Final answer:
Forgetting to add NaOH to the tube can result in the absence of a reaction or incomplete reactions because NaOH is essential for providing necessary hydroxide ions for many chemical processes, such as neutralization reactions and maintaining buffer solutions.
Step-by-step explanation:
If you forget to add NaOH to a test tube in a laboratory reaction, the expected reaction may not occur as designed. In the scenario where you are adding NaOH to a solution of iron(III) chloride, the NaOH is meant to react with the iron(III) chloride to form iron(III) hydroxide and sodium chloride. Without the addition of NaOH, there will be no formation of iron(III) hydroxide, and you will not observe the characteristic changes such as color change or precipitate formation that indicate a reaction has taken place.
NaOH acts as a base and participates in various reactions including neutralization reactions with acids, forming water and salt, or reactions with acidic buffers, effectively maintaining their pH levels. Its role is integral in controlling the pH and driving reactions towards completion. For instance, if NaOH is not added at the equivalence point in a titration, you would not observe the rapid increase in pH that signifies the end of the neutralization process.
In situations where chemical reactions are sensitive to pH changes, the absence of NaOH could lead to incomplete reactions or no reaction at all, as NaOH often provides the necessary hydroxide ions needed for the reaction to proceed.