Tuesday, July 3, 2012

Positively pH Explained

The last experiment we did was on testing the pH of common household items. First off, we hope you had fun. This lab is a wonderful learning experience and it captures your attention with the many different colors. With that, lets get started on understanding this lab!

pH is considered a chemical property. If you haven't learned about those, there are two properties: Chemical and Physical. Physical properties are things such as color, taste, and smell. Chemical properties are ones like pH, boiling point and melting point.

Physical Properties do not change the chemical make up of something.
Chemical Properties do change the chemical make up.

pH is a way to measure how acidic or basic a chemical, compound or material is. Acids have a lower number on the pH scale while bases have higher numbers. The number seven is neutral. A common neurtal substance is... water!

Something to remember, the pH scale is logorithmic. This means that the values are 10 time different than each other. For example: A pH of 2 is 10 times more acidic than a pH of 3. However it is 100 times more acidic than a pH of 4. Why? Because between 2 and 3 is an amount of 10 and between 3 and 4 is another amount of 10. To find how much more acidic 2 is than 4 you multiply the two sets of ten.

So 2 is 10x more acidic than 3.
2 is 100x more acidic (10 times 10) than 4.
2 is 1000x more acidic (10 times 10 times 10) than 5.
Scale for Litmus Paper
There are different ways the pH level of something can be measured. The most common way in a science lab is to use litmus paper.

Litmus paper changes color based on the acidity or alkalinity of a substance.
Acidity: How acidic a substance is, the more acidic the lower the number.
Alkalinity: How basic a substance is, the more basic the higher the number.

Normally there are two colors of litmus paper, red and blue. Each color tests for something different. Red litmus paper checks for basic solutions and blue litmus paper checks for acidic.
A handy way to remember which paper to use:

Red turns Blue if it is Basic.
Blue turns Red it is it Acidic.

For this experiment we used a solution made from red cabbage, so the colors for the ranges of pH are different than litmus.
Basic is Yellow.
Acidic is Pink.
Neutral is Blue.
(Neutral is blue because the cabbage juice is a blue color. )

So how does cabbage juice work to determine whether something is acidic or basic?
Ideal Red Cabbage Colors
Cabbage juice is a neutral substance so it is good for testing the pH of a substance. The reason a red cabbage is the purple color is because of something called a flavin, this is a water soluble pigment. This pigment changes colors in different pH levels. The reason for this change is because of hydrogen.

The indicator changes colors because it detects the ratio of hydroxide to hydronium ions.
Hydroxide: Negatively charged, anion made of one hydrogen and one oxygen. OH-
Hydronium: Positively charged, cation made of three hydrogen and one oxygen. H30+

When something is acidic there are more hydronium ions than hydroxide.
When something is basic there are more hydroxide ions than hydronium.
When something is neutral (like water or red cabbage juice) the amounts are equal.

Simple things like lemons are acidic and we know this because of their sour and sharp taste. Things that are basic are often used as cleaning supplies because they take away protons, or make things negatively charged. Basic substances have a soapy feel when on your hands, common laundry soap is basic.
Lemon, and citrus is the iconic 'fresh scent'
WARNING: When working in the lab it is dangerous to touch acids and bases (especially strong ones) these are dangerous.

Some Lovely Links: These links also tell how to do this experiment. It's common, simple and great!

Please Note: Red Cabbage can have a very strong odor, one that some do not enjoy. Thankfully science can help with this. Over at Spangler Science (lots of supplies!) they have Jiffy Juice, this stuff is great! It is a condensed, power version of Red Cabbage and is odorless! So if you plan on doing this experiment more than once or need a lot of it (especially for schools) this stuff is great. http://www.stevespanglerscience.com/product/1552


  1. I remember doing experiments like this at school - this brings back so many fascinating memories. Fun, engaging experiments like these are what got me and several others into science I think! This post has inspired me to re-create some of these with my son :)

    1. That is wonderful! We plan on updating with some new experiments you can try, but senior year of college gets a little busy! We hope your son has fun with the experiments, we did. :)


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