Understanding pH
The pH scale measures the acidity or alkalinity of an aqueous solution. This scale ranges from 0 to 14, classifying substances by hydrogen ion concentration.
A pH value of 7 indicates a neutral solution, such as pure water. Solutions with a pH below 7 are acidic, with more hydrogen ions. Conversely, solutions with a pH above 7 are alkaline, or basic, with fewer hydrogen ions and more hydroxide ions.
Examples show this range. Highly acidic substances, like stomach acid, have a pH of 1 or 2. Lemon juice, another acidic example, is between pH 2 and 3.
On the basic end, household bleach can have a pH of 12 or 13, and ammonia is around pH 11. Understanding pH helps examine how substances, including salts, interact with water.
Defining Salts
Beyond table salt, “salt” refers to a broad category of chemical compounds. In chemistry, a salt is an ionic compound formed when an acid and a base react in a neutralization reaction.
It involves a cation (positive ion from a base) and an anion (negative ion from an acid). The resulting compound is electrically neutral.
Sodium chloride (NaCl) is well-known, but many other compounds are also salts. For instance, baking soda, sodium bicarbonate (NaHCO₃), is a salt used in cooking and as an antacid.
Epsom salt, or magnesium sulfate (MgSO₄), is another common example, used in bath soaks. These examples show salts are diverse in composition and applications, beyond culinary uses.
How Salts Change pH
The pH of a salt solution, when dissolved in water, depends on the strengths of its parent acid and base. This is due to hydrolysis, where dissolved salt ions react with water.
They produce hydrogen (H⁺) or hydroxide (OH⁻) ions, altering pH. The pH effect relates to whether its ions come from strong or weak acids and bases.
For example, a salt from a strong acid and strong base (like sodium chloride) produces a neutral solution because its ions do not significantly hydrolyze water. Conversely, a salt from a strong acid and weak base yields an acidic solution.
Its cation reacts with water to produce H⁺ ions, lowering pH. Similarly, a salt from a weak acid and strong base results in a basic solution, as its anion hydrolyzes water to generate OH⁻ ions, increasing pH. The parent acid and base strengths dictate the solution’s final acidity or alkalinity.
Examples of Salt Solutions and Their pH
Examples illustrate how salts influence water’s pH. Sodium chloride (NaCl), table salt, forms a neutral solution when dissolved.
It comes from hydrochloric acid (HCl) and sodium hydroxide (NaOH). Neither sodium (Na⁺) nor chloride (Cl⁻) ions significantly react with water, so the solution remains around pH 7.
Ammonium chloride (NH₄Cl) produces an acidic solution in water. It comes from a strong acid (HCl) and a weak base (ammonia, NH₃).
The ammonium ion (NH₄⁺) from the weak base hydrolyzes water, releasing hydrogen ions and making the solution acidic, with a pH around 4.6 to 5.6.
Sodium acetate (CH₃COONa) yields a basic solution. It forms from a weak acid (acetic acid, CH₃COOH) and a strong base (NaOH).
When dissolved, the acetate ion (CH₃COO⁻) from the weak acid reacts with water to produce hydroxide ions. This increases the solution’s pH, making it basic, ranging from 7.8 to 9.2. These examples show how a salt’s chemical origins determine its impact on aqueous solution pH.