Elements, compounds and mixtures

Answer Key - Grade 10 Science Worksheet

Circle the best answer, or write your answer on the line.

1. 1.

   Atomic masses: H = 1.008, O = 16.00. What percent of water, H₂O, is hydrogen by mass, to 0.1%, Ari?

   11.2 %
2. 2.

   Atomic masses: C = 12.01, O = 16.00. What percent of carbon dioxide, CO₂, is oxygen by mass, to 0.1%, Bo?

   72.7 %

3. Read this first

   Boiling points at 1 atm (real data, CRC)

   substance	boiling point (°C)
   acetone	56.05
   ethanol	78.4
   water	100.0
   table salt	1413

4. 3.

   Ethanol and water are distilled. The thermometer at the top holds near 78 °C, then jumps toward 100 °C. What happened, Kira?

   • The ethanol turned into water as the temperature rose
   • Water boiled first near 78 °C, then ethanol joined it at 100 °C
   • Most of the ethanol boiled off near 78 °C, then water's vapor took over
5. 4.

   Salt water is distilled and the vapor is cooled in a clean flask. What collects, and why, Luz?

   • Pure water: salt boils at 1413 °C, so it stays in the first flask
   • Water plus salt vapor, which cools together into a damp white crust
   • Salt water, because some salt evaporates along with the water

6. Cooling two melted samples (example data)

   minute	sample X (°C)	sample Y (°C)
   0	88	88
   1	83	82
   2	80	77
   3	80	73
   4	80	70
   5	80	68
   6	74	65

7. 5.

   Two melted samples cool in the same room. Which is more likely a pure substance, Milo?

   • Sample X: it holds steady at 80 °C for minutes 2 to 5 as it freezes
   • Both are pure, since both cooled from 88 °C to below 80 °C
   • Sample Y: it cools smoothly and steadily each minute, without any pause
8. 6.

   Noah says Y is pure too, just with a lower freezing point. What does the data show?

   • Y is pure, because its temperature keeps on falling each minute of the whole test
   • Y must be a compound, since compounds freeze gradually
   • Y has no flat stretch, so it is not freezing at one fixed temperature
9. 7.

   Pure benzoic acid melts at 122.4 °C. Sample 1 melts sharply at 122 °C. Sample 2 melts from 112 to 118 °C. What does sample 2 show, Pia?

   • It is a different pure compound that happens to melt lower
   • It is impure: impurities lower and widen the melting range
   • Sample 1 is the mixture, because it melts at one temperature

10. Four solids (real data, CRC)

    solid	density (g/cm³)	dissolves in water	magnetic
    iron	7.87	no	yes
    sand (quartz)	2.65	no	no
    aluminum	2.70	no	no
    table salt	2.17	yes	no

11. 8.

    A mix of aluminum filings and table salt needs separating. Which first step works, Ren?

    • Add water: the salt dissolves and the aluminum is filtered out
    • Use a magnet to lift the aluminum out of the salt
    • Float the salt off the top of water, since it is lighter
12. 9.

    Tala stirs a sand-and-aluminum mix into water to float one off. Will density separate them?

    • No: both sink, and their densities differ by only 0.05 g/cm³
    • Yes: sand is less dense than water, so it floats away
    • Yes: aluminum is denser, so it sinks to the bottom and the sand floats on top
13. 10.

    Aluminum is a metal, yet a magnet leaves it behind in a sand mix. Why, Zee?

    • Magnets attract only some metals, such as iron, and aluminum is not one
    • Aluminum is too dense for a magnet to lift
    • The sand's mass blocks the magnet from reaching the aluminum