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.

   Bo finds 4.254 g of chlorine in 20 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%?

   35.1 %
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?

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

   A tank holds 1.00 g of hydrogen gas and 3.00 g of oxygen gas, unreacted. Is its content a compound, Kira?

   • Yes: any ratio of two elements counts as a compound
   • No: 3.00 g of oxygen per gram fits neither fixed ratio, so it is a mixture
   • Yes: it holds hydrogen and oxygen together in one ratio
6. 5.

   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
   • Water mixed with extra oxygen, which would give the same two totals
   • Hydrogen peroxide, H₂O₂, whose fixed ratio is 15.9 g per gram
7. 6.

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

   • Agree: the crystals show the sugar turned into water and back
   • Agree: a clear, even liquid cannot be a mixture
   • It is a solution, a mixture: evaporating separates the sugar from the water
8. 7.

   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?

   • 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
   • No: the mass is kept and the salt comes back unchanged
9. 8.

   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?

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

10. 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

11. 9.

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

    • Marker 1 is proven to be a single pure dye because it left a single spot
    • 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
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 probably not pure gold; a gold-copper mixture fits a density between the two
    • It is pure gold, because a pure sample's density can vary
    • It is pure copper, because gold-colored metal is copper