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1.6.9Inquiry Process

Dichotomous Keys: Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 23 of 160

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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 5How a Key Works

Keys are tools that allow anyone to identify an unknown organism without prior expertise.

1
A dichotomous key is a series of paired statements, each with two choices that lead to another step or an identification.
2
Start at the first pair and choose the statement that matches the specimen.
3
Follow the number or name given to the next pair, repeating until an organism name is reached.
4
Always read both statements before choosing, since the second may match better.
5
Check the final identification against a description to confirm.

Part 2 of 5Building a Key

Choose clear features
  • Choose clear features Use features that are easy to see and measure, such as number of legs, not subjective features like size or colour shade.
Divide into two groups
  • Divide into two groups Each statement should split the organisms so that every specimen fits exactly one choice.
Use yes or no choices
  • Use yes or no choices Features that are present or absent, such as wings or a shell, give unambiguous divisions.
Repeat for each group
  • Repeat for each group Continue splitting each group until every organism stands alone with a name.
Test the key
  • Test the key Try the key on each organism and ask a friend to use it, then correct any confusing wording.

Part 3 of 5Features Often Used

Features Often Used
Animals
  • Animals Presence of a backbone, number of legs, wings, antennae, shell, or segmented body.
Plants
  • Plants Leaf shape, leaf margin, flower structure, arrangement of leaves, and presence of seeds or spores.
Fungi and microbes
  • Fungi and microbes Shape of cell, presence of a cell wall, and growth pattern under the microscope.
Good featurePoor feature
Number of legsBig or small
Wings presentPretty
Leaf edge smooth or toothedDark green

Part 4 of 5Worked Example

Question

  • Use the key below to identify an organism that has a shell, a muscular foot and two shells hinged together.
  • 1a Has a shell, go to 2.
  • 1b No shell, go to 4.
  • 2a One shell, snail.
  • 2b Two shells hinged, go to 3.
  • 3a Shells equal size, clam.
  • 3b Shells unequal, oyster.

  1. Start at 1: the organism has a shell, so choose 1a and go to 2.
  2. At 2 it has two shells hinged together, so choose 2b and go to 3.
  3. At 3 the question of shell size decides the answer; for equal shells choose 3a.

Answer The organism is a clam (assuming equal shells).

Part 5 of 5Exam Checklist & Tips

1.6.9 – Key Points for the Exam

  • Dichotomous Divided into two branches.
  • Key A series of questions leading to identification.
  • Taxon A group of organisms such as a species or genus.
  • Observable feature A characteristic that can be seen and checked.

Common Mistakes

  • Using features that vary within a species, such as age or size.
  • Writing choices that overlap, so a specimen could match both.
  • Skipping the numbering, which makes the key impossible to follow.
  • Using three choices at a step instead of two.

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