Science Buddies: "Ask an Expert"

I am doing an experiment with lemon juice and apples to see how good the lemon juice will act as a preserver for the apple. I am confused about why the citric acid and sulfur dioxide acts as a preserver. Please help to explain simple chemical reactions to me!

Thanks

Hi,

Both citric acid and sulfur dioxide are reducing agents. That means they they are preservatives because they react with oxygen in the air more easily than the apple slices do. When apple slices react with oxygen they turn brown.

You are talking about what are called “redox reactions”. In these reactions a reducing agent is oxidized, and an oxidizing agent is simultaneously “reduced”. In essence this means that electrons are transferred from the reducing agent to the oxidizing agent, and usually, but not always, the chemical bonds of both reactants are rearranged.

This is the same sort of thing that happens when you burn carbon (like coal) in oxygen(or air). The carbon reacts with oxygen ro form carbon oxides. The carbon is oxidized (gives up or donates electrons to oxygen) and the oxygen is reduced (accepts the electrons), and the molecules of both coal and oxygen rearrange so the carbon and oxygen are bonded to each other rather rhan to other carbon or oxygen atoms of the same element. The other thing that happens is that heat is given off (the reaction is “exothermic”).

I hope this helps!

Best regards,

Barrett L Tomlinson

Thank you so much! This helps a lot.

Layla