Fizzing oranges and lemons

We've been doing a bit of cake baking recently, and have made both a lemon cake and an orange cake.  On both occasions we've done a bit of chemistry too!  After zesting and juicing the citrus fruit, we've kept the remainder and I've put them in something to hold the skins such that they can be a container for chemical reactions.  The reaction in question is that of citric acid in the fruit with bicarbonate of soda, producing carbon dioxide.

Acid-base reaction in lemon skins

For the lemons, I used an egg box to hold the skins (which I put a little food colouring inside to make it more amusing) whilst my sons (3 and 1) added bicarbonate of soda.  We used bits of cardboard as spatulas as I didn't want my youngest son to think it was something he should eat, but a teaspoon would actually work better.  They sat down together, listened whilst I explained what to do and added the bicarb a little at a time (with a few spillages, although I was fully expecting a larger mess and some squabbling about sharing!).  

They saw the bubbles and listened to the popping sound - the youngest one said "wow" and his brother taught him the word "fizz".  I'd kept back a bit of the lemon juice and we added a little more of this once the fizzing stopped, and more bubbles appeared.  We talked about how the reaction must have run out of the acid (i.e. it was rate limiting) and when we added more, there was extra bicarb which was able to react to make more carbon dioxide.

Coloured bubbles

My 3 year old then decided to show his brother how colour mixing works and they used their soggy cardboard spatulas to move coloured bubbles between lemon skins to see what colours they could make (the little one has recently learnt several colours and he enjoyed using his new words).  All in all it provided about 15 minutes of entertainment with no disagreements which was pretty miraculous!

Oranges are not as acidic

We've since made an orange cake, and we did a very similar activity.  I put the orange skins in a bun tin as they were bigger, and the boys added bicarb.  There wasn't as much fizzing as for the lemons, and we talked about why this could be - my son thought maybe there wasn't as much acid.  Lemons have a higher citric acid concentration than oranges, and although we kept more orange juice back to add, there weren't many bubbles.  

I asked my son what we could do to make them fizz more and he suggested adding lemon juice.  I found some vinegar instead (cheaper!) and squirted a bit into each orange, which then fizzed to the delight of the little one.  After a couple of minutes I was asked to add colours, which I did, and we also discovered that the red food colouring was acidic as it fizzed when added.  Some more collaborative colour mixing ensued before they decided to go and do something else (which wasn't as harmonious...).  They both enjoyed themselves, so we'll definitely do this again next time we're cooking with citrus fruit!