or rat go blind (why mammals will be “used” in our chemical project experiment) there’s other ways to test

without using animals.

Be careful though, even though they may say “cruelty-free” or “not tested on animals” - it may not be that way through the whole manufacturing process (deceptive advertising). The only way to be sure is to look for the “Leaping Bunny” logo –

an organization that checks that animals are NEVER used from start to finish.