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]

Inquiry Process

6. Inquiry Process — IB Biology SL Revision Notes & Topical Questions

10 min read 13 parts Lesson 1 of 10

📚 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

6. Inquiry Process — IB BIOLOGY SL notes: Plan a biology investigation by writing a focused research question and hypothesis, identifying variables, justifying a method and controlling the experiment.Exploring = the creative start of an investigation, using curiosity and existing…

One relationship
Background research
Starting points
Feasibility
The because clause
Prediction against hypothesis

Part 1 of 11Exploring: the research question

Exploring

Exploring the creative start of an investigation, using curiosity and existing knowledge to make a focused research question and testable hypothesis

It shows independent thinking, initiative and insight.

Insight

Insight a standard experiment can be framed as a comparison
An example isto what extent does the source of amylase (fungal or bacterial) affect its optimum temperature?

One relationship

One relationship investigate one independent variable and one dependent variable in depth
How do light intensity and CO₂ concentration affect photosynthesis? has too many variables.

Research question

Research question it must be focused, specific and state the link between the independent and dependent variables
How do abiotic factors affect plants? is too general.
A focused version iswhat is the effect of soil pH (5.0 to 8.0) on the rate of germination of garden cress seeds?

Background research

Background research use a textbook, the IB data booklet and reliable databases such as PubMed for papers and GenBank for sequences
It helps to understand the theory, find established values and choose a method.

Starting points

Starting points a surprising class result, a different enzyme or organism in a standard practical, or a real-world problem

Feasibility

Feasibility check the equipment, organisms and time, since plant experiments can take days or weeks
Be ready to refine the question after the research.

Part 2 of 11Hypothesis

1Hypothesis

Hypothesis a clear, testable statement that predicts an outcome and gives a scientific reason

It follows an if, then, because structure.

2The because clause

The because clause it must rest on established biology such as enzyme function, osmosis or ecological interactions
If temperature increases, enzyme activity will increase,
- because higher kinetic energy raises the frequency of effective collisions between active site and substrate, up to the optimum.

3Prediction against hypothesis

Prediction against hypothesis a prediction alone is not a hypothesis without the justification

4Provisional

Provisional a hypothesis is a provisional explanation that repeated testing supports or refutes

Part 3 of 11Examples of explored investigations

Investigation	Research question	Hypothesis reason
Enzymes	Effect of pH 4 to 10 on trypsin breaking down casein	Trypsin works in the alkaline small intestine, and pH away from the optimum changes the active site and denatures it (pepsin is best near pH 2 and trypsin near pH 8)
Osmosis	Effect of sucrose concentration 0.0 to 1.0 M on the percentage mass change of potato cylinders after 24 h	A higher concentration lowers the water potential of the solution, steepening the gradient so more water leaves the tissue
Germination	Effect of NaCl 0 to 2.0% w/v on the germination of radish seeds over 7 days	Salt lowers water potential and prevents the water uptake needed for metabolism, though halophytes are adapted to saline conditions

Part 4 of 11Designing: variables

Designing

Designing turning the research question into a step-by-step plan that collects enough high-quality data

Independent variable

Independent variable the single factor deliberately changed
Its range should bracket the expected optimum, such as seven buffers from pH 4 to 10.

Dependent variable

Dependent variable what is measured in response
For the enzyme example it is the time for casein to clear, with rate = 1 / time.
For osmosis it is percentage change in mass, so cylinders of different starting mass can be compared.

Controlled variables

Controlled variables every other factor that could plausibly affect the outcome, with the method of keeping it constant explained
Temperature in a water bath at 37 °C, enzyme and substrate concentrations and the total volume.
For potato, the same species and dimensions, the same solution volume, temperature and time.

Part 5 of 11Designing: range, repeats and method

Range

Range at least five values of the independent variable are recommended to establish a trend
Justify the choice, such as 10 to 50 °C because lower temperatures are too slow and higher ones may denature amylase.

Repeats

Repeats at least three trials per value, and five is better because of the variability of living things
Repeats allow a mean to be calculated and anomalies to be identified and discarded.

Method details

Method details give precise apparatus and quantities
Use 5 cm³ of 1% starch measured with a 10 cm³ graduated pipette and not some solution.
Use a size 5 cork borer to cut three cylinders and trim each to 3.0 cm with a scalpel and ruler.

Creativity

Creativity a colorimeter measuring starch loss with iodine is more objective than judging a colour change by eye

Structure of a method

A list of materials and apparatus with concentrations, sizes and precision.
A risk assessment with specific precautions and waste disposal.
Numbered step-by-step instructions.

Part 6 of 11Pilot studies and other approaches

Pilot study

Pilot study a small trial run to check that the method works

It shows, for example, whether an enzyme reaction is too fast or too slow so concentrations can be adjusted.

Other sources of data

Databases such as GenBank for DNA sequences or the IUCN Red List for conservation status.
Simulations such as PhET for population dynamics or evolution.
Surveys using questionnaires on human physiology or ecological awareness.

Safety

Safety include specific measures
Goggles and a lab coat.
Iodine is an irritant.
Waste goes into designated containers.

Part 7 of 11Design examples

Enzyme investigation

Enzyme investigation pH 4 to 10 for trypsin acting on casein
Seven buffers cover acidic to alkaline conditions and bracket the known optimum.
A water bath at 37 °C keeps temperature constant so only pH affects the rate.

Osmosis investigation

Osmosis investigation six sucrose concentrations of 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 M
The range includes pure water and hypertonic solutions so the isotonic point can be found.
Pat the cylinders dry before weighing, because surface water would cause a systematic error.

Part 8 of 11Controlling variables: calibration

Controlling variables reduces systematic errors, which shift all results in one direction

It makes sure any change is due to the independent variable and the results are valid.

Calibration

Calibration checking an instrument against a known standard and adjusting it

Instruments

pH meter: calibrate with at least two standard buffers such as pH 4.00 and 7.00.
CO₂ and O₂ sensors: calibrate against known gas concentrations in air.
Eyepiece graticulecalibrate with a stage micrometer at each magnification.
Thermometers and probescheck in boiling distilled water (100.0 °C) and crushed ice (0.0 °C).
Balancestare before every use.

Part 9 of 11Controlling the environment and sampling

Temperature

Temperature use a thermostatically controlled water bath or incubator
Let samples acclimatise in it before starting.

Other conditions

Other conditions control light intensity with a lamp at a fixed distance, and humidity
A variable that cannot be controlled perfectly should be monitored and recorded.

Samples

Samples biological material is naturally variable, so use representative samples and replication
Choose samples as similar as possible, such as the same potato variety or organisms of the same age and sex.
Use five or more replicates, such as potato cylinders, at each concentration.
In ecology use random sampling with quadrats.

Control run

Control run a set-up treated exactly like the experimental ones but without the independent variable

It proves the independent variable alone causes the effect.
An antibiotic experiment has a plate of bacteria with no antibiotic.
A starch experiment has starch with no enzyme, to show it does not break down on its own.

Part 10 of 11Control examples

Trypsin and casein

Temperaturetubes in a water bath at 37 °C for 10 minutes before mixing, because enzyme rate is sensitive to temperature.
Concentrationslarge stock solutions and the same volumes measured with graduated pipettes.
Control tubecasein with buffer at pH 8 and boiled trypsin, to show that breakdown is caused by the active enzyme.

Potato and sucrose

Natural variationall cylinders from the same potato with five replicates per concentration.
Surface areaall cylinders cut with the same size 5 cork borer and trimmed to 3.0 cm.
Temperaturetubes kept in the same place away from windows and heat for 24 hours.

Priority

Priority focus on the variables with the most effect, such as temperature in an enzyme experiment and evaporation in an osmosis experiment

Part 11 of 11More from the specification

Key terms

Term	Meaning
Research question	A focused question linking an independent and a dependent variable.
Hypothesis	A testable prediction with a scientific explanation.
Pilot study	A small trial run to test and refine a method.
Controlled variable	A factor kept constant so it does not affect the result.
Systematic error	An error that shifts all results in the same direction.
Calibration	Checking an instrument against a known standard and adjusting it.
Control run	A set-up identical to the experiment except that the independent variable is absent.
Replicate	A repeat of a treatment used to find a mean and spot anomalies.

Avoid these mistakes

Writing a hypothesis with no because scientific reason — a hypothesis must give the reason

Choosing a research question with two independent variables — investigate one relationship in depth

Saying keep it the same without describing how — name the method, such as a water bath, the same volume and the same source

Forgetting to calibrate instruments — this leaves systematic errors

Last updated: 18 October 2026