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]

4.6Unit 4: Energy, Environment, Microbiology and Immunity

Forensics — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 11 parts Lesson 7 of 28 1 quiz

📚 IAL Edexcel Linear 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

4.6 Forensics — IAL EDEXCEL LINEAR notes: Describe how time of death is estimated and how PCR, gel electrophoresis and DNA profiling are used in forensic science.Decomposers — bacteria and fungi break down dead plants and animals and recycle the chemical components needed by livi…

Forensic use
Early stage (a few days)
Later stage (days to weeks)
After a few weeks
Months to years
Rate of decomposition

Part 1 of 9Microorganisms and decay

Decomposers

Decomposers bacteria and fungi break down dead plants and animals and recycle the chemical components needed by living organisms.
They secrete enzymes that break large molecules such as cellulose into small ones such as glucose.
Their respiration releases CO₂ and methane, and CO₂ can be fixed again by plants in photosynthesis.
Waste products released during decomposition provide nutrients to plants.

Forensic use

Forensic use the extent of decomposition is one of several indicators of time of death (TOD).
TOD is only an estimate and cannot be pinpointed exactly.

Part 2 of 9Extent of decomposition

1Early stage (a few days)

Early stage (a few days) decomposers break down cells and tissues and the skin often appears greenish.

2Later stage (days to weeks)

Later stage (days to weeks) microorganisms break down tissues and organs and produce gases such as methane that bloat the body.
The skin blisters and falls off.

3After a few weeks

After a few weeks soft tissues turn to liquid and leave the body.

4Months to years

Months to years only a skeleton remains, which disintegrates after decades or centuries.

5Rate of decomposition

Rate of decomposition it is slower at lower temperatures and in anaerobic conditions and faster at high temperatures.

Part 3 of 9Stage of succession on a body

Succession
- the change in the organisms found on a body over time, with the dead body as the habitat.
- Unlike ecological succession, early arrivals are not out-competed, and all newly arriving species remain as decomposition progresses.

Bacteria are on and in the body immediately after death.

Tissue decomposition creates conditions for flies to lay eggs and larvae to hatch.

As fly larvae consume soft tissue, conditions become favourable for beetles.

As tissue dries out, flies leave because they prefer moisture, whereas beetles can use dry tissue and stay.

When all tissue is gone, most organisms leave.

Location
- stages differ if the body is buried in soil, buried in a coffin or under water, because access for insects and oxygen change.

Part 4 of 9Forensic entomology

1Forensic entomology

Forensic entomology the study of insect colonies on a dead body to estimate TOD.

2Arrival times

Arrival times flies colonise within a few hours of death, but beetles come later.

3Life cycle stage

Life cycle stage blowfly eggs hatch after about 24 hours, so larvae show death was more than 24 hours earlier.
If only blowfly larvae are present, about 24 hours has passed, because insects with longer life cycles are absent.

4Factors

Factors drugs in the body, humidity, oxygen availability and temperature affect insect life cycles.

Part 5 of 9Body temperature and rigor mortis

Algor mortis

Algor mortis cooling of the body after death,

when metabolic heat production stops and the body falls from about 37 °C to the surrounding temperature.
The body cools by about 1.5–2.0 °C per hour, so temperature can estimate TOD.
Rate of cooling change in temperature ÷ time.
Air temperature, surface area to volume ratio, clothing and body fat affect the rate.

Rigor mortis

Rigor mortis stiffening of muscles from about 4–6 hours after death.

Without oxygen, muscle cells respire anaerobically and produce lactic acid, which lowers pH.
The low pH denatures the enzymes that make ATP.
Without ATP, myosin heads cannot detach from actin, so muscles stay contracted.
It begins in small muscles of the head and ends in the large muscles of the lower body.
It is complete in every muscle between 12 and 18 hours and wears off after about 24–36 hours.
Higher temperatures speed it and muscle development also affects it.

Part 6 of 9Polymerase chain reaction (PCR)

DNA profile
- every person except identical twins has a unique DNA sequence, which identifies individuals and shows relationships.

Isolate a DNA sample from saliva, skin, hair or blood.

Amplify the DNA fragments by PCR.

Carry out gel electrophoresis on the product.

Analyse the pattern of fragments.

PCR

PCR an in vitro method of DNA replication that amplifies DNA, even from a single molecule.

Each cycle doubles the DNA, so 20 cycles make about a million copies, and N = N₀ × 2ⁿ.
A thermal cycler provides the temperature and timing of each stage automatically.

Requirements

Requirements template DNA or RNA, primers, Taq polymerase, free nucleotides and a buffer.
Primers are short single-stranded DNA complementary to the 3′ end of the target and mark where polymerase binds.
Taq polymerase comes from Thermus aquaticus, does not denature at 95 °C and has an optimum high enough to prevent uncopied strands re-annealing.

Stage	Temperature	Events
Denaturation	95 °C	Hydrogen bonds between the strands break
Annealing	50–60 °C	Primers bind to the ends of the single strands
Elongation	72 °C	Taq polymerase builds complementary strands

Preparation for electrophoresis
- restriction endonucleases cut the DNA into fragments and a fluorescent tag lets fragments be seen under UV light.

Part 7 of 9Gel electrophoresis

Principle
- DNA is negatively charged because of its phosphate groups, so in an electric field it moves towards the anode (positive pole).
- The pores of the gel let small fragments move faster and further than large ones, so fragments separate by size.

Make an agarose gel plate with wells at one end.

Submerge the gel in a tank of electrolyte, a salt solution that conducts electricity.

Load DNA samples into the wells with a micropipette, with a DNA standard in the first well.

Connect the negative electrode at the well end and the positive anode at the far end, then pass a current.

Add probes, which are single-stranded DNA complementary to regions of interest, labelled with radioactivity or a fluorescent dye.

Take an X-ray image or shine UV light to show the bands and compare them with the standard.

Fragments
- restriction endonucleases cut at specific sites between repeated sections called variable number tandem repeats (VNTRs).
- Microsatellites have fewer repeats than minisatellites.
- Different people have different numbers of repeats, so fragment lengths and banding patterns differ.

Part 8 of 9Uses of DNA profiling

Paternity

Paternity a child shares half of their DNA with each parent,
- so the candidate whose bands match the child's non-maternal bands is the most likely father.
More matching bands mean closer relationship.

Crime scenes

Crime scenes profiles from scene DNA are compared with suspects' profiles using short tandem repeats (STRs), non-coding repeats that vary between people.
The more regions examined, the more reliable the evidence, and 11 or more sites are considered reliable in court.
Few regions risk closely related individuals sharing an identical profile.

Breeding programmes

Breeding programmes profiles identify the most genetically different animals or plants to crossbreed.
Inbreeding builds up harmful recessive alleles and shrinks the gene pool, reducing ability to adapt.

Other uses

Other uses ancestry kits and determining evolutionary relationships between species.

Part 9 of 9More from the specification

Polymerase chain reaction thermal cycling process
- automated thermocycling repeatedly denatures and amplifies target DNA segments in an enzymatic replication process.

Key terms

Term	Meaning
Time of death	The estimated time since a person died, found from several body and insect indicators.
Algor mortis	Cooling of the body after death.
Rigor mortis	Stiffening of muscles after death because no ATP is available to release myosin from actin.
PCR	Polymerase chain reaction, an in vitro method that amplifies DNA.
Taq polymerase	A heat-stable DNA polymerase from //Thermus aquaticus//.
Gel electrophoresis	Separation of DNA fragments by size in an electric field.
VNTR	Variable number tandem repeat, a repeated non-coding sequence that differs between people.
Primer	A short single-stranded DNA sequence that marks where DNA polymerase starts copying.

Avoid these mistakes

Saying a body keeps cooling below its surroundings — it cools only until it reaches the temperature of its surroundings.

Saying DNA moves towards the cathode in electrophoresis — it is negatively charged and moves towards the anode.

Saying rigor mortis is caused by too much ATP — it results from lack of ATP, so myosin heads cannot detach from actin.

Giving time of death as an exact figure — it is only an estimate affected by many factors.

Last updated: 15 October 2026