Question

Why should curd and sour substances not be kept in brass and copper vessels?

Answer 1

Hint : We have studied that acids react with active metals and produce salt and hydrogen . We know that every compound or salt obtained by these reactions are not useful, sometimes they may be harmful. We got this information from chemistry that curd or yogurt contains lactic acid and sour substances for examples lemons contains citric acid which can follow the reaction of acid with metal .We know that ;
                                             Metal + Acid$ \to $ Salt + Hydrogen

Step-By-Step answer:
Lactic acid is an organic acid which contains $ - COOH$ group. Hence it is a weak acid due to the presence of the $ - COOH$ group. Lactic acid develops in cow milk because of breaking down of bacteria. Fresh milk does not contain lactic acid. Dairy products such as yogurt, cheese and condensed milk contain high levels of lactic acid. The sour taste of substances is because they contain acid such as vinegar contains acetic acid, tamarind and grapes contains tartaric acid, lemons and oranges contains citric acid etc. Hence when these acids react with metals they give salt and hydrogen. If these kinds of substances which possess acids are kept in copper containers or vessels they will react and produce salt and hydrogen and the vessel will be corroded. It can also produce harmful products that may spoil the food.

Note: Copper metal is placed below hydrogen in the reactivity series which is a very small difference which can be overcome by the difference in the duration of contact, temperature and oxidative nature of acids etc. Thus it is clear now that we should not keep substances which contain any acid in the metal vessels.