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]

D4.2Theme D — Continuity and change

Stability and change — IB Biology SL Revision Notes & Topical Questions

10 min read 13 parts Lesson 11 of 14 1 quiz

📚 IB Biology 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

D4.2 Stability and change — IB BIOLOGY SL notes: Explain what keeps ecosystems stable, how human activity threatens them, and how they can be studied, harvested sustainably and restored.Features of a stable ecosystemEfficient nutrient cycling, so it is self-supporting.High biodiv…

Ancient rainforests
Stable does not mean static
Supply of energy
Recycling of nutrients
Genetic diversity
Climatic variables within tolerance

Part 1 of 11Stable ecosystems

Stable ecosystems

Features of a stable ecosystem

Efficient nutrient cycling, so it is self-supporting.
High biodiversity.
Resistance to change, so consumer populations stay steady and resources are not overused.
High levels of photosynthesis.

Ancient rainforests

Ancient rainforests the Amazon and Congo forests have lasted tens of millions of years
They hold millions of invertebrate species and thousands of plant, bird and fish species.
Light and moisture are plentiful, so photosynthesis rates are high.
Detritivores such as termites, slugs and worms and decomposers such as fungi recycle organic matter.
Water is cycled as transpiration, condensation and rainfall.

Stable does not mean static

Stable does not mean static natural selection still acts on the species

Part 2 of 11Requirements for stability

1Supply of energy

Supply of energy a reliable source, usually sunlight captured by producers and passed up the food chain

2Recycling of nutrients

Recycling of nutrients decomposers break down compounds in dead matter
Carbon is released as CO₂ and nitrates and phosphates are released to the soil.
Decomposers need enough oxygen, moisture and a suitable temperature.
Hot, dry systems recycle slowly and are less productive.
Removing timber or crops breaks the cycle and lowers productivity.

3Genetic diversity

Genetic diversity the number of different alleles in a population
It lets selection act on favourable alleles, so the population can adapt.
A population with few alleles is less likely to have a favourable one if conditions change.

4Climatic variables within tolerance

Climatic variables within tolerance temperature and rainfall must stay within the range of tolerance
Beyond it a species must migrate or face extinction.
Human-caused climate change is moving some variables beyond tolerance levels.

Part 3 of 11Tipping points

Tipping points

Amazon deforestation

Amazon deforestation it affects the temperature and rainfall of the ecosystem
Transpiration releases water vapour, which cools the air and affects rainfall.
Fewer trees mean less transpiration, less rain and a drier forest.
Temperature and rainfall affect photosynthesis and nutrient cycling.

Tipping point

Tipping point the point beyond which the ecosystem is no longer stable and may become savanna

Nobody knows how much clearance would trigger it.

Percentage loss of forest

(original area - final area) / original area × 100

Part 4 of 11Mesocosms

Mesocosm

Mesocosm an experimental container in which a natural ecosystem is simulated

It is used to study responses to factors such as nutrients, light, temperature or dissolved CO₂.
Land tanks can test sewage on ponds.
Underwater enclosures can test temperature or CO₂ in coastal waters.
Glasshouses can trace carbon through a rainforest.

Realism

Realism the enclosed, controlled nature is unrealistic, so larger mesocosms that allow mixing of water layers are more realistic

Design

The container is clear so light reaches the producers.
It contains autotrophs, decomposers and a few small primary consumers.
It holds no secondary consumers, as there is too little energy and predation would be unethical.
A sealed system is better controlled than an open one.

Control

Control an identical mesocosm in which the test factor is unchanged
Oxygen, pH and population sizes are monitored over weeks.

Part 5 of 11Building a mesocosm

Terrestrial mesocosm

Place gravel in the bottom of a clear container for drainage.

Add a layer of charcoal, which helps prevent mould.

Add moss or filter paper to separate the layers.

Add soil or compost to provide organic matter and microorganisms.

Plant slow-growing producers such as mosses and ferns.

Water it, seal it with a lid and place it in a stable light location.

Aquatic mesocosm

Put pond or lake sediment in the base for nutrients and microorganisms.

Add pond water, not tap water.

Add healthy aquatic plants.

Add a few water fleas or water snails, which are primary consumers only.

Place it in a light location at a stable temperature.

Ethics
- the IB policy allows animals only if there is no alternative
- The best option is to leave animals out.
- If used, include a few herbivores with enough food and a stable temperature, for a short time.
- Return the animals to the wild afterwards.

Part 6 of 11Keystone species

Keystone species

Keystone species species with a disproportionate effect on the structure and function of the ecosystem

Removal can cause loss of other species and ecosystem collapse.
Protecting them stabilises the ecosystem.

Sea otters

Sea otters in Pacific kelp forests they control sea urchins, which would otherwise overgraze the kelp

Beavers

Beavers dams slow the water and create wetlands for many species

Elephants

Elephants in African grassland they eat shrubs and saplings, which keeps the grass for zebras and so lions

Others

Others wolves and some fig trees

Part 7 of 11Sustainable harvesting

Sustainable harvest

Sustainable harvest a resource is replaced as fast as it is harvested, so it does not run out

Assess population size, growth and reproduction and set a quota below the replacement rate.
Use quotas, size limits, closed seasons and protected areas.

Black cherry timber

Black cherry timber a slow-growing hardwood
Use selective felling, not clear felling, which leaves gaps in the canopy.
Leave enough trees to flower and make seeds.
Monitor that regrowth keeps up with logging.

Alaska pollock fishery

Alaska pollock fishery certified sustainable by the Marine Stewardship Council
Pollock grow fast and breed from 3-4 years.
Nets rarely touch the seabed.
Bycatch is under 1% and cannot be sold.
Research vessels and on-board observers monitor it.
Areas with high salmon bycatch are closed.

Part 8 of 11Sustainability of agriculture

Problem	Cause	Consequence	Mitigation
Soil erosion	Bare soil, clearing and overgrazing	Loss of fertile topsoil	Cover crops and terracing
Nutrient run-off	Excess soluble fertiliser washed out by rain	Eutrophication downstream	Small doses, avoiding rain and using organic fertiliser
Fertiliser supply	Expensive and energy-intensive to make	High cost, with price tracking energy costs	Organic fertilisers
Pollution	Pesticides, herbicides and fungicides	Harm to non-target species	Biological control and GM crops
Carbon footprint	Fuel, fertiliser manufacture and livestock methane	Climate change	Renewable energy and efficient machinery

Biological control
- releasing a pest's predators, such as ladybirds that eat aphids, reduces pesticide use

Part 9 of 11Pollution

Eutrophication

Nitrate and phosphate enter the water from fertiliser or sewage.

Algae grow rapidly and form a bloom.

The bloom blocks light, so submerged plants die.

Decomposing bacteria multiply on the dead matter.

Their aerobic respiration raises the biochemical oxygen demand.

Dissolved oxygen falls and fish and invertebrates die, leaving a dead zone.

Biomagnification

Biomagnification the increase in concentration of a persistent pollutant with each trophic level

It differs from bioaccumulation, the build-up within one organism or level.
Top predators eat many prey, so they receive a much larger dose.

DDT

DDT an insecticide that moves from crops to waterways, plankton, fish and birds
It thinned the eggshells of birds of prey so the eggs broke.
It is now banned except for controlling malaria mosquitoes.

Mercury

Mercury released by coal burning and gold mining and converted by microbes to methylmercury
It concentrates in tuna and swordfish, which are risky for people.

Plastics

Plastics they are non-biodegradable
Turtles eat bags that look like jellyfish and albatross chicks are fed plastic and may starve.
Animals become entangled in fishing lines.
Wave action and UV break macroplastics into microplastics under 5 mm that enter food chains.

Science communication

Science communication clear public information has reduced plastic packaging and increased recycling

Part 10 of 11Restoring ecosystems

Restoring ecosystems

Rewilding

Rewilding restoring natural ecosystem processes to improve stability

Strategies

Reintroduce apex predators to control herbivores so vegetation and plant diversity recover.
Reintroduce keystone species to change the structure of the ecosystem.
Connect habitats with wildlife corridors such as hedgerows, which give access to more resources.
Limit human use such as logging, fishing and farming.
Use controlled grazing or burning.

Hinewai Reserve, New Zealand

Hinewai Reserve, New Zealand former farmland where non-native species were removed
Minimal intervention now lets native communities return by succession.

Peat bog

Peat bog draining lets air in, so aerobic decomposers respire and release CO₂

Part 11 of 11More from the specification

Sustainability
- Meeting present needs without harming future generations' ability to meet theirs.

Key terms

Term	Meaning
Tipping point	The point beyond which an ecosystem is no longer stable.
Mesocosm	An experimental container that simulates a natural ecosystem.
Keystone species	A species with a disproportionate effect on the structure and function of its ecosystem.
Sustainable harvest	A harvest that is replaced as fast as it is taken.
Eutrophication	Excess nutrients in water causing an algal bloom, oxygen depletion and a dead zone.
Biomagnification	The increase in concentration of a persistent pollutant at each trophic level.
Rewilding	Restoring an ecosystem so natural processes sustain it.
Biochemical oxygen demand	The demand for dissolved oxygen by respiring decomposers.

Avoid these mistakes

Saying a stable ecosystem never changes — it resists change but natural selection still acts

Putting secondary consumers in a mesocosm — there is too little energy and it is unethical

Mixing up bioaccumulation and biomagnification — biomagnification increases with each trophic level

Using tap water for an aquatic mesocosm — pond water contains the needed microorganisms

Last updated: 18 October 2026