Explain why zinc dissolved in dilute HCl to liberate hydrogen gas but from concentrated H2SO4 the gas evolved is SO2? 

Metals which are more reactive than hydrogen react with acids to form the corresponding metal salt and hydrogen gas. Following is the general chemical equation for the reaction

 Metal + Acid →  Metal Salt + Hydrogen

Zinc is more reactive than hydrogen so placed above hydrogen in the reactivity series. Hence, with both dilute HCl and dilute H2SO4, it will show the same reaction that is formation of the metal salt with evolution of hydrogen gas. 

Zn (s) + 2HCl (aq) → ZnCl2 (aq) + H2 (↑)

Zn (s) + H2SO4 (aq)  → ZnSO4 (aq) + H2 (↑)

With dilute HCl, zinc chloride is formed while with dilute H2SO4, zinc sulphate is formed. In both cases, hydrogen gas is liberated. 

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 metals always reacts with acids to liberate hydrogen.so,Zn when reacts with H2SO4 liberates H2 and not SO2.

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