Why can copper oxide not be used to make an alkaline solution?
Copper oxide is an insoluble base. That means that it does not dissolve in water to give an alkaline solution but it does react with acids to produce a salt plus water. So, if you add copper oxide to water, you just get a cloudy black liquid which is still at pH7. When you filter the liquid, the water goes through the filter paper leaving the black powder on it. However, if you add copper oxide to hydrochloric acid, a reaction takes place producing copper chloride (which is soluble and blue/green in colour) and water. So the copper oxide will initially appear to dissolve forming a blue/green solution but when all the hydrochloric acid has reacted any additional copper oxide added will remain undissolved and make the liquid black and cloudy. If this solution is filtered and then evaporated, green crystals of copper chloride will be formed.
