Video: Applying Knowledge of the Impact of the Introduction of a Base to Water on Its pH and Hydroxide Ion Concentration

For statements I and II state for each if they are true or false. I) When a base is added to pure water, it decreases the water’s pH. II) Adding a base to water raises the hydroxide ion concentration in the water. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) When a base is added to pure water, it decreases the water’s pH. II) Adding a base to water raises the hydroxide ion concentration in the water. If both are true, state if II is a correct explanation for I.

A base is a substance that produces hydroxide or OH⁻ ions when it’s dissolved in water. There are multiple ways to define a base. This particular definition is called the Arrhenius definition of bases. It is probably the simplest way to define a base. And it will work for the purposes of this problem.

A common example of bases are metal hydroxides, such as sodium hydroxide or NaOH, which dissociates in water to form sodium ions and hydroxide ions. Another example of a base is ammonia or NH₃. When ammonia reacts with water, it takes one of water’s hydrogens to form ammonium and hydroxide ions.

We’re going a little bit out of order here. But as we can see, from what we know about bases so far, statement II is true. Adding a base to water raises the hydroxide ion concentration in the water because bases are substances that produce hydroxide ions when they’re dissolved in water.

Now we still have to answer statement I, which is asking us about the relationship between a base and the water’s pH. pH is defined as the negative log of the concentration of hydrogen ions in a solution. You may also see it defined as the negative log of the concentration of hydronium ions, or H₃O⁺. This is because when hydrogen ions are introduced to water, they react with the water almost immediately to form hydronium ions.

We’re interested in the pH of a basic solution. And like we’ve just discussed, bases are substances that produce hydroxide ions. But the pH is defined in terms of the concentration of hydrogen ions. So how do we determine what the pH of the basic solution will be?

In any aqueous solution, whether it’s acidic, basic, or neutral, water can react with itself in a process known as autoionization or self-ionization. In this equilibrium reaction, water reacts with itself to form hydrogen ions and hydroxide ions. This means that, even in a basic solution, both hydrogen ions and hydroxide ions are always going to be present. Since the autoionization of water is an equilibrium reaction, we can write an equilibrium expression for it. This equilibrium expression will be equal to the concentration of hydroxide ions times the concentration of hydrogen ions in the solution.

Since this is an equilibrium reaction, we can use Le Chatelier’s principle, which tells us that the position of equilibrium will move to counteract a change in conditions, to think about what will happen when we add a base to pure water. In pure water, the concentration of hydrogen ions and the concentration of hydroxide ions are equal. When a base is added to the pure water, the concentration of hydroxide ions in the water will increase. The equilibrium will then shift to counteract the addition of the hydroxide ions by decreasing the concentration of hydrogen ions.

We know that the pH is defined as the negative log of the concentration of hydrogen ions in a solution. According to this definition, a decrease in the pH corresponds to an increase in the concentration of hydrogen ions. And conversely, an increase in the pH corresponds to a decrease in the concentration of hydrogen ions. Like we’ve just discussed, when a base is added to water, the concentration of hydrogen ions decreases, which means that the pH will increase.

Statement I says, “When a base is added to pure water, it decreases the water’s pH.” As we’ve just discussed, the opposite is true. When a base is added to water, the pH will increase because the concentration of hydrogen ions decreased. So statement I is false. Since statement I was false and statement II was true, we don’t have to state if II is a correct explanation for I.

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