S ECTION 8–3: P ROPERTIES OF A CIDS AND B ASES Physical Science Coach Kelsoe Pages 240–245

O BJECTIVES Define acid and describe some of the general properties of an acid. Define base and describe some of the general properties of a base. Identify a neutralization reaction, and describe the reactants and products of neutralization. Explain how acids and bases can be defined as proton donors and proton acceptors.

I DENTIFYING A CIDS An acid is a compound that produces hydronium ions (H 3 O + ) when dissolved in water. Recall that when hydrogen chloride gas dissolves in water, it ionizes and forms hydronium and chlorine ions. HCl + H 2 O  H 3 O + + Cl - Acids have certain chemical and physical properties that are similar. Some general properties of acids include sour taste, reactivity with metals, and ability to produce color changes in indicators.

S OUR T ASTE Foods that taste sour often contain acids. Lemons, grapefruits, limes, and oranges all contain citric acid. The vinegar used in salad dressings contains acetic acid, CH 3 COOH. Dairy products that have spoiled contain butyric acid. You should NEVER test an acid by tasting it!!!

R EACTIVITY W ITH M ETALS The reaction between an acid and a metal is an example of a single-replacement reaction. For example, when zinc is added to a test tube containing hydrochloric acid, bubbles form in the tube. As the zinc replaces hydrogen in hydrochloric acid, hydrogen gas and zinc (II) chloride are produced. Zn + 2HCl  H 2 + ZnCl 2

C OLOR C HANGES IN I NDICATORS An indicator is any substance that changes color in the presence of an acid or base. One of the most common indicators used is litmus, a kind of dye derived from plants called lichens. Litmus paper is made by coating strips of paper with litmus. Blue litmus paper turns red in the presence of an acid.

I DENTIFYING B ASES A base is a compound that produces hydroxide (OH - ) ions when dissolved in water. When sodium hydroxide, NaOH, dissolves in water, it dissociates into sodium ions and hydroxide ions. NaOH  Na + + OH - Some general properties of bases include bitter taste, slippery feel, and ability to produce color changes in indicators. Unlike acids, bases usually do not react with metals, but this is not a general property of bases.

B ITTER T ASTE Without sugar, chocolate tastes bitter (like baking chocolate). Cacao beans contain a base called theobromine that gives unsweetened chocolate a bitter taste. Many cough syrups and other liquid medicines contain similar bases. Fruit flavorings are often added to mask the taste of these basic solutions. NEVER test a base by tasting it!

S LIPPERY F EEL Bases feel slippery. Wet soap and many cleaning products that contain bases are slippery to the touch. When wet, some rocks feel slippery because the water dissolves compounds trapped in the rocks, producing a basic solution.

C OLOR C HANGES IN I NDICATORS Bases turn red litmus paper blue. The litmus paper will change back to red if you drop an acidic solution on it. Phenolphthalein is another example of an acid- base indicator. In a solution containing a base, phenolphthalein is red. In a solution containing an acid, phenolphthalein is colorless. Hydrangeas and other flowers contain natural indicators. Hydrangeas will be blue when grown in acidic soil and pink when grown in basic soil.

H YDRANGEAS

N EUTRALIZATION AND S ALTS When people eat fish, they sometimes squeeze lemon juice over the fish. Fish contains bases that can leave a bitter taste. Lemon juice contains citric acid that reacts with the bases in the fish. The reaction between an acid and a base is called neutralization.

N EUTRALIZATION AND S ALTS During neutralization, the negative ions in an acid combine with the positive ions in a base to produce an ionic compound called a salt. At the same time, the hydronium ions from the acid combine with the hydroxide ions from the base to produce water. The neutralization reaction between an acid and a base produces a salt and water.

N EUTRALIZATION AND S ALTS For example, when hydrochloric acid reacts with sodium hydroxide, the following neutralization reaction occurs: (H 3 O + + Cl - ) + (Na + + OH - )  2HOH + (Na + + Cl - ) acid base water salt The products of the reaction are a salt made up of sodium and chloride ions, and water. If you let the water in the resulting solution evaporate, the sodium and chloride ions would begin to crystallize out of solution, forming table salt.

N EUTRALIZATION AND S ALTS Table salt is the most common example of a salt compound. Other common salts include sodium bicarbonate, sodium carbonate, potassium chloride, magnesium chloride, calcium carbonate, and ammonium nitrate. Sodium bicarbonate, NaHCO 3, is produced during the neutralization reaction between sodium hydroxide, NaOH, and carbonic acid, H 2 CO 3. It is better known as baking soda.

P ROTON D ONORS AND A CCEPTORS Recall that hydronium ions (H 3 O + ) are produced when acids dissolve in water. When an acid and a base react in water, a proton from the hydronium ion from the acid combines with the hydroxide ion (OH - ) from the base to form water (H 2 O). Acids lose, or “donate,” protons. Bases “accept” protons, forming water, a neutral molecule.

P ROTON D ONORS AND A CCEPTORS Acids can be defined as proton donors, and bases can be defined as proton acceptors. This definition allows us to classify a wider range of substances as acids or bases. Water is neither an acid nor a base. However, using the proton-donor or proton-acceptor definition, water can act as either an acid or a base depending on the compound with which it reacts.

P ROTON D ONORS AND A CCEPTORS Water as an acid (proton donor): HOH + NH 3  OH - + NH 4 + acid base base acid Water as a base (proton acceptor): HOH + HCl  H 3 O + + Cl - base acid acid base

V OCABULARY Acid Indicator Base Neutralization Salt