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Chemistry · 7. Analysis and practical skills

Identifying substances from converging evidence

Design controlled tests, avoid interfering ions and make appropriately cautious conclusions.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

Is one colour or precipitate enough to identify a substance?

Curriculum coverage

Vietnam chemistry practical work; IGCSE experimental techniques and analysis.

Model scope and limitations

Colours and solubility depend on concentration; real identification needs blanks, standards and safe protocols.

Learning objectives

  • Record observations before inference.
  • Choose reagents that do not introduce the target ion.
  • Combine tests and controls.

Prerequisite knowledge

  • Ions, precipitates and ionic equations.

Core knowledge

Observation

Record “white precipitate” before naming an unconfirmed substance.

Interference

Acidification removes carbonate before a halide test to avoid false precipitates.

Controls

Negative controls check reagent contamination; positive controls verify the procedure.

Worked example

Acidify with dilute HNO₃, then AgNO₃ gives a white precipitate: evidence consistent with Cl⁻. Ammonia solubility adds confidence.

Misconceptions to avoid

  • Do not acidify a chloride test with HCl because it introduces Cl⁻.
  • “No change” is data only when reagents and conditions are valid.

Virtual experiment procedure: Anion analysis

  1. Split and label portions and prepare a blank.
  2. Use small quantities and record each observation immediately.
  3. Use a decision table to exclude and support candidate ions.

Safety and cautions

  • AgNO₃ damages eyes and stains skin; soluble barium salts are toxic. Work under supervision and segregate waste.

Evidence to collect

  • A sample–reagent–observation–inference–confidence table.
Open the virtual experiment

Diagnostic check

1. Which acid is suitable before testing Cl⁻ with AgNO₃?

  1. Dilute HNO₃
  2. HCl
  3. Any acid
Show answer and explanation

Correct answer: Dilute HNO₃

HNO₃ does not introduce an interfering halide.

2. “A white precipitate forms” is what type of statement?

  1. Observation
  2. Final identity
  3. Calculation
Show answer and explanation

Correct answer: Observation

It is observational evidence before inference.

3. What is a blank control for?

  1. Detect reagent contamination
  2. Speed reaction
  3. Concentrate sample
Show answer and explanation

Correct answer: Detect reagent contamination

A blank lacks analyte and should give a negative result.

Application practice

1. Ionic equation for AgCl formation?

  1. Ag⁺ + Cl⁻ → AgCl(s)
  2. Ag + Cl → AgCl₂
  3. AgCl always dissolves
Show answer and explanation

Correct answer: Ag⁺ + Cl⁻ → AgCl(s)

Silver and chloride ions form AgCl precipitate.

2. Is one white precipitate alone definitive?

  1. No, use procedure and confirmation
  2. Always
  3. Only solution colour matters
Show answer and explanation

Correct answer: No, use procedure and confirmation

Different substances can give similar observations.

3. How can cross-contamination be reduced?

  1. Use clean tools and never return excess
  2. Share droppers
  3. Open all bottles
Show answer and explanation

Correct answer: Use clean tools and never return excess

Clean tools and one-way handling protect reagents.

Academic references

  1. OpenStax Chemistry 2e — peer-reviewed general chemistry
  2. IUPAC Gold Book — authoritative chemical terminology
  3. BIPM — SI Brochure, 9th edition

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer