Question

can we get concentrated acid back from dilute acid?

Answer 1

Hey!

aqueous HCl is a solution of HCl gas in water. At 20C and 1013mbar the maximum concentration achievable is 39.1% (i.e. 100g of a concentrated HCl solution in water will contain 39.1g of HCl).  At boiling point (110C) the equilibrium concentration is 20% HCl.  So, heating a solution of concentrated HCl (typically 36.5%) to 110C will cause HCl gas to be removed until its conctration in solution reaches 20%.  Conversely, if one heats a diluted aqueous solution of HCl (e.g. 7%) to 110C, water will be removed until the concentration of HCl in solution has reached 20%.

Similar considerations apply to HNO3; here the equilibrium concentration at boiling point (121C) is 68%.H2SO4 and H3PO4 are the only mineral acids to exist in pure (water free) form though in this form both are very hygroscopic.

Answer 2

$\huge \red {AnsWer}$

Large amount of heat is produced on mixing the acid with water. Hence, the acid is always diluted by adding the acid to water slowly and with constant stirring.

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