كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

Theme E — Nuclear and Quantum Physics

E.3 Radioactive Decay — IB Physics SL Revision Notes & Topical Questions

5 min read 11 parts Lesson 7 of 30 1 quiz

📚 IB Physics SL Hub / Topical Revision

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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Lesson summary

E.3 Radioactive Decay — IB PHYSICS SL notes: Describe nuclear stability, binding energy and mass defect, the properties of alpha, beta and gamma radiation, and the random nature of decay including half-life.Radioactive decay — the random, spontaneous emission of particles or radi…

Conservation
Isotopes and stability
Strong nuclear force
Conversion
Binding energy per nucleon
Beta-minus

Section 1 of 11Nature of radioactive decay

Radioactive decay

Radioactive decay the random, spontaneous emission of particles or radiation from an unstable nucleus

Conservation

Conservation nucleon number and charge are conserved in every decay

Radiations

Alpha is a helium nucleus.
Beta-minus is an electron plus an antineutrino.
Beta-plus is a positron plus a neutrino.
Gamma is a high-energy photon.

Section 2 of 11Nuclear stability

Isotopes and stability

Isotopes and stability light stable nuclei have roughly equal numbers of neutrons and protons
Heavier stable nuclei need extra neutrons to offset the growing repulsion between protons.
Nuclei away from the line of stability decay.

Strong nuclear force

Strong nuclear force an attractive force between nucleons
It has a very short range, a few femtometres.
It is much stronger than the electric force at that range.
It is the same for protons and neutrons.

Section 3 of 11Mass defect and binding energy

1Mass defect

Mass defect a nucleus has less mass than its separate nucleons

2Binding energy

Δmc², the energy needed to separate the nucleus completely into its nucleons

3Conversion

1 u corresponds to 931.5 MeV c⁻²

4Binding energy per nucleon

Binding energy per nucleon it peaks near A ≈ 56–62, the most stable nuclei

Section 4 of 11Alpha decay

Equation

^A_ZX → ^A−4_Z−2Y + ⁴₂α

Energies

Energies alpha particles have discrete energies

Properties

Properties strongly ionising and stopped by paper or a few centimetres of air

Section 5 of 11Beta decay

Beta-minus

Beta-minus a neutron becomes a proton, emitting an electron and an antineutrino, so Z rises by 1

Beta-plus

Beta-plus a proton becomes a neutron, emitting a positron and a neutrino, so Z falls by 1

Energies

Energies beta particles have a continuous range of energies, which was evidence for the neutrino

Properties

Properties stopped by a few millimetres of aluminium

Section 6 of 11Gamma emission

1Process

Process an excited nucleus emits a photon, and A and Z are unchanged

2Properties

Properties weakly ionising and very penetrating, with intensity reduced by thick lead or concrete

Section 7 of 11Radiations compared

Property	Alpha	Beta-minus	Gamma
Nature	Helium nucleus	Electron	Photon
Charge	+2e	−e	0
Ionising ability	High	Medium	Low
Stopped or reduced by	Paper	A few mm of aluminium	Thick lead
Energy spectrum	Discrete	Continuous	Discrete

Section 8 of 11Activity and half-life

Activity

Activity the number of decays per unit time, in becquerel (Bq)

Half-life

Half-life the time for the activity, or the number of undecayed nuclei, to halve

Fraction remaining

Fraction remaining after n half-lives it is (½)ⁿ

Background

Background subtract the background count rate before analysing data

Section 9 of 11Measuring a half-life

Record the background count rate with no source present.

Measure the count rate at regular intervals.

Subtract the background from each reading.

Plot the corrected count rate against time.

Read off the time for the rate to halve at several points and average.

Section 10 of 11Key terms

Term	Meaning
Radioactive decay	The random, spontaneous emission of particles or radiation from an unstable nucleus.
Half-life	The time taken for half the radioactive nuclei in a sample to decay.
Activity	The number of nuclei decaying per unit time.
Mass defect	The difference between the mass of a nucleus and the total mass of its separate nucleons.
Binding energy	The energy required to separate a nucleus completely into its nucleons.
Strong nuclear force	The short-range attractive force between nucleons.
Background radiation	Radiation from natural and artificial sources that is always present.

Section 11 of 11Exam technique

Avoid these mistakes

Saying alpha is the most penetrating radiation — it is the least penetrating.

Forgetting to subtract background radiation from count rates.

Thinking half-life is affected by temperature or chemical state.

Last updated: 18 October 2026