I'm trying to write-up an activity sheet about the "invisible ink" experiment with lemon juice.

 

For those who are not familiar with this -- you take lemon juice and use it as ink to write a "secret message" on a piece of paper. You wait for it to dry up, then when you're ready to read it, you heat it up. The juice becomes brownish.

 

I want to explain WHY this happens, and I can't find any good resources. I keep reading the explanation that the lemon juice "weakens the paper" and when you heat it up the previously lemon'ed parts burn faster (hence turning brown). I am skeptical of this explanation, it sounds weird to me.. if that's true, it would happen on an open flame and not, say, heating by lamp. And yet it DOES happen when you put it on a hot lamp.

 

Is this some chemical reaction due to heat? What's going on here? Anyone? Is it true that the message on the paper is just "burning faster" than the rest of the paper?

 

Help..

 

~mooey

 

p.s, here's an example of this experiment and the proposed explanation http://chemistry.about.com/cs/howtos/ht/invisibleink3.htm

Well it does work - I did it as a child, but we put the paper under the grill; it was all a bit charred but the writing was clear. The impression I got was that the paper had to be on the point of all burning - didn't realise you could do it just by warming it up (and sunlight?? seems like a bad idea for invisible ink). I believe it works with urine as well - perhaps it's an acid thing, but I am already out of my chemical depth so I won't speculate further.

 

wikipedia claim

 

Lemon, apple, orange or onion juice (organic acids and the paper forms ester under heat)

This book claims that the acids break down the cellulose into sugars which then caramelize

Edited June 6, 201114 yr by imatfaal

Well it does work - I did it as a child, but we put the paper under the grill; it was all a bit charred but the writing was clear. The impression I got was that the paper had to be on the point of all burning - didn't realise you could do it just by warming it up (and sunlight?? seems like a bad idea for invisible ink). I believe it works with urine as well - perhaps it's an acid thing, but I am already out of my chemical depth so I won't speculate further.

 

Yeah the explanation that keeps reapeating is that the acidic lemon juice weakens the molecular bonds of the paper. So later when it's burned, the parts that were "dyed" with the lemon juice would burn quicker (and darken quicker). It was just a bit odd to me, and I wanted to make sure that's the right explanation.

Acid catalysed dehydration of the cellulose. Roughly the equivalent of caramelising sugars.

Acid catalysed dehydration of the cellulose. Roughly the equivalent of caramelising sugars.

 

Sorry, chem isn't my strong side. Can you explain (or point me to the direction of more data)?

 

Also, does that mean that ANY heat would do this? not just "burn" ? I shall try.

I always assumed that it is the result of burning the organic substances.

Okay, I found this site: http://www.thenakedscientists.com/HTML/content/kitchenscience/food/exp/invisible-inks/

 

This site goes over many fruit juices and other liquids. Seems it's mostly the sugar. BUT, it doesn't go over lemon juice, so I"m still not sure lemon juice follows that. Though it DOES have sugars in it, so that makes sense.

 

On the other hand, I tried to caramelize the lemon juice on a spoon (heated over flame) and failed... soooo... it doesn't seem to caramelize? I'm not sure what's up.

 

Any ideas?

It wouldn't be the lemon juice that's caramalising. The citric acid would be reacting with the cellulose in the paper to form glucose. The glucose is what you're burning.

For those who don't know, cellulose is a polymer of glucose units.

!!! that makes so much more sense!

 

I knew the whole "burning the paper easier" bit was weird... thanks!

!!! that makes so much more sense!

 

I knew the whole "burning the paper easier" bit was weird... thanks!

 

 

No problems! I have been meaning to reply to this since you posted it actually. Sorry it took so long!

That sounds like a really fun experiment - I might try it with my niece and nephew. I think they'll be a bit young to understand the science but it'll certainly amaze them - no harm in getting them hooked early!

Edited June 9, 201114 yr by Kerry

Kerry, just be extra careful with the heat. I found that the experiment WORKS with an open flame (like a candle) but it gets very tricky to control it from burning the paper. A heat source like a toaster (the heat above it, of course, not inside a toaster oven) will be sufficient and probably safer.

 

Just be careful

 

No problems! I have been meaning to reply to this since you posted it actually. Sorry it took so long!

 

I can't find resources on this, and I'd love to add something about this to the kit in the "Extra resources to read" section (links and such)... any ideas ? links will be very welcome.

Toaster it is then! Thanks