6.4 Acids and Bases in Solution One of the most venomous creatures in the world is a jellyfish. Some jellyfish stings can permanently scar and even kill their victims. They use their venom to stun and paralyze their prey, including humans. The venom of some jellyfish contains bases. Bases can be neutralized by an acid. The best way to treat some jellyfish stings is to rinse the area with vinegar. Vinegar is a solution containing acetic acid, which is a weak acid that is safe for your skin. What ions do acids and bases form in water? Acids in Solution – Notice that the formulas below for some common acids begins with the letter H, the symbol for hydrogen. In a solution with water, most acids separate into hydrogen ions and negative ions. A hydrogen ion (H+) is an atom of hydrogen that has lost its electron. In the case of hydrochloric acid, for example, hydrogen ions and chloride ions form. HCl → H+ + Cl- Acid Formula Hydrochloric Acid HCl Nitric Acid HNO₃ Sulfuric Acid H₂SO₄ Acetic Acid HC₂H₃O₂ The production of hydrogen ions helps define an acid. An acid produces hydrogen ions (H+) in water. These hydrogen ions are responsible for corroding metals and turning blue litmus paper red. Acids may be strong or weak. Strength refers to how well the acid dissociates, or separates, into ions in water. A strong acid, like nitric acid, would form many hydrogen ions in water, but weak acetic acid, would form very few. Bases in solution – in the table below, many bases shown are made of positive ions combined with hydroxide ions. The hydroxide ion (OH-) is a negative ion made of oxygen and hydrogen. When some bases dissolve in water, they separate into positive ions and hydroxide ions. Sodium hydroxide in water does this: NaOH →Na+ + OH¯ Base Formula Sodium hydroxide NaOH Potassium hydroxide KOH Calcium hydroxide Ca(OH)₂ Aluminum hydroxide Al(OH)₃ Ammonia NH₃ Calcium oxide CaO Not every base contains hydroxide ions. For example, ammonia (NH₃) does not. In solution, ammonia reacts with water to form hydroxide ions. NH₃ + H₂O → NH₄+ + OH¯ Notice that both reactions produce negative hydroxide ions. A base produces hydroxide ions (OH¯) in water. Hydroxide ions are responsible for the bitter taste and slippery feel of bases, and for turning red litmus paper blue. Strong bases, like sodium hydroxide, readily produce hydroxide ions in water. Weak bases, such as ammonia, do not. Measuring pH – to determine the strength of an acid or base, chemists use a scale called pH, which ranges from 0 to 14. It expresses the concentration of hydrogen ions in a solution. pH stands for “power of Hydrogen.” The most acidic substances are found at the low end of the scale, while basic substances are found at the high end. A low pH tells you that the concentration of hydrogen ions is high and hydroxide is low. A high pH tells you the opposite is true. You can find the pH of a solution using indicator paper, which changes a different color for each pH value. A pH lower than 7 is acidic. A pH higher than 7 is basic. If the pH is 7, the solution is neutral, meaning it’s neither an acid, nor a base. What are the Products of Neutralization? What happens when you mix an acid and a base? Salt water! Look at the equation below for the reaction between equal concentrations and amounts of hydrochloric acid and sodium hydroxide: HCl + NaOH → H₂O + Na+ + Cl¯ If you tested the pH of the mixture, it would be close to 7, or neutral. In fact, a reaction between an acid and a base is called neutralization. Reactants – after neutralization, an acid-base mixture is not as acidic or basic as the individual starting solutions were. The final pH depends on the volumes, concentrations, and strengths of the reactants. Products – “Salt” may be the familiar name of the stuff you sprinkle on food, but to a chemist, the word refers to a specific group of compounds. A salt is any ionic compound that can be made from a neutralization reaction. A salt is made from the positive ion of a bases and the negative ion of an acid. The reaction of a metal with a nonmetal is a neutralization reaction and produces a salt. Look at this equation for the neutralization reaction of nitric acid with potassium hydroxide that forms potassium nitrate salt: HNO₃ + KOH → H₂O + K+ + NO₃¯ In a neutralization reaction, an acid reacts with a base to produce a salt and water. Some common salts are shown below: Salt Neutralizaion Reaction Salt Formula Use Sodium chloride NaOH + HCl → H₂O + Na+ + Cl¯ NaCl Table salt Potassium iodide KOH + HI → H₂O + K+ + I¯ KI Disinfectants Potassium chloride KOH + HCl → H₂O + K+ + Cl¯ KCl Salt substitute Calcium carbonate Ca(OH)₂ + H₂CO₃ → 2 H₂O + Ca²+ + CO₃²¯ CaCO₃ Limestone .