YouTube: https://youtube.com/watch?v=XpapV4Q2_iI
Previous: Hurricane Sandy FAQs
Next: Secrets of the Vampire Squid

Categories

Statistics

View count:224,116
Likes:5,092
Dislikes:40
Comments:855
Duration:03:04
Uploaded:2012-11-01
Last sync:2018-04-26 14:30
Do science at home with Hank in this episode of SciShow - you'll learn how to make your own litmus paper, what it's good for, and how it works.

Like SciShow on Facebook! http://www.facebook.com/scishow
Follow SciShow on Twitter! http://www.twitter.com/scishow

References
http://www.madehow.com/Volume-6/Litmus-Paper.html#b
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC125091/
http://en.wikipedia.org/wiki/Anthocyanin#pH_indicator
http://en.wikipedia.org/wiki/Litmus
Hello, this is Hank with SciShow Experiments.  People often talk about a Litmus test, to mean a way of getting a really quick check on something or someone.  We use it most often in pretty superficial situations like politics or speed dating.  "Is the person liberal or conservative?  Are they on Team Jacob or Team Edward?  Plain or peanut M&Ms?"  But if you've taken any chemistry at all you know that a Litmus test is actually a method for determining the pH of a substance (that's its alkalinity or its acidity).

pH is the concentration of protons or hydrogen ions in a solution.  Protons and hydrogen ions are the same thing.  Since hydrogen is just an electron and a proton, if a hydrogen atom loses an electron to become an ion, all that's left is the proton.  Same thing!

The more protons you have in a solution, the more acidic it is.  If you have a high concentration of protons or low pH (sorry, counterintuitive) it will freak out and burn your hand off.  A really low concentration or protons, or high pH, and the solution will want more protons so bad that, again, it will freak out and burn your hand off.  You'll want a pH of about 7.  That's what we call "neutral," and that's what water is: a nice, stable amount of protons.

Now you don't need a laboratory supply company to get everything you need to conduct your own Litmus test.  All you need is a trip to the grocery store.  Get yourself a purple cabbage and a coffee filter...it's wearing a hat!

Cut the raw cabbage into small pieces until you have about a cup's worth if you spread it evenly on a plate.  Cook the cabbage in the microwave for three to five minutes until the leaves are nice and soft.  Now, use the coffee filter to soak up the juice, or strain it onto another plate that has the coffee filter on it.  If you need more juice, cook another batch or two until the filter is soaked.  Then, let it dry.

You can now use the pieces of the filter to test the pH of any solution you like.  If the paper turns green or yellow, which it does with this baking soda solution, the solution is a base, or doesn't have many protons.  If it turns pink, like with this lemon juice, it's acidic, meaning it has lots of protons.  So, how does this work?

Purple cabbage contains a large amount of anthocyanin, one of the most common pigments of the plant kingdom.  It's what makes pansies purple and blood oranges bloody and autumn leaves turn red, and as it happens, anthocyanin changes color depending on the pH of its environment, turning reddish in acidic solutions and greenish-yellow in basic solutions.  But the pH of cabbage juice itself is about neutral, so the anthocyanin in the juice appears in its neutral color, purple.

This all makes sense when you consider that the Litmus paper used in labs is made from a similar process.  It's infused with natural dye found in some lichens, that works in much the same way.

Now, you can conduct your own real Litmus tests at home, and while it's not gonna help you pick a Supreme Court justice, you may have enough cabbage left over to make a nice coleslaw.  Enjoy!

Thank you for watching this SciShow Experiment.  If you have other ideas for experiments we should do, or questions, or comments, or anything else, we're on Facebook and Twitter, and we're in the comments below.  And if you want to keep getting smarter with us here at SciShow, you can go to youtube.com/scishow and subscribe.