Elements, compounds and mixtures

Answer Key - Grade 10 Science Worksheet - Challenge Me

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

1. 1.

   Ari finds 3.545 g of chlorine in 10 g of sand and salt. In NaCl, 35.45 g of every 58.44 g is chlorine. What percent of the mix is NaCl, to 0.1%?

   58.4 %
2. 2.

   Esme finds 8.862 g of chlorine in 15 g of sand and salt. In NaCl, 35.45 g of every 58.44 g is chlorine. What percent of the mix is NaCl, to 0.1%?

   97.4 %

3. Read this first

   Two compounds of hydrogen and oxygen (real data, IUPAC masses)

   compound	formula	grams of oxygen per gram of hydrogen
   water	H₂O	7.94
   hydrogen peroxide	H₂O₂	15.9

4. 3.

   Jun says water and peroxide are one mixture of hydrogen and oxygen in different amounts. What does the table show?

   • The ratios differ, so both are mixtures that could hold any amounts whatever
   • Each has its own fixed ratio, so each is a compound, not a mixture
   • Both hold the same two elements, so they are the same compound
5. 4.

   Sample Z is one pure substance made of 1.00 g hydrogen joined with 15.9 g oxygen. Which is it, Luz?

   • Water, because its formula also holds hydrogen and oxygen
   • Hydrogen peroxide, H₂O₂, whose fixed ratio is 15.9 g per gram
   • Water mixed with extra oxygen, which would give the same two totals
6. 5.

   Milo says clear, even sugar water is one substance. Boiling off the water leaves sugar crystals. Which reply fits?

   • It is a solution, a mixture: evaporating separates the sugar from the water
   • Agree: the crystals show the sugar turned into water and back
   • Agree: a clear, even liquid cannot be a mixture
7. 6.

   Noah says dissolving makes a new substance. 20.0 g salt plus 100.0 g water weighs 120.0 g, and boiling away the water leaves 20.0 g of salt. Verdict?

   • No: the mass is kept and the salt comes back unchanged
   • Yes: the salt vanished, so it must have been changed or destroyed
   • Yes: the 20.0 g of crystals that came back must be new salt
8. 7.

   Pia says air is a compound, since it is about 78% nitrogen and 21% oxygen. Liquid air is distilled and the gases boil off one at a time. Verdict?

   • Yes: its makeup is nearly the same everywhere, so air has a fixed ratio like a compound
   • No: the gases keep their own properties and boil off at different temperatures
   • No: air is an element because gases are elements

9. Paper chromatography of three black markers (example data)

   marker	spots found at (fraction of solvent distance)
   1	0.82
   2	0.30, 0.55, 0.82
   3	0.30, 0.82

10. 8.

    Ren runs three black markers on paper. Which statement does the chromatogram support?

    • Markers 2 and 3 each hold at least two dyes, and 3's dyes are both in 2
    • Marker 3 is a compound because its spots sit at fixed distances
    • Marker 1 is proven to be a single pure dye because it left a single spot
11. 9.

    Well water passes a filter and looks clear. Boiling away 1.00 L leaves 0.50 g of white solid. Is the water pure, Tala?

    • Yes: the filter removed everything that could have been in the water
    • Yes: 0.50 g in a whole liter is too small to matter
    • No: 0.50 g per liter of dissolved solids stays behind, though unseen
12. 10.

    A gold-colored coin has a density of 15.5 g/cm³. Pure gold is 19.3 and copper is 8.96 g/cm³. What does 15.5 suggest, Zee?

    • It is pure gold, because a pure sample's density can vary
    • It is probably not pure gold; a gold-copper mixture fits a density between the two
    • It is pure copper, because gold-colored metal is copper