With aluminum we will deliver the first two simple experience, which is quite good broken aluminum spoon. Place a small piece of metal in the tube with any acid, even with salt. Aluminum will immediately begin to dissolve, vigorously displacing the hydrogen of the acid is formed aluminium salt АlСl3. Another piece aluminum lower in concentrated alkali solution, for example, caustic soda (be careful!). And again, the metal will begin to dissolve and release of hydrogen. Only this time formed another salt, namely aluminum salt of the acid, the aluminate NaAlO2.
Oxide and aluminum hydroxide are simultaneously both basic and acidic properties, i.e., they react with both acids and alkalis. They are called amphoteric. Compounds of tin, by the way, too atterny; check it yourself, unless of course you already took the tin from cans.
There is a rule: more than metal, so it is more likely to oxidize, corrode. Sodium, for example, can not be left in the air, it is stored under kerosene. But well-known fact: the aluminum is much more active than, for example, iron, but iron rusts quickly, and aluminum, as its no hold on the air and in water, practically does not change. That's the exception to the rule?
Put the experience. Attach a piece of aluminum wire in an inclined position over the flame of a gas burner or spirit lamp to heat up the bottom part of the wire. At 660°C. this metal melts; it would seem, it can be expected that the aluminum will start to drip on the burner. But instead of melting, the heated end of the wire suddenly slack. Look better, and you will see a thin plastic inside the molten metal. This case - alumina Al2O3, substances durable and very resistant.
Oxide thin and dense layer covers the surface of aluminum and does not allow him to continue to oxidize. This property is used in practice. For example, for plating metals on metal surfaces apply a thin aluminum layer, the aluminum immediately covered with oxide, which protects the metal from corrosion.
And two more metal, with whom we will deliver the experience, chromium and Nickel. In the periodic table they are far from each other, but there is reason to consider them together: and the chromium and Nickel cover metal products, so that they Shine, not rusted. So, backless metal bed cover is usually Nickel, automotive bumpers - chrome. Is it possible to know exactly what metal is made the floor?
Let's try to analyze. Chip a piece of flooring from old parts and leave him in the air for a few days, he managed to be covered by the oxide film, and then put into a test tube with concentrated hydrochloric acid (handle with care! Acid should not get on your hands and clothes!). If it was a Nickel, it will immediately begin to dissolve in the acid forming the salt NiCl2; this will be released hydrogen. If a brilliant layer of chromium, the first time there will be no change and only then the metal will begin to dissolve in the acid with the formation of chromium chloride СrСl3. Removing this piece of coatings from acid tweezers, rinsing with water and drying in air, after two or three days, you will again see the same effect.
Explanation: on the surface of the chromium forms a thin film of oxide, which prevents interaction of the acid with the metal. However, it dissolves in the acid, though slowly. Air chrome again covered with an oxide film. But the Nickel protective layer no.
But in this case why we kept the metals in the air before the first experience? Because chrome was already covered with a layer of oxide! And then that was covered only the outer side and the inner facing the product with oxygen in contact is not entered.