Alkalis are soluble bases. An alkaline solution can be formed when a metal oxide is dissolved in water. An acidic solution can be formed when a non-metal oxide is dissolved in water. For example, magnesium oxide dissolves to form alkaline solutions. Sulfur dioxide and nitrogen dioxide however, will dissolve to form acidic solutions. Observe the colour of the indicator in each sample. Record the results in a suitable table showing the name of the oxide, the colour of the universal indicator, the pH, and whether the oxide is acidic, alkaline or neutral in water.
Additional information This is a resource from the Practical Chemistry project , developed by the Nuffield Foundation and the Royal Society of Chemistry. Level years years. Use Practical experiments. Category Acids and bases. Carbon dioxide as an acidic oxide. Those bases that dissolve in water form alkaline solutions. Related articles.
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Metal oxides which exhibit this behavior are termed basic oxides because they act as bases. The oxides of nonacidic cations are so water reactive that they are seldom seen. These oxides cannot be prepared by dehydrating the hydroxide at high temperature.
The oxides of feebly acidic cations are more common. Lime, CaO is an example. Lime can be prepared comercially by the thermal decomposition of limestone. The oxides of feebly acidic cations react exothermically with water producing the hydroxide. The hydroxides of feebly acidic cations are not deliquescent. Magnesium oxide reacts with acids as you would expect any simple metal oxide to react.
For example, it reacts with warm dilute hydrochloric acid to give magnesium chloride solution. Describing the properties of aluminium oxide can be confusing because it exists in a number of different forms. One of those forms is very unreactive. It is known chemically as alpha-Al 2 O 3 and is produced at high temperatures.
Aluminium oxide doesn't react in a simple way with water in the sense that sodium oxide and magnesium oxide do, and doesn't dissolve in it. Although it still contains oxide ions, they are held too strongly in the solid lattice to react with the water. Note: Some forms of aluminium oxide do, however, absorb water very effectively. I haven't been able to establish whether this absorption just involves things like hydrogen bonds or whether an actual chemical reaction to produce some sort of hydroxide occurs.
If you have any firm information on this, could you contact me via the address on the about this site page. Aluminium oxide contains oxide ions and so reacts with acids in the same way as sodium or magnesium oxides. That means, for example, that aluminium oxide will react with hot dilute hydrochloric acid to give aluminium chloride solution. In this and similar reactions with other acids , aluminium oxide is showing the basic side of its amphoteric nature.
Aluminium oxide has also got an acidic side to its nature, and it shows this by reacting with bases such as sodium hydroxide solution. Various aluminates are formed - compounds where the aluminium is found in the negative ion. This is possible because aluminium has the ability to form covalent bonds with oxygen. In the case of sodium, there is too much electronegativity difference between sodium and oxygen to form anything other than an ionic bond. But electronegativity increases as you go across the period - and the electronegativity difference between aluminium and oxygen is smaller.
That allows the formation of covalent bonds between the two. Note: If you aren't happy about electronegativity you will find it explained if you follow this link. Use the BACK button on your browser to return quickly to this page later.
With hot, concentrated sodium hydroxide solution, aluminium oxide reacts to give a colourless solution of sodium tetrahydroxoaluminate. Note: You may find all sorts of other formulae given for the product from this reaction.
These range from NaAlO 2 which is a dehydrated form of the one in the equation to Na 3 Al OH 6 which is a different product altogether. What you actually get will depend on things like the temperature and the concentration of the sodium hydroxide solution.
In any case, the truth is almost certainly a lot more complicated than any of these. This is a case where it is a good idea to find out what your examiners quote in their support material or mark schemes, and stick with that. If necessary, get this sort of information from your examiners if you are doing a UK-based course by following the links on the syllabuses page. By the time you get to silicon as you go across the period, electronegativity has increased so much that there is no longer enough electronegativity difference between silicon and oxygen to form ionic bonds.
Silicon dioxide has no basic properties - it doesn't contain oxide ions and it doesn't react with acids. Instead, it is very weakly acidic, reacting with strong bases.
Silicon dioxide doesn't react with water, because of the difficulty of breaking up the giant covalent structure. Silicon dioxide reacts with sodium hydroxide solution, but only if it is hot and concentrated. A colourless solution of sodium silicate is formed. You may also be familiar with one of the reactions happening in the Blast Furnace extraction of iron - in which calcium oxide from the limestone which is one of the raw materials reacts with silicon dioxide to produce a liquid slag, calcium silicate.
This is also an example of the acidic silicon dioxide reacting with a base. For the remainder of the oxides, we are mainly going to be considering the results of reacting them with water to give solutions of various acids. When we talk about the acidity of the oxides increasing as you go from, say, phosphorus V oxide to sulphur trioxide to chlorine VII oxide, what we are normally talking about is the increasing strengths of the acids formed when they react with water.
Phosphorus III oxide reacts with cold water to give a solution of the weak acid, H 3 PO 3 - known variously as phosphorous acid, orthophosphorous acid or phosphonic acid. Its reaction with hot water is much more complicated. Note: Notice the "-ous" ending in the first two names. That's not a spelling mistake - it's for real! It is used to distinguish it from phosphoric acid which is quite different see below. The names of the phosphorus-containing acids are a bit of a nightmare!
In fact, as far as I'm concerned, the phosphorus acids in general have always been and continue to be a complete nightmare! Don't get too worried about these names at this level. Just be sure that you can write the formulae if you need to - and be grateful that you don't need to know all that much else about them!
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