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7.2.7Acids, Bases and Salts

Half-Life and Risk — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 8 of 11 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 5Short and Long Half-Lives

Short half-life
  • Short half-life Nuclei decay quickly, so the isotope emits a lot of radiation in a short time and loses its activity fast.
Long half-life
  • Long half-life Nuclei decay slowly, so the isotope emits less radiation per second but stays radioactive for a long time.
Large amounts
  • Large amounts A large quantity of a short half-life isotope can be extremely dangerous to handle because its activity is high.
Long-term risk
  • Long-term risk Materials with long half-lives present a contamination risk for many years, so they need long-term storage.

Section 2 of 5Medical Tracers

1
A tracer is a radioactive isotope introduced into the body so its movement, for example in the blood, can be followed.
2
Gamma emitters are chosen because gamma rays penetrate the body and can be detected from outside it.
3
The half-life is chosen to be a few hours, long enough for the procedure but short enough to limit long-term harm.
4
The amount used is kept as small as possible to reduce the patient's exposure to radiation.

Section 3 of 5Choosing an Isotope

Choosing an Isotope
Radiation type
  • Radiation type Gamma is preferred for tracers because it can leave the body;
    • alpha is absorbed too quickly to be detected outside.
Half-life
  • Half-life Short for medical use, so the activity falls quickly, and long for waste that must be stored for many years.
Amount
  • Amount Using less material gives less radiation, which reduces the hazard from any isotope.
Waste disposal
  • Waste disposal Long half-life waste is buried underground so that it cannot reach people or the environment.

Section 4 of 5Worked Example

Question

  • A medical tracer has a half-life of 4 hours.
  • What fraction of the original activity remains after 12 hours?

  1. Number of half-lives = 12 ÷ 4 = 3.
  2. Fraction remaining = (1/2)³ = 1/8 = 0.125.

Answer 1/8, or 12.5 % of the original activity

Section 5 of 5Exam Checklist & Tips

7.2.7 – Key Points for the Exam

  • Tracer A radioactive isotope used to follow the movement of a substance through a system.
  • Gamma emitter An isotope that emits gamma radiation, which can be detected outside the body.
  • Half-life The time taken for the activity of a sample to halve.
  • Activity The number of decays per second in a sample of radioactive material.
  • Disposal The safe storage or burial of radioactive waste until it is no longer hazardous.

Common Mistakes

  • Stating that a short half-life is always safer;
    • a large amount of a short half-life isotope can be very dangerous.
  • Choosing an alpha emitter as a medical tracer;
    • alpha is absorbed in very little distance and cannot be detected outside the body.
  • Assuming a long half-life means more radiation;
    • for the same number of nuclei, a long half-life gives a lower activity.
  • Confusing activity with half-life;
    • activity is the rate of decay and half-life is the time for that rate to halve.

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