Chemistry · Ch 6 — States of Matter
The Gaseous State
The Gaseous State
Gases all around us
The gaseous state is the simplest of the three states of matter. We live our entire lives immersed in a mixture of gases — air — specifically within the troposphere, the lowest layer of the atmosphere, held to Earth's surface by gravity. This thin layer is essential to life: it shields us from harmful radiation and supplies substances such as dioxygen, dinitrogen, carbon dioxide, and water vapour.
Which elements are gases?
Looking at the periodic table, only eleven elements exist as gases under normal conditions of temperature and pressure (Fig. 5.4) — hydrogen; the halogens nitrogen, oxygen, fluorine and chlorine; and all six noble gases (helium, neon, argon, krypton, xenon, radon).
Defining physical properties of the gaseous state
- Gases are highly compressible.
- Gases exert pressure equally in all directions.
- Gases have much lower density than solids and liquids.
- Gases have no fixed volume or shape — they take the volume and shape of whatever container holds them.
- Gases mix evenly and completely in all proportions, without needing any mechanical stirring.
Why gases behave so simply …
What this figure shows. A fragment of the periodic table drawn as a grid of coloured boxes. Header row reads 'Group number' then columns '1', a wavy break, '15', '16', '17', '18'. Row 2: under Group 1 a pink box 'H'; under Group 18 an orange box 'He'. Row 3: under 15,16,17,18 the blue boxes 'N', 'O', 'F', and orange 'Ne'. Row 4: blue box 'Cl' (under 17) and orange 'Ar' (under 18). Remaining rows show only the Group-18 column continuing downward with orange boxes 'Kr', 'Xe', 'Rn'. Hydrogen is shaded pink (distinct), the noble gases (He, Ne, Ar, Kr, Xe, Rn) are shaded orange, and N, O, F, Cl are shaded blue — the e …