Module 1 · What Is Chemistry?Practice · 化学是什么

Chemistry studies what substances are made of, how they are built, their properties, and how they change. Rusting is iron reacting with oxygen and water to form a new substance, so it’s a chemistry question. A falling ball and the Moon’s distance are physics questions, and how bees find flowers is a biology question.

Beakers and flasks have wide bottoms that heat unevenly and can crack over a bare flame, so they sit on a wire gauze. Test tubes and evaporating dishes can be heated directly. A graduated cylinder is for measuring volumes and must never be heated.

When the balance is level, left pan = right pan + rider reading. The 10 g of masses is on the left and the rider reads 0.5 g, so 10 g = salt + 0.5 g, and the salt is only 10 − 0.5 = 9.5 g. The object belongs on the left pan and the masses on the right.

A torn filter paper or liquid above its edge lets cloudy liquid bypass the paper, and a dirty beaker contaminates the filtrate. If the funnel tip doesn’t touch the beaker wall, the filtrate splashes; if the paper isn’t flat against the funnel, filtering is slow. Neither makes the filtrate cloudy.

Use two conversion factors: 1 L = 1000 mL and 1 h = 60 min.

Put each unit you want to cancel in the denominator, and L and h cancel out.

Start counting at the first nonzero digit (2): 2, 0, 5, 0, so 4 significant figures. Leading zeros only locate the decimal point and don’t count; a zero between digits counts, and a trailing zero after the decimal point counts because it shows the measurement was made to that place.

Precision is how close the results are to each other; accuracy is how close they are to the true value, 10.00 g. Set B varies by only 0.02 g, so it’s precise, but its average, 10.52 g, is about 0.5 g off, so it isn’t accurate (perhaps the balance wasn’t zeroed). Set A is both precise and accurate; set C averages 9.99 g, close to the true value, but its readings are far apart, so it isn’t precise; set D is neither.

T (K) = t (°C) + 273, so t = T − 273 = 77 − 273 = −196 °C. Kelvin temperatures are never negative, but a Celsius temperature below zero is perfectly normal.

The variable you change is the water temperature (the independent variable), and what you measure is the time to dissolve (the dependent variable). Everything else that could affect dissolving, such as the volume of water, the grain size, and stirring, must be kept the same, so any difference can be blamed on temperature.

First rinse with plenty of water to wash the acid away, dilute it, and carry off the heat; then a weakly basic sodium bicarbonate solution neutralizes what’s left. Sodium hydroxide is a strong base and corrosive itself, and hydrochloric acid would only add more acid.