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Because it was written on my textbook that Zn and Al dissolves in acid and alkali, I tried. When I put Al in NaOH solution it reacted immediately. But when I put Zn in the same concentration of NaOH solution it didn't react. I heated and added NaOH and at last it emitted some bubbles.
This is what I observed when I reacted alkali and Al or Zn.
On the contrary, when I put Zn into 9.5% HCl solution it reacted at once. But when I put Al into the same concentration of HCl solution, it took so long until it starts to emit bubbles.
Could anyone explain why there is such a difference?

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1 Answer 1

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A "piece of aluminum" is not pure aluminum, but a sample of pure $\ce{Al}$ with a strong protective layer of $\ce{Al2O3}$. Interestingly, pure aluminum oxide forms complexes and reacts with NaOH, but $\ce{Al2O3}$ is only sluggishly attacked by $\ce{HCl}$. Here is a supporting reference:

In aqueous solutions of HCl acid aluminum wire does not dissolve due to presence of oxide film on surface.

And, a more detailed answer, per the same source:

Theoretically, the oxide film on Al is soluble in low pH solutions. However, in practice, the cathodic reaction, hydrogen reduction, results in a diffusion layer with a pH at the surface that is much higher than the bulk solution pH. This retards the rate of dissolution and can even bring it to a virtual stop if it enables a uniform passive film to form. In high pH solutions, the reverse is the case and dissolution is accelerated, instead of retarded, by the pH shift that accompanies hydrogen evolution. In less pure Al, the inclusions become dedicated sites for the hydrogen reduction reaction restricting the pH shift to their locations, and this allows dissolution to continue at anodic sites.

On the topic of zinc and HCl per an educational site:

Let’s ex­am­ine the ex­am­ple of the in­ter­ac­tion be­tween zinc and hy­drochlo­ric acid. Zinc also re­acts with HCl, re­leas­ing small bub­bles of hy­dro­gen and form­ing zinc chlo­ride Zn­Cl₂.

But why small bubbles? My answer, there is also hydrogen embrittlement occurring on the zinc surface, as highlighted in recent research, "The Effects of Zinc Alloy Electroplating on the Hydrogen Embrittlement of High Strength Steels", to quote:

Hydrogen permeation measurements were performed and indicated that the quantity of hydrogen absorbed by the steel substrates is critically dependent on the composition of the initial deposit. Large quantities of hydrogen were trapped in the coatings and continued to diffuse into the steel over extended times after the plating process was completed.

I trust this helps.

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  • $\begingroup$ Thank you very much for taking time to answer my question. I tried again with alminium foil and copper into HCl. This time bubbles formed on copper and aluminium dissolved very fast. Your answer was very helpful! $\endgroup$ Oct 3, 2022 at 6:46

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