To which of the following would the addition of an equal volume of \(0.60 \mathrm{M} \mathrm{NaOH}\) lead to a solution having a lower pH: (a) water, (b) \(0.30 \mathrm{M}\) HCl, (c) \(0.70 M \mathrm{KOH}\) (d) \(0.40 M \mathrm{NaNO}_{3} ?\)

The pH scale is a measure of how acidic or basic a solution is. It runs from 0 to 14, with 7 being neutral. Solutions with a pH less than 7 are considered acidic, while those with a pH greater than 7 are considered basic or alkaline. The scale is logarithmic, meaning each whole pH value below 7 is ten times more acidic than the next higher value, and each whole pH value above 7 is ten times more basic than the next lower value.

For example, a pH of 3 is ten times more acidic than a pH of 4. This is important because chemical reactions in solutions, including biological systems, often depend on pH. Enzymes in the human body, for instance, work optimally at specific pH levels.

Strong acids and bases are substances that completely dissociate into ions in water, contributing to the acidity or basicity of a solution. Examples of strong acids include hydrochloric acid (HCl) and sulfuric acid (H2SO4), while examples of strong bases include sodium hydroxide (NaOH) and potassium hydroxide (KOH).

Because strong acids and bases dissociate fully, they shift the pH of a solution significantly, even at low concentrations. This is contrasted with weak acids and bases, which only partially dissociate in solution and have a less pronounced effect on pH.

A neutralization reaction occurs when an acid and a base react to form water and a salt. This type of reaction can be thought of as a double replacement reaction where the hydrogen ions (H+) from the acid combine with the hydroxide ions (OH−) from the base to make water (H2O), and the remaining ions form the salt.

The general equation for a neutralization reaction is: \(\text{acid} + \text{base} \rightarrow \text{water} + \text{salt}\). If a strong acid reacts with a strong base, the resulting solution is often neutral. However, if a strong acid reacts with a weak base, or vice versa, the resulting solution may still be acidic or basic, depending on the strengths of the reactants.

The concentration of a solution refers to the amount of solute that is dissolved in a certain volume of solvent. It is typically expressed in units of molarity (M), which is moles of solute per liter of solution. Knowing the concentration of solutions is crucial for predicting the outcomes of reactions, including neutralization.

For instance, when you add a strong base like NaOH to a neutral solution such as water, the concentration of hydroxide ions increases, increasing the solution's pH. If added to an acidic solution, the hydroxide ions from the base will react with the hydrogen ions from the acid to form water, reducing the solution's acidity. The change in pH after mixing depends on the original concentrations of the acid and base.