This is part of a series of Instructables detailing free science kits and activities developed by the spectrUM Discovery Area in Missoula, MT. The kits and activities are for use at home, in the classroom, or in a distance learning setting with teachers.

For more on how our museum shares these activities with students and teachers, see this video on YouTube.

Did you know that you can do a chemistry experiment and create works of art with nothing but a sheet of paper and a few mild household acids and bases? We'll be using a special property of regular goldenrod-colored paper to explore chemistry and art in this Instructable. We'll also use another special type of paper in the form of pH test strips to explore the properties of different solutions.

This is a great experiment for all ages to determine the pH of various safe household items. We'll use acids and bases to create different colors on goldenrod paper that reacts differently to different pH levels, and also show you how to write a secret message that is only revealed when the paper is exposed to a solution that is alkaline.

This Instructable is entered into the Paper Contest, so please vote for it if you find it useful or inspirational either at home or in the classroom.

Supplies

Genuine goldenrod paper - it used to be very available in copy shops and such, but because it is hard to recycle you may have to get it from science suppliers like Educational Innovations or on Amazon. It can't be just yellow or orange construction paper, must be goldenrod for the chemistry to work!
A mild basic solution - we love baking soda because it is very safe, but things like ammonia, Windex, and soapy water work as well.
A mild acid solution - we're using citric acid dissolved in water here, you can also use vinegar or lemon juice.
A paintbrush or cotton swabs and cotton balls
Parrafin wax from a candle or a yellow crayon
Bowls to hold your solutions
Water

Optional:

It's very handy to test the pH of your solutions with regular pH test strips and a color chart. This adds a bit more exploration to the chemistry component of this activity.
If you don't want to buy goldenrod paper specifically for this, you can make something similar using turmeric and regular paper - see this instruction guide from Educational Innnovations.

Step 1: Mix Acid and Base Solutions

Warning: we are using things like baking soda and citric acid which are safe to handle and only mildly acidic or basic. If in doubt about a household chemical, DON'T use it. Mixing some chemicals in your house like bleach and ammonia can create toxic fumes and harm or kill you, so use common sense and don't mix chemicals together if you are uncertain of the results.

The first step in creating our paper chemistry artwork is to mix up two solutions - one of which is acidic, meaning it has a lower pH, and the other basic, meaning a higher pH. We'll be using baking soda for our basic solution and citric acid for our acid solution. Take two bowls and add about 1/8 to 1/4 cup of water to each. You don't really need much water here. Mix in about a teaspoon of baking soda to one of the bowls, and a teaspoon of citric acid to the other bowl. Keep them separate so you know which one is which! Be careful not to inhale the citric acid powder or baking soda, and don't drink either of these solutions.

Notice that both of these solutions are clear when the powder is mixed with the water, and might be hard to tell apart. But they are very different in their chemical composition, and we'll be using that difference to create our different colors.

Other safe household solutions you can try with this activity are soapy water for the basic solution like using dish soap and water or laundry detergent, and vinegar or lemon juice for the acidic solution. Again, stick to things that are safe to handle with your bare hands or are explicitly edible.

Step 2: Test the PH of Your Solutions

We like to test the pH of our solutions to see how acidic or basic they are, and learn a bit about the pH scale. Test strips normally have a color chart to show the corresponding pH of a chemical you are testing to the color the paper changes to. pH stands for "power of Hydrogen" or "potential of Hydrogen" and refers to the concentration of free Hydrogen ions there are in a solution - more of them means a more acidic solution, fewer of them means more basic. Pure water is 7 on the scale, right in the middle between the range of 0-14, and is a neutral pH. The paper strips and the the goldenrod paper we are using both react somewhat similarly to acids and bases - they change color based on how acidic or basic the solution is.

The test strips are a universal indicator, meaning that they measure most or all of the full range of the pH scale from 0-14 and change through different colors of the rainbow based on how acidic or basic the solution is. Your pH strips should have a color chart that comes with them, and we included one in the kit instructions to refer to. If you are using this with a class of students, the American Chemical Society has a .pdf you can download with multiple images of the scale on one sheet to cut out and distribute to students.

To test the pH of your solutions, take a paper test strip and simply dip it in the solution. It will soak up the solution, and will change color a bit. If the solution is a neutral pH - like pure water - it won't change much in color, indicating that the solution has a neutral pH. If it's acidic, it will be more orange or red and have a lower pH number. If it's basic it will have a higher number and will be darker green to dark purple. It helps to hold it up next to the chart so you can directly compare the colors. Record your pH readings if you desire.

Step 3: Create Some Chemistry Art!

Once we've mixed up our solutions and have done some exploration as to their pH, it is time to get creative. We like to use cotton balls and cotton swabs for our paintbrushes, but you are welcome to use other absorbent items. Soak up some of the basic solution in a cotton ball, swab, or regular paintbrush and brush it on the paper. What happens? It should change to a dark orange or deep red hue - remember that this sort of depends on how basic your solution is. Try painting a few strokes to see what happens.

To create a different color - basically back to the original color of the paper - you can paint on the acidic solution. It changes the color of the paper to the goldenrod, maybe even a bit paler yellow. See what happens when you switch these back and forth!

If you would like to write a secret message on the paper to only be revealed in the presence of a basic solution, draw it on the paper with paraffin wax or a yellow crayon - the yellow color of the crayon is almost impossible to see, and we find crayons much easier to use for younger learners than a chunk of wax. Draw a picture or message then wipe some of the basic solution over the top of it. What happens? This artistic process is called a wax resist method, meaning that the wax prevents the paper from soaking in the chemicals and it retains its orange color while the paper around it changes to red with the basic solution soaked in. Making thicker lines here helps, as the solution likes to wick underneath the wax a bit, especially if you put a lot of liquid on the paper.

Step 4: Questions to Consider

Here are some questions to consider and extensions to use when facilitating this in the classroom:

Are there some other safe solutions you can think of to test? What about orange juice or tomato juice? Milk? How might the colors of those juices interact with the colors produced by the chemical reaction?
How many times do you think this paper can switch back and forth between red and yellow?
If you have test strips left over from this experiment, try them with some other items in your fridge or cupboards - how basic or acidic are they?
Can you fold the paper in an interesting pattern, then spread the basic solution onto it to create some interesting designs?
You can easily make a "bloody handprint" by putting your hand in the baking soda solution and slapping it onto the paper (be sure to wash your hands afterward!). Can you make another kind of "stamp" out of sponges or foam?
Scientists often use pH as a way to measure the health of rivers and lakes. See this amazing video published (and embedded here with express permission) by our friends over at the Clark Fork Watershed Education Program about Phenomenal pH