Plan and carry out chemistry investigations, distinguish independent, dependent and controlled variables, and report measurements using significant figures, units and dimensional analysis (MA STE practices).

What this topic is asking

The Massachusetts STE framework is built on the same eight science and engineering practices as the NGSS, so a Massachusetts chemistry course is not only a set of facts but a way of working. Before you can succeed at moles, gas laws, or titrations, you need to plan an investigation, identify and control variables, take measurements that carry the right number of significant figures, and convert between units with dimensional analysis. These skills are woven through every later topic, and good chemistry assessments test them in the context of real data rather than in isolation.

Variables and a fair test

If a student investigates how temperature affects reaction rate, the temperature is the independent variable, the rate (or time) is the dependent variable, and the concentration, volume, and amount of each substance are controlled. Leaving more than one variable free to change means you cannot tell which one caused the result, so the conclusion is not valid. A control group (a set-up with no treatment) gives a baseline for comparison.

Accuracy and precision

These two words are often confused but mean different things. Accuracy is how close a measurement is to the true or accepted value. Precision is how close repeated measurements are to one another, regardless of the true value. A balance that always reads 12.40 g when the true mass is 12.41 g is precise (reproducible) and nearly accurate. A balance that gives 11.8 g, 12.6 g, and 12.2 g for the same object is neither precise nor reliable. You want both, but they are independent: a reading can be precise yet inaccurate if the instrument is wrongly calibrated, which produces a consistent systematic error. Random error scatters readings around the true value and is reduced by repeating and averaging.

Significant figures

Significant figures show how precisely a quantity is known. The rules you need are:

• All non-zero digits are significant (45.2 has three).
• Zeros between non-zero digits are significant (4002 has four).
• Leading zeros are not significant (0.0034 has two).
• Trailing zeros after a decimal point are significant (12.40 has four).

When you multiply or divide, the answer carries the same number of significant figures as the measurement with the fewest. When you add or subtract, the answer is rounded to the fewest decimal places. The point is honesty: an answer cannot be more precise than the data it came from, so writing 12.41672 g from a balance that reads to 0.01 g is wrong.

SI units and dimensional analysis

Chemistry uses the SI base units: the meter (length), kilogram (mass), second (time), kelvin (temperature), and mole (amount of substance). Volume is usually the liter or milliliter, and density combines mass and volume. Dimensional analysis (the factor-label method) converts between units by multiplying by fractions equal to one, arranged so the unwanted unit cancels.

Reading data the way the framework expects

Because the STE framework is practice-based, a question is far more likely to give you a data table or a graph than to ask for a bare definition. Practice identifying the variables in someone else's experiment, spotting the control, judging whether a result is reliable, and reading a value off a graph with the correct units. When you evaluate an investigation, look for uncontrolled variables, too few trials, or a measuring instrument that is not precise enough for the job.

Try this

Q1. A student changes the concentration of acid and measures the volume of gas produced. Name the independent and dependent variables. [2]

• Cue. Independent: acid concentration. Dependent: volume of gas produced.

Q2. How many significant figures are in $0.00560$? [1]

• Cue. Three (the leading zeros do not count; the trailing zero after the decimal does).