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.

   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 %

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?

   • 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
   • Both hold the same two elements, so they are the same compound
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?

   • No: 3.00 g of oxygen per gram fits neither fixed ratio, so it is a mixture
   • Yes: any ratio of two elements counts as a compound
   • 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?

   • Hydrogen peroxide, H₂O₂, whose fixed ratio is 15.9 g per gram
   • Water, because its formula also holds hydrogen and oxygen
   • Water mixed with extra oxygen, which would give the same two totals
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
   • It is a solution, a mixture: evaporating separates the sugar from the water
   • Agree: a clear, even liquid cannot be a mixture
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
   • No: the mass is kept and the salt comes back unchanged
   • Yes: the 20.0 g of crystals that came back must be new salt
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?

   • 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
   • No: the gases keep their own properties and boil off at different temperatures
10. 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?

    • No: 0.50 g per liter of dissolved solids stays behind, though unseen
    • Yes: 0.50 g in a whole liter is too small to matter
    • Yes: the filter removed everything that could have been in the water
11. 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 copper, because gold-colored metal is copper
    • 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