Diving deeper: secrets hiding in aluminum cans

What’s hiding inside aluminum cans? Just the drink, or…?

Safe­ty pre­cau­tions

  • Wear pro­tec­tive gloves, glass­es, and a mask.
  • Work in a well-ven­ti­lat­ed area.
  • Per­form this ex­per­i­ment un­der adult su­per­vi­sion only!

Reagents and equip­ment

  • var­i­ous soft drinks in alu­minum cans;
  • sand­pa­per;
  • beakers;
  • drain clean­er;
  • wood­en rods;
  • glass con­tain­er.

Step-by-step in­struc­tions

Buff the top lay­er of paint off of the alu­minum cans. Open the cans and use the wood­en rods to sus­pend them in suit­ably-sized beakers. Add some drain clean­er (usu­al­ly a 10% sodi­um or potas­si­um hy­drox­ide so­lu­tion). Ob­serve a tu­mul­tuous re­ac­tion and re­lease of gas. Wait two hours. Take the cans out of the liq­uid. The alu­minum may have dis­solved, but the drinks haven't leaked out!

Process de­scrip­tion

These or­di­nary alu­minum cans are con­ceal­ing a de­vi­ous se­cret – they are cov­ered with a pro­tec­tive lay­er not only on the out­side, but on the in­side as well! When we re­move their paint coat­ings, we ex­pose the alu­minum, which eas­i­ly re­acts with a drain clean­er that con­tains an al­ka­line com­po­nent. But even when the alu­minum has dis­solved com­plete­ly, the drinks will not leak. The cans con­tain a se­cret sec­ond ma­te­ri­al – a lay­er of plas­tic that keeps the drinks them­selves from in­ter­act­ing with the alu­minum.

Car­bon­at­ed drinks con­tain what are known as “acid­i­ty reg­u­la­tors,” the most pop­u­lar of which are or­thophos­pho­ric and cit­ric acids. They are used to acid­i­fy foods, giv­ing them a sour and slight­ly bit­ter taste. These acids can re­act with var­i­ous met­als, in­clud­ing the alu­minum com­pris­ing the soda cans. A spe­cial plas­tic coat­ing is there­fore ap­plied to the in­side of the cans to pre­vent the two from in­ter­act­ing. This plas­tic is most of­ten an epoxy resin (poly­eth­yl­ene, polypropy­lene, polyester, or oth­er poly­mers may be used in some cas­es). These ex­treme­ly-preva­lent epoxy resins con­tain bisphe­nol A (BPA), a sub­stance that usu­al­ly evokes a mixed re­ac­tion from con­sumers. Bisphe­nol A has the abil­i­ty to mim­ic nat­u­ral hu­man hor­mones; name­ly, it is struc­tural­ly sim­i­lar to es­tro­gen. Over the past 20 years, it has un­der­gone thor­ough in­ves­ti­ga­tion to de­ter­mine any ef­fects it may have on the hu­man body. Ac­cord­ing to ex­ten­sive re­search and risk as­sess­ments con­duct­ed by both gov­ern­men­tal and in­de­pen­dent or­ga­ni­za­tions world­wide, bisphe­nol A is safe at nor­mal ev­ery­day lev­els of ex­po­sure from food and bev­er­age con­tain­ers. A Cana­di­an De­part­ment of Health re­port con­clud­ed that an av­er­age adult would need to con­sume 940 canned drinks in one day to even get close to the “tol­er­a­ble dai­ly in­take” lim­it for BPA ex­po­sure.

When the alu­minum cans are pro­cessed for re­cy­cling, the films burn up at the high tem­per­a­tures in the con­vec­tion fur­naces.

2Al + 2NaOH + 6H₂O = 2Na[Al(OH)₄] + 3H₂

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