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]

2cStructures and Functions in Living Organisms

Biological molecules — Edexcel Linear - OL Revision Notes & Topical Questions

5 min read 7 parts Lesson 3 of 25 1 quiz

📚 Edexcel Linear - OL 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

Biological molecules Edexcel O Level Linear Biology notes: identify the chemical elements present in carbohydrates, proteins and lipids (fats and oils)Haemoglobin is a globular protein with 4 chains and haem groups (quaternary).Collagen is fibrous3 chains in a triple helix, stron…

Biological molecules Edexcel O Level Linear Biology: what to do next
Condensation
Hydrolysis
Glycosidic bond
Ester bond
Peptide bond

Part 1 of 6Key ideas

Haemoglobin is a globular protein with 4 chains and haem groups (quaternary).

Collagen is fibrous

3 chains in a triple helix, strong and insoluble.

Water has high specific heat capacity and high latent heat of vaporisation.

Sucrose = glucose + fructose; lactose = glucose + galactose.

Part 2 of 6Syllabus points

identify the chemical elements present in carbohydrates, proteins and lipids (fats and oils)

describe the structure of carbohydrates, proteins and lipids as large molecules made up from smaller basic units

starch and glycogen from simple sugars
protein from amino acids
and lipid from fatty acids and glycerol

practical: investigate food samples for the presence of glucose, starch, protein and fat

understand the role of enzymes as biological catalysts in metabolic reactions

understand how temperature changes can affect enzyme function, including changes to the shape of active site

practical: investigate how enzyme activity can be affected by changes in temperature

understand how enzyme function can be affected by changes in pH altering the active site

Part 3 of 6Process & sequence

Primary: amino acid sequence

Secondary: α-helix / β-sheet (H-bonds)

Tertiary: 3D folding (ionic, disulfide, H, hydrophobic)

Quaternary: several chains

Heat sample with Benedict’s – no colour change: no reducing sugar.

Boil a new sample with dilute HCl (hydrolysis).

Neutralise with sodium hydrogencarbonate.

Heat with Benedict’s again.

Brick-red now → non-reducing sugar present.

Part 4 of 6Summary

Summary

Monomers join by condensation; polymers break by hydrolysis.

Cellulose is made of β-glucose.

Biuret reagent turns purple with protein.

Part 5 of 6Keep going

Biological molecules Edexcel O Level Linear Biology: what to do next

These Biological molecules Edexcel O Level Linear Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Biological molecules Edexcel O Level Linear Biology understanding with the practice questions below, then reread any Biological molecules Edexcel O Level Linear Biology point you were unsure about before moving on.

Practice: Biological molecules Edexcel O Level Linear Biology practice questions

Previous lesson: 2b Cell structure — Notes

Next lesson: 2d Movement of substances into and out of cells — Notes

Course: Edexcel Linear - OL — all units

Official syllabus: Pearson Edexcel International GCSEs

Part 6 of 6Compare: Compare

Biological molecules compared
Molecule	Monomer	Bond	Structure	Function	Test
Starch (amylose)	α-glucose	α-1,4 glycosidic	Unbranched helix	Energy store in plants – compact	Iodine → blue-black
Amylopectin / glycogen	α-glucose	α-1,4 and α-1,6	Branched (glycogen more)	Rapid glucose release	Iodine
Cellulose	β-glucose	β-1,4 glycosidic	Straight chains, H-bonds → microfibrils	Cell wall strength	—
Triglyceride	Glycerol + 3 fatty acids	Ester	Hydrophobic	Energy store, insulation	Emulsion test
Phospholipid	Glycerol + 2 fatty acids + phosphate	Ester	Hydrophilic head, hydrophobic tails	Membrane bilayer	—
Protein	Amino acids	Peptide	Primary → quaternary	Enzymes, haemoglobin, collagen	Biuret → purple
Water	—	Hydrogen bonds	Polar molecule	Solvent, coolant, high SHC	—

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Monomer · Condensation · Hydrolysis · Glycosidic bond · Ester bond · Peptide bond · Reducing sugar · Hydrogen bond

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).

Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

Avoid vague words

“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

Last updated: 18 October 2026