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]

2.7.5Inquiry Process

The Transpiration Stream — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 77 of 160

📚 IB Biology HL 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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Part 1 of 5The Cohesion-Tension Mechanism

Transpiration is the loss of water vapour; the transpiration stream is the resulting upward flow of water.

Water evaporates from the moist walls of mesophyll cells into the air spaces of the leaf.

Water vapour then diffuses out through open stomata down a gradient into the drier atmosphere.

Loss of water from the cell walls pulls more water from adjacent cells and from the xylem, creating tension, or negative pressure, in the xylem.

Hydrogen bonds between water molecules give cohesion, so the water column is pulled up as an unbroken chain.

Adhesion between water and the hydrophilic walls of xylem vessels helps hold the column up and prevents it from falling back.

Part 2 of 5Why It Matters

Mineral ions

Mineral ions The flow of water carries dissolved mineral ions from the roots to the leaves and other parts of the shoot.

Cooling

Cooling Evaporation takes heat from the leaf, which helps prevent overheating in sunlight.

Turgor

Turgor Water entering cells keeps them turgid, supporting non-woody stems and keeping leaves expanded.

Photosynthesis

Photosynthesis Some of the water is used in the light-dependent reactions, although this is a tiny fraction of the water that is transpired.

Part 3 of 5Factors Affecting Transpiration Rate

Factors Affecting Transpiration Rate

Light

Light Light opens stomata, so higher light intensity generally increases the rate.

Temperature

Temperature Higher temperature increases evaporation and diffusion speed and lowers the humidity of the air, raising the rate.

Humidity

Humidity High humidity reduces the water vapour gradient, so transpiration slows.

Wind

Wind Moving air carries away water vapour near the leaf and keeps the gradient steep, increasing the rate.

Factor	Change	Effect on rate
Light intensity	Increase	Increases
Humidity	Increase	Decreases
Wind speed	Increase	Increases

Part 4 of 5Worked Example

Question

In a potometer with a capillary tube of radius 0.5 mm, an air bubble moves 45 mm in 10 minutes.
Calculate the volume of water taken up per minute.

Cross-section area of tube = π × 0.5² = 0.785 mm².
Volume taken up = 0.785 × 45 = 35.3 mm³.
Rate = 35.3 ÷ 10 = 3.53 mm³ per minute.

Answer Water uptake is approximately 3.5 mm³ per minute.

Part 5 of 5Exam Checklist & Tips

2.7.5 – Key Points for the Exam

Transpiration The loss of water vapour from the aerial parts of a plant, mainly through stomata.
Cohesion The attraction between molecules of the same substance, such as water held by hydrogen bonds.
Adhesion The attraction between water and the walls of the xylem.
Potometer Apparatus used to estimate water uptake of a cut shoot.
Transpiration pull The tension in the xylem generated by evaporation in the leaf.

Common Mistakes

Saying water is pushed up by root pressure; in tall plants the main force is transpiration pull.
Writing that cohesion is between water and xylem walls; that is adhesion.
Forgetting that stomata must be open for most transpiration to occur.
Stating that high humidity increases transpiration.

Last updated: 27 October 2026