Chemical solutions that change over time?

Question

Could some please name a few chemical solutions (ie: carbonate, bromide, hydroxide) which change over time. As in, their chemical compositions change so if they were to be used in a reactions lab, they would produce different reactions then if a new solution had been used.

Answer 1

Solutions can change over time by reacting with air. In particular, hydroxide solutions take up carbon dioxide from the air to make carbonate.

The solution of a reducing agent (e.g. iron(II)) can get oxidised by air.

Potassium permanganate solutions slowly decompose, especially if light gets at them, to give MnO2 and oxygen.

Changes like this would indeed lead to different chemistry.

Answer 2

In analytical chemistry, there is no such thing as a stable solution. Solvent evaporates, gases may bring impurities that change the analyte, precipitation may occur, reactions with the vessel (e.g. absorption, ion exchange) may change the effective concentration.

Light exposure can decompose substances in solution. So can temperature changes.

(Not to mention the possibility of pollution by direct exposure to contaminated lab equipment or extreme atmospheric conditions, like HSO4 vapours in a suction cabinet.)

Any solution with an exact concentration has a limited period of usability. As a rule of thumb, solutions with low concentrations degrade faster, as do solutions with volatile or redox-active substances.

Rather than dump slightly overtime solutions, one can measure the current concentrations and assign a corrective factor to the nominal concentration in the solution.

For a solution to provide completely different reactions, the changes would have to be dramatic. This can happen by bad choices for storage containers or storage shelves.