Let us organize representative aqueous solutions around us using the knowledge we have learned so far.
Basic Knowledge
Familiar aqueous solutions and their properties:
・Saltwater: Salt (NaCl) dissolved in water. Neutral. Conducts electricity (electrolyte).
・Sugar water: Sugar dissolved in water. Neutral. Does not conduct electricity (nonelectrolyte).
・Hydrochloric acid: An aqueous solution of hydrogen chloride (HCl). Strongly acidic. Dissolves iron and zinc.
・Sodium hydroxide solution: Strongly alkaline. Dissolves proteins (dangerous).
・Carbonated water: An aqueous solution of carbon dioxide (CO2). Weakly acidic.
・Limewater: An aqueous solution of calcium hydroxide. Alkaline. Turns cloudy with CO2.
・Ammonia water: An aqueous solution of ammonia (NH3). Alkaline. Has a pungent odor.
・Vinegar: An aqueous solution of acetic acid. Acidic.
・Copper sulfate solution: Blue, acidic, and contains copper ions.
Evaporate it (saltwater and sugar water → a solid remains; hydrochloric acid and ammonia water → nothing remains)
Smell it (hydrochloric acid, ammonia water, and vinegar → pungent odor; others → odorless)
Check whether it conducts electricity (saltwater, hydrochloric acid, and sodium hydroxide solution → conduct electricity; sugar water → does not conduct electricity)
Litmus and BTB (identify acidic, neutral, and alkaline solutions)
Color (copper sulfate = blue; many others are colorless)
React with limewater (turns cloudy with an aqueous solution containing CO2)
Further Study (Background and Significance)
A problem asking students to identify five aqueous solutions (saltwater, sugar water, hydrochloric acid, sodium hydroxide solution, and ammonia water) is a standard middle-school question. An efficient identification procedure is: ① observe the color → ② smell it → ③ test it with litmus paper → ④ evaporate it → ⑤ test electrical conductivity.
Aqueous solutions containing dissolved gases (hydrochloric acid, carbonated water, and ammonia water) are characterized by leaving nothing behind when heated because the gas escapes. Aqueous solutions containing dissolved solids (saltwater, sugar water, and sodium hydroxide solution) leave a solid behind when heated. Another basic property is that electrolytes (substances that produce ions) conduct electricity, whereas nonelectrolytes (substances that dissolve as molecules) do not.
- Saltwater = neutral, conducts electricity, solid remains
- Sugar water = neutral, does not conduct electricity, solid remains
- Hydrochloric acid = acidic, has an odor, no solid remains
- Sodium hydroxide solution = alkaline, solid remains
- Ammonia water = alkaline, pungent odor, no solid remains
- Carbonated water = acidic, nothing remains after the gas escapes
- Limewater turns cloudy with CO2 (detection)
Points to Note (Easily Confused)
① Aqueous solutions containing dissolved gases (hydrochloric acid, ammonia water, and carbonated water) leave nothing behind when heated. ② Sodium hydroxide solution is odorless, whereas ammonia water has a pungent odor, so they can be distinguished by smell. ③ Saltwater and sugar water are both neutral, and a solid remains after evaporation, but they can be distinguished by whether they conduct electricity. ④ Limewater itself is colorless, but turning cloudy with CO2 is a detection reaction.
Practice
- Think of a procedure for identifying five aqueous solutions (saltwater, sugar water, hydrochloric acid, sodium hydroxide solution, and ammonia water).
- Name one aqueous solution containing a dissolved gas and state what happens when it is heated.
- Give one method for distinguishing saltwater from sugar water.