General Properties of Acids and Bases

Acids are substances that taste sour, turn blue litmus red, react with active metals to release hydrogen, and neutralize bases, while bases taste bitter, feel slippery, turn red litmus blue and neutralize acids; both conduct electricity when dissolved in water. In brief: vinegar (acetic acid), lemon juice (citric acid) and stomach acid (hydrochloric acid) are acids, and soap, baking soda and household ammonia are bases; acids have a pH below 7, bases have a pH above 7, and pure water at 25 °C is neutral with a pH of 7.

Acids and bases are among the most important classes of chemicals. They are found in food, in the human body, in cleaning products, in industry and in the environment. This article describes the characteristic properties of each, compares them in a table, lists common examples and their uses, explains how acids and bases react with each other and with metals and carbonates, discusses safety, works through examples and gives the history of how the ideas developed.

What are the properties of acids?

• Sour taste. The word acid comes from the Latin acidus, meaning sour. Never taste chemicals in the laboratory.
• Color changes of indicators. Acids turn blue litmus paper red, and keep phenolphthalein colorless.
• Reaction with active metals. Acids dissolve metals such as magnesium, zinc and iron and release hydrogen gas: Mg + 2HCl → MgCl₂ + H₂.
• Reaction with carbonates and hydrogencarbonates. They produce carbon dioxide: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
• Reaction with bases (neutralization) to give a salt and water.
• Electrical conductivity. Aqueous solutions of acids contain ions, so they conduct electricity.
• Corrosive nature. Strong and concentrated acids attack skin, clothes and many materials.
• Hydrogen ions. All acids release hydrogen ions (H⁺, in water as H₃O⁺) in solution.

What are the properties of bases?

• Bitter taste (do not taste in the laboratory).
• Slippery or soapy feel, because bases react with the oils of the skin to form soap-like compounds.
• Color changes of indicators. Bases turn red litmus paper blue, and phenolphthalein pink.
• Reaction with acids to give salts and water.
• Electrical conductivity in solution.
• Caustic nature. Strong bases such as sodium hydroxide cause severe burns and dissolve fats and proteins.
• Hydroxide ions. Many bases release OH⁻ in solution. Others, such as ammonia, produce it by reacting with water.

Soluble metal hydroxides are called alkalis. Substances that can act as both an acid and a base, such as water and aluminum hydroxide, are called amphoteric.

Acids and bases compared

Common acids and bases

What are the typical reactions?

1. Acid + metal → salt + hydrogen. Zn + H₂SO₄ → ZnSO₄ + H₂.
2. Acid + metal oxide or hydroxide → salt + water. CuO + H₂SO₄ → CuSO₄ + H₂O.
3. Acid + carbonate → salt + water + carbon dioxide.
4. Acid + base → salt + water (neutralization): HCl + NaOH → NaCl + H₂O.
5. Base + ammonium salt → ammonia gas. NaOH + NH₄Cl → NaCl + H₂O + NH₃.
6. Base + fat → soap (saponification).

Worked examples

1. Identify the type from tests. A solution turns blue litmus red, fizzes with sodium carbonate, and releases a gas that burns with a pop when reacted with magnesium. It is an acid: the fizz is CO₂ from the carbonate, and the pop is hydrogen.
2. Classify by pH. Lemon juice has a pH of about 2.2 (acidic), milk 6.6 (slightly acidic), blood 7.4 (slightly basic), and household ammonia 11.5 (basic).
3. Antacid. A tablet has 500 mg of Mg(OH)₂ (58.32 g/mol), which is 8.57 mmol. It neutralizes 2 × 8.57 = 17.1 mmol of HCl. If a stomach contains 250 mL of 0.0300 mol/L HCl (7.50 mmol), 7.50 ÷ 17.1 = 0.44 of a tablet is enough in principle.

4. Neutralization volume. 25.0 mL of 0.100 mol/L HCl requires 25.0 mL of 0.100 mol/L NaOH, as the mole ratio is 1 : 1 and the concentrations are equal.
5. Gas produced. 2.43 g of magnesium (0.100 mol) reacting with excess acid gives 0.100 mol of H₂, which has a volume of 2.24 L at STP.
6. Why vinegar fizzes on baking soda. CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂. The acid reacts with a hydrogencarbonate, and the CO₂ escapes as bubbles.

How do you handle acids and bases safely?

• Wear goggles, gloves and a lab coat. Splashes of strong bases can be more dangerous to the eyes than acids, since they penetrate tissue.
• When diluting a concentrated acid, always add the acid to the water.
• Rinse skin or eyes immediately with plenty of water for at least 15 minutes.
• Never mix cleaning products. Bleach and acid release chlorine gas, and bleach with ammonia releases toxic chloramines.
• Store acids and bases separately, in labeled containers.

How did ideas about acids and bases develop?

Robert Boyle listed the properties of acids and alkalis in 1661, including the color changes of plant extracts. Antoine Lavoisier proposed in 1776 that acids contain oxygen, which was disproved when Humphry Davy showed in 1810 that hydrochloric acid contains no oxygen. Justus von Liebig defined acids as compounds with replaceable hydrogen in 1838. Svante Arrhenius defined acids and bases in terms of ions in 1884 and 1887, and the later theories of Brønsted, Lowry and Lewis extended the definitions.

Which mistakes are common?

Mistake 1: Thinking that all acids are dangerous or all bases are harmless

Dilute acetic acid (vinegar) and citric acid are safe, while concentrated sodium hydroxide is highly dangerous.

Mistake 2: Confusing strength with concentration

A strong acid is completely ionized. A concentrated acid has a large amount of acid per volume. Dilute hydrochloric acid is strong but dilute, and concentrated acetic acid is weak but concentrated.

Mistake 3: Believing that neutral means harmless

A neutral solution has a pH of 7, but it can still contain toxic substances.

Mistake 4: Tasting or touching to test a chemical

Use indicators or a pH meter instead.