Disappearing Ink
Just because you can’t see it, doesn’t mean it isn’t there.

Concepts
observations, inferences, acids and bases, neutralization reactions

Materials
  • 1 bottle of disappearing ink
  • Paper towel
  • Stopwatch
  • Optional: 1 bottle of ammonia

Procedure
1. Lay a paper towel flat on a table top. Place a couple of drops of ink in the center of the paper towel. Describe how the ink looks initially on the paper towel.

2. Let the ink sit for five minutes. Describe how the ink looks after five minutes have passed.

3. Make an inference which explains what you observed.

Optional because of the use of ammonia
1. After the ink has disappeared place a couple of drops of ammonia on the spot of the paper towel where the ink had previously been. Describe what happens.


2. Make an inference to explain what occurred when ammonia was placed on the paper towel.


Explanation
Disappearing ink basically consists of three things: water, sodium hydroxide (a base), and an acid-base indicator, either thymolphthalein (for blue ink) or phenolphthalein (for red ink). When the ink is dropped onto a porous material such as a paper towel it spreads out and the water in the ink reacts with the carbon dioxide in the air to form carbonic acid. The carbonic acid reacts with the sodium hydroxide to form sodium carbonate (a salt) and more water. Once the sodium hydroxide is neutralized the acid-base indicator loses its color since the base is no longer present. Hence, the ink “disappears”. In reality the acid-base indicator is still present and the addition of another base, such as ammonia or lime water will cause it to reappear.

An acid-base indicator is typically a weak acid or a weak base which changes color depending on the hydrogen ion concentration (or pH) of the solution. Changing the pH of the solution causes the indicator to change color. Some indicators have a range of colors (cabbage juice) while other indicators simple appear as a single color or don’t appear (thymolphthalein).

In a typical neutralization reaction an acid and a base react to create a salt and water. For the specific neutralization reaction occurring with disappearing ink a word equation and a chemical equation are shown below.

sodium hydroxide plus carbonic acid yields sodium carbonate and water
2 Na(OH) + H2CO3 Na2CO3 + 2 H2O

figure21.jpg
Figure 21: Disappearing ink immediately after dropped onto a paper towel (left). Disappearing ink 3 minutes later (right).

figure22.jpg
Figure 22: A base such as ammonia dropped onto the disappeared ink brings it back.

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