Introduction to the Particulate Nature of Matter · Paper 2 — Free Chemistry HL Practice Questions

A student investigates the physical properties of three substances, X, Y, and Z, at $298\,\text{K}$ and $1\,\text{atm}$. - Substance X: brittle, white crystalline solid; does not conduct electricity when solid, but its aqueous solution conducts electricity. - Substance Y: shiny, grey solid; conducts electricity and can be hammered into thin sheets without breaking. - Substance Z: colourless liquid; does not conduct electricity, boiling point $353\,\text{K}$, immiscible with water.

A student measures the density of a sample of liquid ethanol ($\text{C}_2\text{H}_5\text{OH}$) at $20\,^\circ\text{C}$ and finds it to be $0.789\,\text{g cm}^{-3}$. The student then cools the ethanol to $-120\,^\circ\text{C}$, where it becomes a solid with a density of $0.817\,\text{g cm}^{-3}$.

A student investigates the properties of a pure liquid substance, X. The student places a sample of liquid X into an open beaker and heats it gently, recording temperature every 30 seconds. The temperature rises steadily from $25\,^\circ\text{C}$ to $78\,^\circ\text{C}$, then remains constant at $78\,^\circ\text{C}$ for several minutes despite continued heating. After all the liquid has disappeared, the temperature of the vapour begins to rise again.

A student investigates two solid substances: Substance X (silicon dioxide, $\text{SiO}_2$) and Substance Y (iodine, $\text{I}_2$). Both are pure substances at room temperature ($25\,^\circ\text{C}$). Property — Substance X — Substance Y Melting point — $1710\,^\circ\text{C}$ — $114\,^\circ\text{C}$ Electrical conductivity (solid) — none Vapour colour — colourless — purple Electronegativity values: $\text{Si} = 1.9$, $\text{O} = 3.4$, $\text{I} = 2.7$

Oxygen gas, $\text{O}_2$, and ozone gas, $\text{O}_3$, are both allotropes of oxygen. At room temperature and pressure, oxygen is a colourless gas while ozone is a pale blue gas with a distinct sharp odour.

A student investigates the physical properties of three unknown substances A, B, and C at $25\,\text{°C}$. The following observations are recorded: - Substance A: brittle solid, melting point $> 1000\,\text{°C}$, non-conductor in solid state, conducts electricity when molten. - Substance B: soft solid, melting point $113\,\text{°C}$, non-conductor in solid state and when molten. - Substance C: shiny solid, can be hammered into sheets, conducts electricity in solid state, melting point $660\,\text{°C}$.

Liquid bromine, $\text{Br}_2\text{(l)}$, and liquid mercury, Hg(l), are both dense liquids at room temperature.

This question is about the particulate nature of matter and the limitations of early atomic models. Dalton's atomic model (1808) proposed that atoms are solid, indivisible spheres. Thomson's "plum pudding" model (1897) revised this by suggesting atoms contain negatively charged electrons embedded in a diffuse positive sphere of charge. Data from Geiger–Marsden experiment: - A beam of alpha particles is directed at a thin gold foil - Most alpha particles pass straight through with negligible deflection - Approximately 1 in 8000 alpha particles are deflected by more than $90°$ - Alpha particle charge: $+2e$ - Gold atomic number: $Z = 79$ - Radius of gold atom: $r_{\text{atom}} = 144\,\text{pm}$ - Radius of gold nucleus: $r_{\text{nucleus}} = 7.3\,\text{fm}$ *

A student places a sealed syringe containing $20.0\,\text{cm}^3$ of liquid water at $25\,°\text{C}$ into a freezer at $-10\,°\text{C}$. After several hours, the water has completely frozen, and the volume of the ice is measured as $21.8\,\text{cm}^3$. The density of liquid water at $25\,°\text{C}$ is $0.997\,\text{g cm}^{-3}$.

A student investigates three pure substances, X, Y, and Z, at $25\,^\circ\text{C}$. - Substance X: melting point $= 801\,^\circ\text{C}$; boiling point $= 1413\,^\circ\text{C}$; brittle solid; does not conduct electricity when solid; molten form conducts electricity. - Substance Y: melting point $= -39\,^\circ\text{C}$; boiling point $= 357\,^\circ\text{C}$; shiny, malleable liquid; conducts electricity in both liquid and solid states. - Substance Z: melting point $= 0\,^\circ\text{C}$; boiling point $= 100\,^\circ\text{C}$; colourless liquid; does not conduct electricity; when electrolysed, gases form at both electrodes.

A sample of radium-226 ($^{226}_{88}\text{Ra}$) undergoes alpha decay to form radon-222 ($^{222}_{86}\text{Rn}$) and an alpha particle ($^{4}_{2}\text{He}$). - Mass of $^{226}_{88}\text{Ra}$ atom: $226.0254\,\text{u}$ - Mass of $^{222}_{86}\text{Rn}$ atom: $222.0176\,\text{u}$ - Mass of $^{4}_{2}\text{He}$ atom: $4.0026\,\text{u}$ - $1\,\text{u} = 1.66 \times 10^{-27}\,\text{kg}$ - $c = 3.00 \times 10^{8}\,\text{m\,s}^{-1}$

Diamond and graphite are allotropes of carbon. Diamond is one of the hardest naturally occurring substances, while graphite is soft and can act as a lubricant.

A sample of solid iodine ($\text{I}_2$) is heated gently in a fume cupboard. The solid sublimes directly into a purple vapour without melting. The vapour is then cooled on a cold surface, where it reforms as solid iodine crystals.