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aluminium oxide reacts with sodium hydroxide
Using moderately concentrated sodium hydroxide at temperatures of between 140 and 240degC, and pressures of up to 35 atmospheres, the bauxite is treated to produce alumina from the iron oxides in the ore. The alumina is amphoteric and so dissolves in the sodium hydroxide leaving the impurities less soluble at high pH such as iron oxides behind in a “red mud”.
This reaction releases heat from aluminium and sodium in an endothermic process. This also gives off hydrogen gas which bubbles off the surface of the mixture into the air.
Other metals that can also react with aqueous NaOH are Beryllium, zinc and lead. These dissolve in excess of sodium hydroxide to give a clear solution of Na2ZnO2 or Na2PbO2.
What are the chemical properties of aluminium oxide?
Aluminium oxide has an electronegativity of 1.5 and so it is very refractory to water. It has an ionic bond to water and so easily grabs its hydrogen atoms from the water and forms hydroxide ions.
The trend is from strongly basic oxides on the left hand side to strongly acidic ones on the right, via an amphoteric oxide (aluminium oxide) in the middle. This is because an amphoteric oxide has both acidic and basic properties.
The reactivity of the aluminium oxide with sodium hydroxide is very slow - but it still happens! A few flakes of the aluminum oxide are formed, but not many of them get into the solution. The aluminium oxide is then filtered out. It is then cooled and "seeded" with some of the previously produced aluminium hydroxide to provide something for it to precipitate around. This is called the Bayer process and it produces the alumina needed to make aluminium metal.
