We will learn how to produce and identify the properties of representative gases studied in middle-school chemistry, such as oxygen, carbon dioxide, hydrogen, and ammonia.
Basic Knowledge
Oxygen (O2): Produced by adding hydrogen peroxide solution (Oxydol) to manganese dioxide. It has no color or odor and dissolves poorly in water. It supports combustion (combustion-supporting property).
Carbon dioxide (CO2): Produced by adding hydrochloric acid to limestone. It has no color or odor, dissolves slightly in water, and shows acidic properties. It turns limewater cloudy white (a detection method).
Hydrogen (H2): Produced by adding hydrochloric acid to zinc. It has no color or odor and is the lightest gas. When a flame is brought near it, it burns with a popping sound and becomes water.
Ammonia (NH3): Produced by mixing ammonium chloride and calcium hydroxide and heating the mixture. It has a pungent odor, dissolves readily in water, shows alkaline properties, and is lighter than air (upward displacement of air).
There are three methods of collecting gases: collection over water (gases that dissolve poorly in water), downward displacement of air (gases heavier than air), and upward displacement of air (gases lighter than air).
Combustion-supporting property（the property of supporting the burning of other substances. A characteristic of oxygen）
Limewater（an aqueous solution of calcium hydroxide. It becomes cloudy with CO2）
Collection over water（collecting a gas by replacing water. Used for gases that dissolve poorly in water）
Downward displacement of air（collecting a gas heavier than air from below）
Upward displacement of air（collecting a gas lighter than air from above）
BTB solution（an indicator that changes to yellow in acidic, green in neutral, and blue in alkaline solutions）
Further Exploration (Background and Significance)
The properties of each gas can be understood from the structures of molecules, which we will study later. Ammonia (NH3) is highly polar and forms hydrogen bonds with water, so it dissolves extremely well in water (about 700L dissolves in 1L of water). Oxygen and hydrogen are nonpolar and dissolve poorly in water.
The method of collecting a gas is selected according to its properties. If hydrogen is collected by downward displacement of air, it escapes because it is lighter than air. Conversely, if a poisonous gas heavier than air, such as chlorine, is collected by upward displacement of air, it can fill the room and become dangerous. This is the experimental wisdom needed to achieve both safety and efficiency.
- Oxygen = Oxydol + manganese dioxide; supports combustion
- Carbon dioxide = limestone + hydrochloric acid; turns limewater cloudy
- Hydrogen = zinc + hydrochloric acid; lightest gas; burns with a pop
- Ammonia = pungent odor, dissolves readily in water, alkaline
- Collection over water = gases that dissolve poorly in water (O2, H2)
- Downward displacement = gases heavier than air (CO2, chlorine)
- Upward displacement = gases lighter than air (NH3)
Points to Note (Easily Confused)
① Oxygen supports combustion; it does not burn by itself. Hydrogen burns. ② Carbon dioxide dissolves slightly in water, so collection over water is possible, but downward displacement of air produces a purer gas. ③ Ammonia dissolves readily in water, so collection over water is not possible. ④ Limewater (aqueous calcium hydroxide, Ca(OH)2) and limestone (CaCO3) are different substances.
Practice
- What happens when carbon dioxide is passed through limewater?
- Answer how ammonia is collected, including the reason.
- What happens when a flame is brought near hydrogen?