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8th grade Elements and compounds worksheet

6 printable versionsBuilding Confidence10 questions · answer keyFree · no account

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Elements and compounds

Grade 8 Science Worksheet - Building Confidence

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

  1. A box holds 2 molecules of C₂H₆O. How many atoms are in the box altogether, Ari?

  2. Bo's sheet shows 2 molecules of H₂SO₄. How many atoms is that altogether?

  3. Baking soda is NaHCO₃. A tray holds 3 units of it. How many atoms is that in all, Esme?

  4. Jun says this box holds a compound because the atoms are joined. Is that right?

    • No: it is a mixture, since the balls are separate from each other
    • Yes: joined atoms always make a compound, whatever kind they are
    • Yes: each particle has two atoms in it, and two atoms make a compound
    • No: every ball is one kind, so it is an element

  5. What kind of matter is in this box, Luz?

    • A compound, since it has two kinds of atoms in it
    • A compound of He and N, since both are in one box
    • A mixture of elements: He and N₂ are not joined
    • A compound, since it has a joined pair of atoms

  6. Baking soda is NaHCO₃. How many kinds of atoms does it have, and what is it, Noah?

    • Six kinds, since it has six atoms in total
    • Two kinds: Na and HCO₃, so it is a compound
    • Three kinds: Na, H and CO, so it is a compound
    • Four kinds (Na, H, C, O): a compound

  7. Pia sees CO and Co on a chart. Are they the same substance?

    • Yes: both are cobalt, whichever letters are capitals
    • No: both are compounds made of two elements
    • No: CO is carbon and oxygen; Co is cobalt
    • Yes: capital letters do not change the substance

  8. Read this first

    Baking soda and vinegar (example data)

    observationbefore mixingafter mixing
    what is in the cupwhite powder and clear liquidbubbling liquid
    temperature (°C)2217

  9. Baking soda meets vinegar in a cup. Which clue best shows a new substance formed, Ren?

    • Gas bubbles formed that were not there before
    • Nothing new formed, since the same atoms are in the cup
    • The temperature changed, and any change means a new substance
    • The powder was gone, so it must have melted

  10. Ice melts, wood burns and sugar dissolves in tea. Which one is a chemical change, and why, Tala?

    • Ice melting, since it changed from a solid to a liquid
    • Wood burning, since ash and gas are new substances
    • Sugar dissolving, since the sugar seemed to disappear
    • The three together, since each change makes new substances

  11. Heating sugar (example data)

    propertybefore heatingafter heating
    colorwhiteblack
    dissolves in wateryesno
    smellnoneburnt

  12. Zee heats sugar in a spoon, and the table shows what happened. Is this a chemical change?

    • Yes: color, solubility and smell all changed
    • No: only the color changed, which is just looks
    • No: it is the same sugar, just hotter and darker
    • Cannot tell without weighing the sample first
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8th grade Elements and compounds worksheet – printable
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About this worksheet

Elements and compounds. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

  • I can sort a substance as an element (one kind of atom) or a compound (two or more kinds joined), from its formula or a particle box, and give the reason.
  • I can count atoms: all the atoms in a number of molecules, and the atoms of one element in a formula with a coefficient and brackets (3 Mg(NO₃)₂).
  • I can tell a compound from a mixture in a particle box, and say that a compound has its own properties.
  • I can use before-and-after data (melting point, mass, gas, color) to decide whether a new substance formed.
  • I can explain with a particle model why atoms are rearranged, not made or lost, and use a fixed mass ratio (water is about 1 g of hydrogen to 8 g of oxygen).
  • I can judge test evidence: separating points to a mixture, splitting points to a compound, and a test that fails to split a substance does not prove it is an element.

Words to know

element: A pure substance made of one kind of atom

compound: A pure substance of two or more kinds of atoms joined together

molecule: A group of atoms joined together; it can be one kind or several

kind of atom: Which element an atom is: every H atom is one kind

mixture: Substances stirred together but not joined

property: Something you can observe or measure, like color, state or melting point

coefficient: The number in front of a formula: how many units there are

Common mistakes

Your child counted the atoms in one molecule only. A number in front of a formula says how many molecules there are, so multiply by it.

Your child counted the kinds of atoms instead of all the atoms. Kinds are the different symbols; the question wants every atom, so add the subscripts.

Your child called a molecule a compound because it has several joined atoms. An element can have joined atoms if they are all the same kind.

Questions parents ask

What is the most common mistake with elements and compounds?

Your child counted the atoms in one molecule only. A number in front of a formula says how many molecules there are, so multiply by it.

Are these 8th grade science worksheets free to print?

Yes. All 6 printable versions at each level are free to print, with the answer key, and no account is needed.

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