Sodium hydroxide, an inorganic compound with the chemical formula $\text{NaOH}$, is commonly known as lye or caustic soda. It is a powerful chemical that exists as a white, odorless crystalline solid. $\text{NaOH}$ is highly soluble in water, dissolving easily to form an aqueous solution. This compound is widely utilized across industrial and household applications due to its strong reactivity. In its solid form, sodium hydroxide readily absorbs moisture and carbon dioxide from the air.
The pH Scale and the General Answer
The acidity or alkalinity of an aqueous solution is measured using the $\text{pH}$ scale, which ranges from 0 to 14. A value of $\text{pH}$ 7 is considered neutral, while values below 7 indicate acidity, and values above 7 signify basicity (alkalinity). When dissolved in water, sodium hydroxide forms a solution that typically registers a $\text{pH}$ between 13 and 14, indicating an extremely alkaline solution. The scale is logarithmic, meaning a single unit change represents a tenfold difference in concentration. Solutions in this upper range possess a significant concentration of hydroxide ions, which is the defining characteristic of a strong base.
Understanding Strong Bases and Hydroxide Ions
Sodium hydroxide is classified as a strong base because it undergoes complete dissociation when introduced to water. A strong base is defined as a compound that breaks apart entirely into its constituent ions in an aqueous solution. The $\text{NaOH}$ molecule splits into a sodium cation ($\text{Na}^+$) and a hydroxide anion ($\text{OH}^-$). This complete ionization is the reason for the compound’s high $\text{pH}$ value, as the resulting solution is flooded with hydroxide ions responsible for alkalinity. Since the $\text{pH}$ scale is inversely related to the $\text{H}^+$ concentration, the high concentration of $\text{OH}^-$ ions drives the hydrogen ion concentration to an extremely low level, resulting in the maximum $\text{pH}$ of 14 for concentrated solutions.
How Concentration Influences Exact pH
The exact $\text{pH}$ of a sodium hydroxide solution depends entirely on the concentration of $\text{NaOH}$ dissolved in the water. Chemists use molarity (moles of solute per liter) to determine the $\text{pH}$. A standard 1 molar (1 M) $\text{NaOH}$ solution has a $\text{pH}$ of 14 because the $\text{OH}^-$ concentration is 1 mole per liter. For any aqueous solution, $\text{pH}$ and $\text{pOH}$ are related by the equation $\text{pH} + \text{pOH} = 14$. As the $\text{NaOH}$ solution becomes more dilute, the $\text{OH}^-$ concentration decreases, causing the $\text{pH}$ value to drop closer to 7 (e.g., 0.001 M $\text{NaOH}$ has a $\text{pH}$ of 11).
Common Uses and Safety Precautions
Common Uses
Sodium hydroxide’s powerful alkalinity makes it indispensable in a wide range of applications, especially those requiring the breakdown of organic material. It is used extensively in the manufacturing of paper, textiles, and rayon, and it is also a key ingredient in the process of making soaps and detergents. In the home, it is a common component in drain cleaners because it reacts with fats and grease to convert them into a form of soap that is water-soluble, thus clearing blockages.
Safety Precautions
Given its high $\text{pH}$ and corrosive nature, sodium hydroxide requires careful handling. The compound readily hydrolyzes proteins and lipids, meaning it can dissolve organic tissue, causing severe chemical burns on contact with skin or eyes. When working with $\text{NaOH}$, wear appropriate personal protective equipment, including chemical-resistant gloves, clothing, and eye protection. Always add the caustic soda slowly to water, never the reverse, because the dissolving process generates significant heat that can cause the solution to boil and splatter.