Lesson 10: Useful Information
Here is some information that will be useful as you try to analyze your unknown solutions. (These are also given in Examples 4 and 5 in your workbook.)
Acid Formulas
It is very useful to be able to identify which chemicals are acids and which are bases. H+ in a formula or equation indicates an acid. HCl is the formula for hydrochloric acid. Hydrogen with chlorine or any other halogen or sulfur makes an acid. Also, chemicals with formulas like HXO where X is nonmetal are generally acids.
Acid Properties
Here are two very convenient properties of acidic solutions. They change the color of litmus paper from blue to red. Acidic solutions will also change the color of pH paper to various shades of red and orange that correspond to pH values of less than 7.
Base Formulas
Bases can also be recognized by their formulas. Hydroxide ion is a base. Chemicals with formulas like MOH where M is a metal are bases. Also, ammonium hydroxide is a base, as is ammonia.
Base Properties
Here are some of the convenient properties of basic solutions. Litmus paper will turn from red to blue and phenolphthalein will turn pink in the presence of basic solutions. Also, pH paper will turn to various shades of blue and green with basic solutions.
Reaction of Ammonium Ion
If you have an ammonium ion in a compound and you add base (that is, hydroxide ion) to it, you will form NH4OH, which will decompose to give NH3, which comes off as a gas. That arrow pointing up in the equation indicates that it is released as a gas. You won't see any bubbling, but you should be able to smell the ammonia. Sometimes it is necessary to warm the solution to help force the NH3 into the gaseous state. The presence of ammonia vapor can also be detected by its reaction with wet red litmus paper held above the test tube (not in the solution.)
Reaction of Carbonate Ion
Another very special and useful reaction is the reaction of carbonate and bicarbonate ions with acids. If a solution contains carbonate or bicarbonate, it can react with acids to form H2CO3, which then decomposes to give off carbon dioxide gas. In this case, you will see bubbles.
Precipitation Reactions
A more general kind of reaction, but still very useful, is the precipitation reaction. If you use the complete formula for each compound, then the precipitation reaction can be viewed as a double displacement reaction in which one of the products happens to be insoluble in water. You start with two compounds which are both soluble in water and by mixing them together, you form a compound which is insoluble in water. Because it is insoluble it will form a solid and drop out of solution.
You will be using this kind of reaction extensively, along with the solubility rules in the following way. Select a pair of compounds to work with. Use the formulas of the compounds to predict what the products of a reaction would be by completing and balancing the equation. Then determine whether the products are soluble or insoluble in water by looking at the solubility rules. If one of the products is insoluble, then you can predict that a precipitation reaction will take place when the solutions are mixed.
Amphoteric Hydroxides
Another very useful phenomenon is amphoterism. Amphoterism has to do with the ability of a material to act either as an acid or as a base, depending on the conditions in which it finds itself.
The reaction of lead(II) ion with hydroxide ion gives an example of amphoterism. Lead(II) ion will react with two hydroxide ions to form lead(II) hydroxide. That compound is insoluble in water and will precipitate. Lead(II) hydroxide is an amphoteric compound because it can act as either an acid or a base. |
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It is a base because it contains hydroxide ions. As a base, it can react with acids. When it does, the precipitate dissolves and the lead ion goes back into solution. |
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However, if you add more hydroxide, the extra hydroxide will react with the lead(II) hydroxide to form a complex ion, lead(II) tetrahydroxide ion, which is soluble in water. Therefore, lead (II) hydroxide, is also an acid, because it can react with a base (OH-). {The term complex ion refers to an ion which surrounded by and bonded to other molecules or ions forming a cluster which, as a whole, has a charge.} |
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Pb(OH)2(s)
What happens experimentally is that if you add a little bit of base to lead(II) ion, you form a product that's insoluble in water, but if you add enough additional base to that, you can get it to dissolve again. It is a very useful property at times, if you're trying to identify certain kinds of compounds, those which contain lead ion, for example.