Module 8 · Common Substances Around UsPractice · 身边的常见物质

$\ce{CO2}$ turns limewater cloudy, forms carbonic acid in water that turns purple litmus red, and doesn’t burn. CO does none of the first two but burns with a blue flame. Both gases are odorless, and CO is poisonous, so smelling them can’t tell them apart and isn’t safe.

Let x be the mass of copper. CuO and Cu both have coefficient 1, so write 80 under CuO and 64 under Cu: $\frac{80}{64} = \frac{8.0\,\text{g}}{x}$, so x = 6.4 g. Or think of it directly: copper is 64/80 of the mass of copper(II) oxide, and 8.0 g × 64/80 = 6.4 g.

Fires go out when you remove the fuel, cut off the oxygen, or cool the fuel below its ignition temperature. A lid, a burner cap, and sand all cut the flame off from the air. A firebreak removes the fuel, and water on wood mainly cools it.

A coal stove holds a large mass of very hot fuel; fanning carries away too little heat to cool it below its ignition temperature and supplies more oxygen, so it burns hotter. A candle flame is small, so fanning carries away enough heat to drop it below its ignition temperature, and it goes out. The ignition temperature is a property of the substance and can’t usually be lowered.

Metals above hydrogen in the activity series can displace hydrogen from acids: Mg, Zn, and Fe are above H, so they react, for example $\ce{Fe(s) +}$ $\ce{2HCl(aq) ->}$ $\ce{FeCl2(aq) +}$ $\ce{H2(g)}$. Cu and Ag are below H and don’t react with dilute hydrochloric acid.

X reacts with dilute sulfuric acid and Y doesn’t, so X is above hydrogen and Y below it: X > Y. Y displaces Z from a solution of its salt, so Y > Z. Together: X > Y > Z.

Concentrated hydrochloric acid is volatile: HCl gas keeps escaping, so the solute and the solution both lose mass and the mass percent falls. For example, if 100 g of 37% acid loses 5 g of HCl, it becomes (37 − 5) ÷ (100 − 5) ≈ 33.7%. Choice A describes concentrated sulfuric acid, which absorbs water from the air: its mass goes up and its mass percent goes down.

At first the sodium hydroxide solution is basic, pH > 7. The acid neutralizes the base, so as base is used up the pH falls; when they have exactly reacted, the solution is sodium chloride solution with pH = 7; adding more acid makes it acidic, pH < 7.

A double-replacement reaction happens only if swapping partners makes a precipitate, a gas, or water. B forms a $\ce{BaSO4}$ precipitate; C forms $\ce{H2O}$ and $\ce{CO2}$ gas; D is acid plus base, forming water. Swapping in A gives KCl and $\ce{NaNO3}$, and in E $\ce{CuCl2}$ and $\ce{Na2SO4}$: all soluble, with no precipitate, gas, or water, so they don’t react.

Let x be the mass of $\ce{CO2}$. $\ce{Na2CO3}$ and $\ce{CO2}$ both have coefficient 1, so write 106 under $\ce{Na2CO3}$ and 44 under $\ce{CO2}$: $\frac{106}{44} = \frac{10.6\,\text{g}}{x}$, so x = 4.4 g. The acid is in excess, so work from the sodium carbonate; the 2 in front of HCl isn’t needed here.