Alkaline solutions can be formed if a metal oxide dissolves in water. But do oxides always dissolve to produce alkaline solutions?

Watch the following video showing the combustion of sulfur in oxygen, producing sulfur dioxide. Observe what happens when the sulfur dioxide bubbles into a solution of Universal Indicator.

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Now consider what might happen when other elements, both metals and non-metals, burn in oxygen and the products are mixed with a little water and Universal Indicator. When a little water is mixed in, and some Universal indicator is added, what would you expect to see? Predict the results here by dragging the appropriate labels to match each element.

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[We do not have a suitable video of other example elements burning in oxygen then water and UI being added. This activity encourages teachers to think through other examples, then consider the trends, to cover this core idea]

If these experiments were carried out, what would the resultant solutions look like?

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What do you notice about the oxides of the non-metals when dissolved in water?

Non-metal oxides dissolve to give acidic solutions, for example sulfur dioxide and carbon dioxide each give acidic solutions, as shown by the indicator turning red.

What do you notice about the pH of metal oxides when soluble?

Metal oxides that are soluble or sparingly soluble give alkaline solutions.

The metals in Group I and Group II of the Periodic Table give soluble metal oxides that dissolve in water to give alkaline solutions due to hydroxide ions, for example sodium hydroxide and potassium hydroxide. These are familiar laboratory alkalis. (Note that the oxides of magnesium and calcium are only very sparingly soluble, and calcium hydroxide solution is familiar as limewater.)

What is the result of testing copper oxide in water? And why?

Copper oxide is an example of other insoluble metal oxides. The water remains neutral as no oxide is dissolved. Later, we shall consider how copper oxide behaves with acids despite its lack of solubility in water.