2.7.6Atoms, Elements and Compounds
Prepare a Soluble Salt II — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 14 of 26 Unit quiz
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.
Section 1 of 5Titration Method
A titration finds the exact volumes needed for neutralisation, so the salt can be made without any leftover acid or alkali.
1
Use a pipette to measure 25.0 cm³ of the alkali into a conical flask and add a few drops of indicator such as phenolphthalein.
2
Add the acid from a burette slowly, swirling the flask, until the indicator just changes colour, and record the volume of acid used.
3
Repeat the mixing with the same volume of acid and alkali, this time without any indicator, so no colour is left in the salt.
4
Heat the resulting solution to partially evaporate it, leave it to crystallise, decant the liquid and dry the crystals.
Section 2 of 5Key Points of the Method
Each stage of the method has a clear purpose, which examiners often test.
Indicator
- Indicator The indicator shows the exact volume of acid needed, but it is removed by repeating the mix without it.
Volume used
- Volume used The volume of acid is the final burette reading minus the starting reading, which gives the volume that neutralises the alkali.
Partial evaporation
- Partial evaporation Heating only partly evaporates the water, which leaves a saturated solution that crystallises as it cools.
Crystal water
- Crystal water Some water is left behind so that water of crystallisation is kept in the crystals and the salt does not overheat and decompose.
Section 3 of 5Choosing Indicator and Salt
Choosing Indicator and Salt
Phenolphthalein
- Phenolphthalein Colourless to pink, a sharp change that suits a strong acid with a strong alkali.
Methyl orange
- Methyl orange Red to yellow, a sharp change that also suits this type of titration.
Products
- Products The salt formed depends on the acid used, such as sodium chloride from hydrochloric acid and sodium hydroxide.
Water
- Water Water formed in the reaction is removed by evaporation, leaving only the salt.
Section 4 of 5Worked Example
Question
- In a titration, the burette reading starts at 0.50 cm³ and ends at 24.80 cm³.
- Calculate the volume of acid added, and state the volume used when the repeat is carried out without the indicator.
- Volume of acid = final reading − starting reading = 24.80 cm³ − 0.50 cm³ = 24.30 cm³.
- The repeat uses the same volume of acid, 24.30 cm³, added to the same volume of alkali without the indicator.
- This ensures the salt solution is made with exactly the same proportions, but without indicator contamination.
Answer 24.30 cm³ of acid, added to the alkali again without indicator.
Section 5 of 5Exam Checklist & Tips
2.7.6 – Key Points for the Exam
- Titration A method of finding the exact volume of one solution needed to react with another.
- Conical flask A flask with a flat base and narrow neck, used to hold the solution during a titration.
- Water of crystallisation Water molecules that are part of the crystal structure of a hydrated salt.
- Saturated solution A solution that cannot dissolve any more solid at a given temperature.
- Decant To carefully pour off the liquid above settled crystals.
Common Mistakes
- Forgetting to repeat the mixing without indicator, leaving an indicator in the final salt.
- Evaporating the solution to complete dryness, which can overheat and decompose the salt.
- Using the burette reading as the volume of acid added without subtracting the starting reading.
- Using universal indicator to find the end-point, which does not give a sharp enough colour change.
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