Module 2 · Classifying Matter and ChangePractice · 物质的分类与变化

Boiling is a physical change: the water molecules stay whole; only the spaces between them become much larger and they move around freely, so the answer is C. In A the molecules are close together, which is liquid water. B splits water into $\ce{H2}$ and $\ce{O2}$, and D into single atoms: both would mean new substances, a chemical change such as electrolysis, which boiling isn’t.

A substance is liquid when the temperature is above its melting point and below its boiling point. At 35 °C, $\ce{Br2}$ (−7.2 °C to 58.8 °C) and Cs (28.4 °C to 671 °C) are in that range, so they are liquids. $\ce{Cl2}$ boils at −34 °C, below 35 °C, so it’s a gas; $\ce{I2}$ and Na melt above 35 °C, so they are solids.

When alcohol evaporates, liquid alcohol just becomes alcohol vapor; no new substance forms, so it’s a physical change. Rusting forms rust, spoiling food forms new bad-smelling substances, and burning coal forms carbon dioxide and more: all chemical changes.

“Can burn” describes something ethanol is able to do, which shows itself only in a chemical change: a chemical property. “Is burning” describes a process happening now that forms carbon dioxide and water: a chemical change. Words like “can,” “will,” and “easily” usually signal a property.

First count the kinds of molecules: one kind means a pure substance, two or more a mixture. Then look at each kind: molecules of a single kind of atom are an element, and molecules of different atoms are a compound. A is a mixture of two elements ($\ce{H2}$ and $\ce{O2}$); B is only $\ce{H2O}$, a pure compound; C is only $\ce{N2}$, a pure element; D contains two compounds, $\ce{CO2}$ and $\ce{H2O}$, so it’s a mixture of compounds.

A compound is a pure substance made of two or more elements. Dry ice ($\ce{CO2}$) and potassium permanganate ($\ce{KMnO4}$) both contain more than one element, so they are compounds. Liquid oxygen ($\ce{O2}$) and mercury (Hg) contain one element each, so they are elements. Air contains nitrogen, oxygen, and other substances, so it’s a mixture, not a compound.

The salt in seawater is dissolved, so filtering or letting it settle only removes undissolved matter, not the salt. Evaporation drives the water off and leaves the salt behind. Distillation boils the water off, then condenses and collects the vapor, leaving the salt in the flask: that liquid is fresh water.

First the volume: V = 2.0 × 3.0 × 5.0 = 30 cm³. Then $\rho = \dfrac{m}{V}$: ρ = 237 g ÷ 30 cm³ = 7.9 g/cm³, close to that of iron.

The volume of water pushed aside equals the pebble’s volume: V = 26.5 mL − 20.0 mL = 6.5 mL = 6.5 cm³. ρ = 17.2 g ÷ 6.5 cm³ ≈ 2.65 g/cm³. Using 26.5 mL as the volume is a common mistake.

Going from solid to liquid, liquid to gas, or straight from solid to gas, the particles have to pull apart against their attractions, which takes heat: melting, vaporization, and sublimation absorb heat. The reverse changes, freezing and condensation (and deposition), release heat.