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🔬 Science · Scientific Reasoning · Lesson 3 of 6 · 5 min

Measurement tools, metric units and precision

📏 You will be asked which tool to use, how to read it, and whether a set of numbers is accurate, precise or both.

💡 Key idea

Accuracy = close to the true value; precision = close to each other.

🧠 The idea

Match the tool to the quantity. Mass: a BALANCE (grams). Liquid volume: a graduated cylinder for general work, a pipette for small exact amounts. Length: a ruler or meter stick. Temperature: a thermometer (°C in science). In a narrow tube water curves into a MENISCUS; read the bottom of the curve at eye level. The metric system uses base units (meter, gram, liter) and prefixes that are powers of 10: kilo- = 1,000, centi- = 1/100, milli- = 1/1,000, micro- = 1/1,000,000. So 1 km = 1,000 m, 1 m = 100 cm = 1,000 mm, 1 g = 1,000 mg, 1 L = 1,000 mL. ACCURACY is how close a measurement is to the true value. PRECISION is how close repeated measurements are to each other, and how fine the scale is (0.01 g is more precise than 0.1 g). Results can be precise but not accurate, such as a scale that always reads 2 g high.

✏️ Worked example

A 50.0 g standard mass is weighed three times on two balances. Balance A reads 48.1 g, 48.0 g and 48.1 g. Balance B reads 49.6 g, 50.5 g and 50.0 g. Balance A is precise (the readings agree within 0.1 g) but not accurate (all about 2 g low). Balance B is less precise (spread of 0.9 g) but its average, (49.6 + 50.5 + 50.0) / 3 = 50.0 g, is accurate. Conversion check: 48.1 g × 1,000 mg/g = 48,100 mg.

📌 Takeaways

  • Balance for mass, graduated cylinder or pipette for volume, thermometer for temperature.
  • kilo = 1,000; centi = 1/100; milli = 1/1,000; read a meniscus at its bottom, at eye level.
  • Precise readings can still be inaccurate if the tool is off.

✅ Check yourself

Question 1 of 3

Which tool should a student use to measure the mass of a rock?

  1. Thermometer
  2. Graduated cylinder
  3. Balance
  4. Meter stick
Show the answer and rationale

Answer: C

A balance measures mass in grams. A graduated cylinder measures liquid volume, a thermometer measures temperature, and a meter stick measures length.

Question 2 of 3

How many milliliters are in 2.5 liters?

  1. 2,500 mL
  2. 25,000 mL
  3. 25 mL
  4. 250 mL
Show the answer and rationale

Answer: A

Milli- means one-thousandth, so 1 L = 1,000 mL and 2.5 L × 1,000 = 2,500 mL. The answer 250 mL comes from multiplying by 100 instead of 1,000, which is the centi- relationship.

Question 3 of 3

Three readings of a known 10.0 cm object are 11.2 cm, 11.3 cm and 11.2 cm. How would you describe them?

  1. Not accurate and not precise
  2. Accurate but not precise
  3. Precise but not accurate
  4. Accurate and precise
Show the answer and rationale

Answer: C

The readings agree closely with each other (precise), but all are more than 1 cm from the true 10.0 cm (not accurate). Being close to each other does not mean being close to the true value.

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