Answer Key Ws 7.2 Half-life

  1. How old is the oldest tree?

The oldest tree is more than 4780 years old.

  1. DYN. How old is the oldest rock? What kind of rock is it?

4.55 billion years old. A meteorite

  1. Carbon Dating: Explain why all plants and animals contain the element carbon?

Plants use carbon dioxide to make their food. Animals take in the carbon when they eat plants

  1. What is the ratio of C-14 to C-12 when an organism is alive?

When an organism is alive,theratio of the number of C-14 atoms to the number of C-12 atoms in the organism remains nearly constant.

  1. What happens to this ratio when the organism dies?

When an organism dies, its C-14 atoms decay without being replaced; the ratio of C-14 to C-12 then decreases with time.

  1. Why is this ratio important?

Be measuring this ratio, the age of an organism's remains can be estimated. The older the sample is, the less C-14 there will be in the sample because it has changed into C-12

  1. Define radiocarbon dating.

Radiocarbon dating is the process of determining the age of an object by measuring the amount of C-14 remains in that object

  1. What is the age limit that radiocarbon dating can be used?

Organisms that lived within the past 50 000 years contains enough C-14 to be measured using radiocarbon.

  1. Fg 7.15A shows?

A sample is removed from bone for carbon dating. The bone is a human femur that is more than 500 years old.

  1. Define half-life.

A half-life is a constant for any radioactive isotope and is equal to the time required for half the nuclei in a sample to decay.

  1. Using table 7.5 p. 304, answer the next 3 questions. How many grams of strontium-90 will remain after 2 half lives have past if you started with 10g?

2.50 grams of strontium-90 will remain after 2 half lives have past if you started with 10g

  1. How many years are 3 half-lives of strontium-90?

87

  1. If you started with 20g of strontium-90, what percentage will remain after 4 half-lives

6.25% of strontium will remain after 4 half-lives have past if you started with 20g

  1. Using a Decay Curve: Define decay curve.

A decay curve is a curved line on a graph that shows the rate at which radioisotopes decay.

  1. What is the only difference between the decay of any radioisotope?

The only difference between the decay of any radioisotope will be the length of the half-life.

  1. Compare the half-lives of radon-222 with uranium-238.

Radon-222 = 3.8 daysU-238 = 4.5 billion years

  1. What does fg. 7.16 p. 305 show?

The graph shows how the amount of strontium-90 in a sample changes over time

  1. What % of St-90 remains after 3 half-lives?

12.5% of St-90 remains after 3 half-lives

  1. How many grams are there after 4 half-lives?

0.625 grams of St-90 remains after 4 half-lives

  1. What is the half-life of I-131?

I-131 has a half-life of eights days

  1. Why must I-131 be prepared in or near a hospital?

It decays so quickly it must be used immediately

  1. Why is 16 days 2 half-lives?

Because 8 days is one half life of I-131

  1. Why does only 5 grams remain after 2 half-lives?

Because you started with 20g, after 1 half life you have 10g, after another half-life you have half of that = 5 g

  1. What does 3 half-lives mean?

The original mass of isotope will be reduced by half three times

  1. In ex 2, how many grams will there be after 4 half-lives?

4g

  1. Common Isotope Pairs: What does table 7.6 show?

Shows what each radioactive isotope turns into during decay

  1. What is the difference between a parent isotope and a daughter isotope?

Parent is what you start with, daughter is what the parent turns into

  1. Give an example of each.

C-14 turns into N-14

  1. What is the longest half-life?

Rubidium-87 at 47 billion years

  1. The Potassium-40 Clock: What is the relationship between K-40 and Ar-40?

K-40 is the parent and Ar-40 is the daughter. This happens at a specific rate (half life)

  1. Argon is a gas. Why does lava set the K-40 / Ar-40 clock back to zero?

In liquid phase, all the Ar-40 rises out of the rock because it is less dense. The molen rock has no Ar-40 in it.

  1. What happens to the molten rock as it cools?

As the rock hardens, the constantly forming daughter Ar-40 is trapped inside the solid rock. When the rock hardens, the clock begins as it starts to trap the constantly forming Ar-40

  1. Where did the Ar-40 gas come from?

From the K-40 parent during half life decay

  1. Using data from a potassium-40 clock: What does table 7.7 show?

As half lives pass, there is less K-40 and more Ar-40 (the ratio of Ar-40 to K-40 increases)

  1. If a sample of rock contained equal masses of K-40 and Ar-40, why would you say the sample is 1.3 billion years old?

It is exactly 1 half life old

  1. Table 7.7: What does a ratio of Ar-40 to K-40 of 0:1 mean?

There is no Ar-40 formed yet ( time 0)

  1. What does it mean if there is more Ar-40 compared with K-40 in a sample of rock?

The age of the sample is more then 1 half life old

  1. What does it mean if the daughter curve is above the parent curve?

The age of the daughter is more then 1 half life

  1. Fg 7.17 p. 308. what do the red and blue lines indicate?

% of substance remaining in sample

  1. How many years are 2 half-lives of K-40?

2.6 billion years

  1. If a ratio of 15:1 Ar-40 to K-40 exists, how old is the sample?

5.2 billion years

  1. If you have 25% of K-40 remaining in the sample, how old is it?

2 half lives old = 2.6 billion years

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