VARDAAN LEARNING INSTITUTE | POWERED BY VARDAAN COMET
Hydrogen
ICSE Class 8 Chemistry • Chapter 7 (Detailed Master Notes)
Chapter Overview
Hydrogen is the very first element in the periodic table and the most abundant element in the universe. Our Sun and the stars run on it. It is the lightest gas known to mankind and holds incredible potential as the clean fuel of the future. Let us explore its chemical properties and how we prepare it in the laboratory.
7.1 Introduction and Position
Discovered by Henry Cavendish, Hydrogen gets its name from Greek words meaning "Water-former" because it produces water when burned.
- Symbol: $H$
- Formula: $H_2$ (It exists as a diatomic molecule).
- Atomic Number: $1$
- Electronic Configuration: $1$ (K-shell only).
- Valency: $+1$ (sometimes $-1$).
Unique Position in the Periodic Table:
Hydrogen's position is highly debated. Its valency of $+1$ allows it to behave chemically like the active Alkali Metals (Group 1: Lithium, Sodium). However, because it needs just $1$ electron to complete its shell (forming $H^-$), it also perfectly mirrors the chemical behavior of the non-metallic Halogens (Group 17: Fluorine, Chlorine). Due to this dual nature, it sits alone at the top of the table.
7.2 Laboratory Preparation of Hydrogen Gas
In the lab, Hydrogen is easily produced by reacting an active metal with dilute acid.
Standard Reactants: Granulated Zinc and Dilute Hydrochloric Acid (or Dilute Sulphuric Acid).
Chemical Reaction:
$\mathbf{Zn + 2HCl (dilute) \rightarrow ZnCl_2 + H_2 \uparrow}$
Zinc chemically displaces the hydrogen from the acid. Bubbles of hydrogen gas immediately rise upward.
Why Granulated Zinc?
We specifically use the granulated form of zinc because it provides a much larger surface area compared to a solid chunk. This allows the acid to react faster and more efficiently. We avoid ultra-pure zinc because impurities uniquely catalyze (speed up) the reaction.
Collection Method: Downward Displacement of Water
The formed gas bubbles through a delivery tube and is collected in an inverted jar filled with water. The gas rises and pushes the water completely downward. This method works perfectly because:
- Hydrogen is practically insoluble in water.
- Hydrogen is far lighter than water.
AI Image Prompt: A detailed diagram of a standard chemistry laboratory setup. A round-bottom flask containing grey zinc granules and liquid acid. A thistle funnel passes through a cork into the flask. A glass delivery tube leads out of the flask into a glass trough filled with blue water. Gas bubbles rise inside an inverted gas jar placed over a beehive shelf in the trough.
7.3 Physical Properties of Hydrogen
- Color & Odor: It is a colorless, odorless, and tasteless gas.
- Density: It is the lightest known gas. It is $14.4$ times lighter than air.
- Solubility: It is almost completely insoluble in water.
- Toxicity: It is totally non-poisonous, but animals cannot survive breathing strictly pure hydrogen (due to a lack of oxygen).
7.4 Chemical Properties of Hydrogen
- Combustibility: Hydrogen is a highly combustible gas. It easily catches fire and burns
with a distinct pale blue flame, producing massive amounts of heat and water vapor.
$\mathbf{2H_2 + O_2 \rightarrow 2H_2O}$ - Supporter of Combustion: Hydrogen itself burns easily, but it does NOT support the combustion of other things. If you place a burning splint into a jar of pure hydrogen, the gas catches fire, but the splint immediately extinguishes.
- Reducing Agent: Hydrogen has a strong affinity for oxygen. It can chemically steal
oxygen away from metal oxides, reducing them back to pure metals.
$\mathbf{CuO (Black) + H_2 \rightarrow Cu (Red-brown) + H_2O}$
Q1. Why is hydrogen gas collected by the downward displacement of water, rather than air?
Answer: Hydrogen is virtually insoluble in water, so it does not mix with it. Also, hydrogen forms a highly explosive mixture when mixed directly with oxygen in the air. Collecting it over water prevents it from accidentally mixing with air, ensuring safe collection.
Q2. State the primary reason Hydrogen is considered a powerful "fuel of the future".
Answer: Hydrogen possesses the highest calorific value of any fuel (it produces massive energy when burned). Most importantly, its only combustion byproduct is pure water vapor ($H_2O$), meaning it causes absolutely zero pollution.