Elements and compounds

Answer Key - Grade 8 Science Worksheet - Challenge Me

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

1. 1.

   Water holds hydrogen and oxygen in a fixed ratio by mass, about 1 g to 8 g. How many grams of water form from 2 g of hydrogen and plenty of oxygen, Ari?

   18 g
2. 2.

   In water, hydrogen and oxygen are in a fixed ratio by mass, about 1 g to 8 g. Bo burns 3 g of hydrogen in plenty of oxygen. How many grams of water form?

   27 g
3. 3.

   Water is about 1 g of hydrogen for every 8 g of oxygen. Esme's sample of water was made from 5 g of hydrogen. What is the sample's mass in grams?

   45 g

4. Read this first

   Use this

   (particle model).

6. 4.

   Which statement does the particle model support about the atoms, Jun?

   • Four new O atoms formed to make the water
   • The hydrogen was used up, so 8 atoms are gone
   • Atoms were only rearranged: 8 H and 4 O before and after
   • The H and O are still separate in the after box, like a mixture
7. 5.

   How do the kinds of atoms and the kinds of substances change from before to after, Kira?

   Use the picture in question 4.

   • Two kinds of atoms before and after; elements became a compound
   • Two kinds of atoms before and three after the reaction
   • Two kinds of atoms before and one after, since one substance remains
   • Nothing changes: it is still two substances after
8. 6.

   Luz says the after box is a mixture of hydrogen and oxygen. Which evidence from the model answers that best?

   Use the picture in question 4.

   • Every O is joined to two H, forming one kind of molecule
   • A mixture always has exactly two substances in it
   • The after box has fewer molecules than before, so atoms were lost
   • Water is common, so it cannot be a mixture of gases

9. Test results (example data)

   sample	separated by a magnet	split by electricity
   X	yes (two solids)	no
   Y	no	yes (two gases)
   Z	no	no

10. 7.

    Which sample is most likely a compound, Milo?

    • Y: a magnet cannot separate it, but electricity splits it
    • X, since a magnet pulled two different solids out of it
    • Both Y and Z, since neither was separated by a magnet
    • Z, since it resisted both tests, so it must be joined tightly
11. 8.

    Noah says sample Z must be an element, since it did not split in either test. How strong is that evidence?

    • Strong: Z passed the magnet test, which shows it is an element
    • Strong: any pure substance that does not split is an element
    • Wrong: Z must be a mixture, since nothing came out
    • Weak: Z may be a compound that these two tests cannot split

12. Two salt samples (example data)

    sample	source	sodium (% by mass)	chlorine (% by mass)
    A	sea water	39.3	60.7
    B	a mine	39.3	60.7

13. 9.

    What do the two rows show about sodium chloride, Pia?

    • Each sample is a mixture, since percentages are shown for two parts
    • Salt from different places has different amounts of sodium
    • Chlorine is the larger part, so the salt is mostly chlorine gas
    • A compound has a fixed ratio of elements, wherever it comes from

14. Heating sugar (example data)

    observation	result
    formula of sugar	C₁₂H₂₂O₁₁
    before heating	white crystals that dissolve in water
    after strong heating	black solid (carbon) and water vapor

15. 10.

    Tala says sugar is a mixture of carbon and water, since both come out when it is heated. Which reply is best supported?

    • Agree: whatever comes out of it must have been in it as a mixture
    • Agree: sugar looks like water and charcoal stirred together
    • Disagree: sugar is a compound; heating rearranges its joined atoms
    • Disagree: sugar is an element, since it has a formula of its own