Chemistry daily facts

a)brown stains on Chinese wok = rust = Metal Oxide
Lemon juice contain citric acid
Acid + Metal Oxide = salt + water
Hence, Metal Oxide has removed from Chinese wok.

b)Fizzy drink tablets contain hydrogencarbonate
Heat and water cause decomposition of hydrogencarbonate to produce carbon dioxide
To prevent fizzy drink tablets become useless, it should be kept in cool dry place

a) brown stains on wok = rust = Fe2O3
lemon contain citric acid
acid dissolve the oxide.

b) contain hydrogencarbonate, wet heat and water cause decomposition.