Acids, Bases, And the pH Scale

Name some acids and bases that are familiar to you. Pause on the question and ask the students to answer orally, then click to show the pictures. Compare to the students’ answers.

What are some Properties of Acids? Chemical formula always begins with “H” HNO3 Sour taste Turn litmus paper red Conduct electricity * Very corrosive to metals React with metals * If a solution conducts electricity, it is called an electrolyte. Litmus Paper Before showing the list, ask students to answer the question orally to check for pre-knowledge of the subject. The pictures may trigger some of their ideas. Zinc reacting with hydrochloric acid

What Are Some Properties of Bases? Chemical formula always ends with “OH” NaOH Taste bitter Turn litmus paper blue Conduct electricity Very corrosive Feel slippery

What Are Some Names of Common Acids? Acetic acid is found in vinegar Ascorbic acid is found in citrus fruits Hydrochloric acid is found in your stomach Phosphoric acid is found in fertilizers and the soft drinks you drink Nitric acid is used in some explosives like TNT Before showing the list, ask students to name some specific names of acids.

What Are Some Names of Common Bases? Ammonia (NH3) is often found in cleaning products Potassium hydroxide (KOH) has many uses in industry, and can be a precursor to making soaps Sodium bicarbonate (NaHCO3) is found in baking soda Calcium carbonate (CaCO3) is found in antacids Sodium hydroxide (NaOH) is found in lye KOH in pure form Before showing the list, ask students to name some bases. The pictures may help. Antacids neutralize stomach acids

What examples of acids and bases do you have around your house? Have students answer the question orally before you click on the pictures.

The Strength of Acids A strong acid ionizes, or dissociates, completely in aqueous solutions to form H3O+. A strong acid is a strong electrolyte – it conducts electricity easily. A weak acid will be a weak electrolyte. Explain that an electrolyte is a substance, that when dissolved in water, will conduct electricity. Sulfuric acid is a strong acid. Phosphoric acid is a weak acid.

The Strength of Bases Sodium hydroxide is a strong base. Strong bases completely dissociate into its ions (OH-) in aqueous solutions. Strong bases are strong electrolytes. Sodium hydroxide is a strong base. Ammonia is a weak base.

Measuring the Strength of Acids and Bases The amount of hydronium ions and hydroxide ions in solution can be detected by various chemicals and an electronic device, called a pH meter. A pH meter measures the pH value of a substance.

Measuring the Strength of Acids and Bases Acid-base indicators are liquid chemicals which will change color in the presence of these ions. Some acid-base indicators are: phenolphthalein, methyl red, bromthymol blue, phenol red, methyl orange, red wine, cabbage juice, and beet juice. Liquid acid-base indicators will give you an approximate pH value.

Measuring the Strength of Acids and Bases Litmus paper will also give an approximate value of pH. Some litmus paper indicates approximate pH values, thus approximate strength. Some litmus paper only indicates if it’s an acid or a base, not its strength or its pH.

The pH Scale pH stands for the French words pouvoir hydrogene, meaning hydrogen power. Strong Acids Acids Discuss with students the range of the pH scale, point out that as the value decreases, the substance is more acidic, and as the value increases, the substance is more basic. Strong Bases Bases

Weak or Strong pH? Strong Acids pH = 0 – 2.9 Weak Acids pH = 3.0 – 5.9 Neutral pH = 6.0 – 8.0 Weak Bases pH = 8.1 – 11.9 Strong Bases pH = 12 – 14

Neutralization Reactions When a strong acid and a strong base solution are mixed, A neutralization reaction occurs, and The products do not have characteristics of either acids or bases. Instead, a neutral salt and water are formed. HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq) Explain to students that it takes equal values of acid and base to neutralize the substances. A strong acid and a weak base added together may make a weaker acid. And the same is true for bases – a strong base added to a weak acid will make a weaker base. Strong Acid Strong Base Water Salt

Neutralization Antacids relieve indigestion, usually caused by excess stomach acid. But, taking an antacid, which is a weak base, will react with the strong acid in the stomach to produce a weaker acid, and thus the person will feel better.

Acids Bases Acids are high in Hydrogen ion concentration Bases are high in Hydroxide ion concentration

Compare the amount of H+ ions with the amount of OH- ions in Solution Compare the amount of H+ ions with the amount of OH- ions in Solution. Notice the Relationship to the pH Scale. Spend a moment here to review the picture. It is a great visual showing how as hydronium ions are decreasing, the hydroxide ions are increasing, and vice-versa. Low pH = Acid = High amount of H+ ion, low amount of OH- ion High pH = Base = Low amount of H+ ion, high amount of OH- ion

Review – True/False

Acids dissociate in water to form H3O+ ions.

Acids taste bitter and bases taste sour.

Acids taste sour, and bases taste bitter.

A strong acid is a strong electrolyte.

Ammonia is a stronger base than antacid.

The pH scale measures values from 0-14, with 7 being neutral.

pH stands for the power of Hydroxide.

pH stands for the power of Hydrogen.

Bases produce hydroxide ions in aqueous solutions.

Liquid pH indicators give exact pH values.

Liquid pH indicators give approximate pH values.

Adding acids and bases together always results in a neutral solution.

Adding acids and bases together can produce a neutral solution but only if they are of equivalent strengths and amounts.