Tools &
Evidence
Evidence
A solid is shiny and has a mass of 16.4 g. Which observation is qualitative, and which is quantitative?
Shiny is qualitative (descriptive).
16.4 g is quantitative (a measurement with a unit).
What is the difference between accuracy and precision?
Accuracy: closeness to an accepted/reference value.
Precision: closeness of repeated measurements to one another
How many milliliters equal 1 liter? How many milligrams equal 1 gram?
1 L = 1000 mL
1 g = 1000 mg
Classify pure copper and distilled water (H₂O) as element or compound.
Copper: element, one type of atom.
Distilled water: compound, hydrogen and oxygen chemically combined in a fixed ratio.
State the density equation and one appropriate density unit.
D = m / V
Common units: g/mL or g/cm³.
Which tool measures mass, and which tool measures liquid volume precisely? Give the usual unit for each.
Mass: an electronic balance, in grams (g).
Liquid volume: a graduated cylinder, in milliliters (mL).
How many significant figures are in 0.004060? Explain the zeros.
Four significant figures.
Leading zeros do not count. The zero between 4 and 6 and the final decimal zero count.
Convert 0.180 L to mL. Show a conversion factor that cancels liters.
0.180 L × (1000 mL / 1 L) = 180 mL
L cancels. To show three significant figures explicitly: 1.80 × 10² mL.
Classify saltwater and granite as homogeneous or heterogeneous mixtures.
Saltwater: homogeneous, with uniform composition when fully dissolved.
Granite: heterogeneous, with different visible components.
Water rises from 28.0 mL to 36.5 mL when a solid is fully submerged. What is the solid’s volume?
V = 36.5 mL − 28.0 mL = 8.5 mL
The change in water volume gives the object’s volume.
How should you read the volume of water in a graduated cylinder? Explain why.
Read the bottom of the meniscus at eye level.
Eye-level reading reduces parallax error from viewing at an angle.
Write 0.00000860 in scientific notation while preserving significant figures.
8.60 × 10⁻⁶
The coefficient preserves all three significant figures.
Convert 4.25 g to mg using dimensional analysis.
4.25 g × (1000 mg / 1 g) = 4250 mg
With three significant figures: 4.25 × 10³ mg.
Is density a physical or chemical property? What about flammability? Explain.
Density is physical: measurement does not change chemical identity.
Flammability is chemical: it describes the ability to react by burning.
A solid has mass 48.6 g and volume 18.0 mL. Calculate its density with units.
D = 48.6 g ÷ 18.0 mL = 2.70 g/mL
Both measurements support three significant figures.
Two scientists report 24.8 g and 25 g for the same sample. Can you decide which is more accurate from the written values alone? Explain.
No. More digits do not establish accuracy.
Check the reference value, tool resolution, calibration, technique, and repeated measurements.
A team measures 18.4 g, 18.5 g, and 18.4 g. The reference value is 20.0 g. Describe accuracy and precision using the data.
Precise, but not accurate relative to 20.0 g.
The trials span only 0.1 g, but each is about 1.5–1.6 g below the reference.
Starting with D = m/V, solve for mass and for volume.
Mass: m = D × V
Volume: V = m / D
Ice melts and an iron nail rusts. Identify the type of change in each and explain at the substance level.
Melting ice: physical change. It remains H₂O.
Rusting iron: chemical change. Iron reacts to form new substances.
Two samples of the same pure material have different masses. Under the same conditions, should their densities differ? Explain.
Their densities should be the same within measurement uncertainty.
Density is intensive. A larger sample has proportionally greater mass and volume.
A team uses a beaker for precise volume, reads from above, and records mass as ‘14.7’. Identify three repairs to its data collection.
Use a graduated cylinder for precise volume.
Read the water meniscus at eye level.
Record the mass unit, such as 14.7 g.
A team repeats measurements three times and gets nearly identical results. It claims: ‘Our results must be accurate.’ Evaluate that claim.
Consistency supports precision, not necessarily accuracy.
A calibration error can shift every trial. Compare results with a reference and check the instrument.
A sample has a mass of 0.0360 kg and density of 3.00 g/mL. Calculate its volume. Show compatible units.
0.0360 kg × (1000 g / 1 kg) = 36.0 g
V = m/D = 36.0 g ÷ 3.00 g/mL
V = 12.0 mL
A liquid bubbles when heated. A scholar says bubbles always prove a chemical reaction. Evaluate the claim and identify the liquid-to-gas phase change.
Bubbles alone do not prove a chemical reaction. Boiling can produce bubbles during a physical change.
Liquid to gas is vaporization. Chemical change requires evidence of new substances.
An air bubble sticks to a submerged solid during water displacement. How will this affect the calculated density, and how can the method improve?
The bubble increases apparent displaced volume.
With mass unchanged, D = m/V becomes too low.
Remove trapped bubbles and repeat the measurement