Video Transcript
A metallic element reacts slowly with water to form a dilute colorless solution, which is tested as shown. In which of the test tubes A, B, and C would a precipitate be observed?
First, let’s analyze the reaction between the unknown metal and water. We know that this reaction was slow. This tells us that the metal in question is reactive enough to produce an aqueous solution and a colorless gas when in water. However, it is likely not a very reactive metal, such as an alkali metal, since the reaction was not rapid or vigorous. A metal reaction with water generally produces a hydroxide whose chemical formula depends on the identity and charge of the metal cation. Hydrogen gas is also produced, which is likely the colorless gas indicated in the diagram. The solution was tested using litmus paper, which turned blue. Litmus paper turns blue in the presence of a basic solution. This test confirms that a metal hydroxide is likely produced since metal hydroxide solutions are basic.
Additionally, a flame test was conducted on the alkali solution. Flame tests are qualitative tests used in chemistry to determine the identity of a metal cation in a solution or salt based on the color of flame produced. When heated over a Bunsen burner, the solution produced a red flame. A red flame can indicate the presence of lithium ions or strontium ions. Because the initial reaction with water was slow, it is unlikely that the metal is an alkali metal, so it is unlikely that lithium ions produced the red flame. So the solution probably contains strontium ions. Using this information, we can deduce that the metal reacted with water was strontium and the aqueous solution produced is strontium hydroxide.
Now let’s have a look at the three test tubes to determine which would produce a precipitate with strontium hydroxide. Test tube A contains sodium hydroxide. When strontium hydroxide and sodium hydroxide are combined, these two soluble reactants would remain as ions in solution and no visible reaction would occur. Test tube B contains aqueous sulfuric acid and when combined with strontium hydroxide would result in an acid–base reaction. Since strontium hydroxide is an alkali solution, it would behave as a base in this reaction. In an acid–base reaction, a salt and water are produced. The salt produced by this reaction would be strontium sulfate. Strontium sulfate is an insoluble compound and would form a white precipitate. We would observe a precipitate in test tube B.
Test tube C contains a solution of sodium carbonate. Strontium hydroxide and sodium carbonate would react in a double-replacement reaction, producing the compound strontium carbonate and sodium hydroxide. Strontium carbonate is another insoluble compound that would appear as a white precipitate. Therefore, the test tubes in which a precipitate would be observed would be B and C.
