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9th grade Find a rock's age from radioactive decay worksheet

6 printable versionsBuilding Confidence10 questions · answer keyFree · no account

Level: Building Confidence · On Level · Challenge Me

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Find a rock's age from radioactive decay

Grade 9 Science Worksheet - Building Confidence

NameDate

Circle the best answer.

  1. What is a half-life, by Ari's reading (USGS)?

    • A time that changes with sample size
    • The time for the whole parent to decay
    • Half the age of the rock
    • The time for half of the parent to decay

  2. A rock has 100 g of a parent isotope. How much is left after one half-life, by Bo's math?

    • 0 g
    • 25 g
    • 75 g
    • 50 g

  3. Two samples of potassium-40, one big and one small. A big sample and a small sample of potassium-40 are compared. Which has the longer half-life, by Kira's reading?

    • It changes with temperature
    • They have the same half-life
    • The small sample
    • The big sample

  4. After two half-lives, what fraction of the parent remains, by Luz's math?

    • 1/4
    • 1/3
    • 1/2
    • It has fully decayed

  5. Read this first

    Half-lives (USGS)

    ParentDaughterHalf-life
    uranium-238lead-2064.5 billion years
    uranium-235lead-207704 million years
    potassium-40argon-401.25 billion years
    carbon-14nitrogen-145,730 years

  6. What is the half-life of carbon-14, by Milo's reading?

    • 5,730 years
    • 1.25 billion years
    • 4.5 billion years
    • 5,730 million years

  7. Which clock fits a piece of old wood from a campfire site, by Noah's reading (USGS)?

    • Potassium-40
    • Carbon-14
    • Uranium-238
    • Uranium-235

  8. Which clock fits a volcanic rock about 2 billion years old, by Pia's reading?

    • Counting rock layers
    • Carbon-14
    • Uranium-238 or potassium-40
    • Any clock gives the same result

  9. Up to about what age can carbon-14 date material, by Ren's reading (USGS)?

    • About 4.5 billion years
    • Any age
    • About 5 years
    • About 50,000 years

  10. A sample has equal numbers of parent and daughter atoms (1:1). How many half-lives have passed, by Tala's reading?

    • Two
    • Zero
    • Half of one
    • One

  11. A sample has 1 parent atom for every 3 daughter atoms (1:3). How many half-lives have passed, by Zee's math?

    • Four
    • One
    • Two
    • Three, from the 3
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9th grade Find a rock's age from radioactive decay worksheet – printable
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About this worksheet

Find a rock's age from radioactive decay. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

  • I can turn a fraction left or a parent-to-daughter ratio into half-lives, and half-lives into an age.
  • I can choose the right clock: carbon-14 for once-living material up to about 50,000 years, uranium or potassium for old rock.
  • I can bracket a sedimentary layer between dated ash or dikes and explain why agreeing methods build confidence.

Standard: HS-ESS1-5, HS-ESS1-6

Words to know

parent isotope: The radioactive atom that decays

daughter isotope: The stable atom it decays into

half-life: The time for half of the parent to decay

bracketing: Dating layers above and below to fix a layer's age range

radiocarbon: Carbon-14, used for once-living material

Common mistakes

Your child treated decay as steady subtraction. Each half-life halves the parent that is left: 1/2, 1/4, 1/8 (USGS).

Your child picked the wrong clock. Carbon-14 (half-life 5,730 years) dates once-living material up to about 50,000 years; uranium and potassium clocks date rocks millions to billions of years old (USGS).

Your child mixed up the parent-to-daughter ratio with the fraction left. 1:1 means 1/2 left (one half-life); 1:3 means 1/4 left (two); 1:7 means 1/8 left (three).

Questions parents ask

What is the most common mistake with find a rock's age from radioactive decay?

Your child treated decay as steady subtraction. Each half-life halves the parent that is left: 1/2, 1/4, 1/8 (USGS).

Are these 9th 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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