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1CH0 · CP3.6Core practical investigationsCore

CP3.6: Solid Base Addition and pH

Pearson’s specified context adds powdered calcium hydroxide or calcium oxide to a fixed volume of dilute hydrochloric acid. Record the initial acid volume, concentration and pH, then the actual cumulative mass after each approved addition. Do not substitute an unrelated liquid–liquid titration and assume the same practical evidence has been collected.

35 min Video by Science School Core practical investigations
Edexcel Core Practical 9-1 GCSE C3.6 Investigate the change in pH to a Volume of Hydrochloric acidCurated by Kepler, not produced by us. Watch it before the notes below.
Open on YouTube

Before you start

  • Use your teacher’s tier checklist: this overview can include Higher-only material, and is not a tier-specific revision list.
  • Pearson Edexcel separate GCSE Chemistry 1CH0. Preserve the specification’s core-practical reference and any explicitly required subparts.
  • Recall neutralisation, the pH scale and cumulative mass.
  • Use the centre’s approved supervised method, apparatus, heating and disposal arrangements. Written preparation does not replace performing and recording the practical activities.

Watch, pause, try

Video by Science School. Use the demonstration alongside this lesson’s specification reference and written explanations.

Viewing focus: Control solid-base additions to a fixed acid sample

After watching, attempt these questions before opening the answers. You can also use the written lesson below.

After watching 1: Which horizontal-axis quantity is required here?

Show answer and reasoning

Cumulative solid mass. Track the total mass added to the fixed initial sample.

After watching 2: Why allow an appropriate reading to stabilise?

Show answer and reasoning

Mixing and dissolution can affect the sampled pH. Use a consistent observation rule.

What you'll be able to do

  • Control solid-base additions to a fixed acid sample
  • Record pH against cumulative mass with appropriate resolution
  • Evaluate a neutralisation estimate without assuming a linear pH scale
1

Make cumulative solid mass the intended variable

Pearson’s specified context adds powdered calcium hydroxide or calcium oxide to a fixed volume of dilute hydrochloric acid. Record the initial acid volume, concentration and pH, then the actual cumulative mass after each approved addition. Do not substitute an unrelated liquid–liquid titration and assume the same practical evidence has been collected.

Keep starting acid conditions, the chosen solid, mixing and observation timing comparable. The oxide and hydroxide are different compounds; switching between them halfway through the series would change more than mass alone. Use the centre’s approved handling and addition procedure and measure actual additions rather than assuming every spatula has the same mass.

2

Observe pH consistently

Use the specified indicator-paper method or an appropriately checked pH probe. With paper, transfer a sample using the approved clean rod and compare under consistent conditions; with a probe, use appropriate rinsing, calibration and stabilisation. Record the resolution supported by the method instead of inventing extra decimal places.

Stirring helps make the sampled liquid representative, while dissolution and equilibration affect readings. Excess solid and a pH plateau do not mean that no particles or reactions exist. Distinguish a measured acid–base signal from a direct count of every dissolved ion or unreacted solid particle.

3

Interpret the shape and limits of the graph

Plot pH against cumulative mass, with units and a suitable scale. The change reflects neutralisation and the evolving solution conditions. The pH scale is not a linear amount-of-acid scale, so equal vertical changes do not necessarily represent equal reacted amounts.

Use enough points near the important transition to support an estimate. A pH reading below neutral followed by one above neutral can bracket a transition under suitable assumptions, but does not establish an exact mass from an arbitrary straight line. Independent repeats, finer additions and more precise measurement can improve a justified estimate.

Cumulative added mass / gApproximate indicator pH
0.001
0.101
0.202
0.303
0.4012
0.5012
Scroll horizontally to see the full code or table.
Original schematic indicator readings for a controlled comparison, not a preparation recipe or measured student results. The deliberately coarse readings illustrate why finer sampling near the transition is needed.
4

Worked applications

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.

Example 1Five measured additions are 0.100 g each, followed by 0.050 g. Find the total added mass.
1Five additions give 0.500 g.
2Add the final 0.050 g.
Answer
0.550 g cumulative mass.
Example 2Coarse indicator readings are about pH 3 at 0.30 g and pH 12 at 0.40 g. Does a straight interpolation establish exactly 0.344 g for neutralisation?
1The readings can bracket the transition in the stated controlled model.
2The pH scale, curve shape and indicator resolution do not justify an exact linear result.
Answer
Use additional readings near the transition and a stated estimate with uncertainty; an exact 0.344 g claim is not established.

Common mistakes to avoid

Plotting each increment instead of cumulative mass.
Keep both values in the raw record and plot the required total.
Assuming a large pH transition is exactly linear.
Use sufficient points and an explicit estimate with uncertainty.

Key takeaways

  • You can control solid-base additions to a fixed acid sample.
  • You can record pH against cumulative mass with appropriate resolution.
  • You can evaluate a neutralisation estimate without assuming a linear pH scale.

Apply it in an exam

Question 1: Coarse indicator readings are about pH 3 at 0.30 g and pH 12 at 0.40 g. Does a straight interpolation establish exactly 0.344 g for neutralisation?

Show answer and reasoning

Use additional readings near the transition and a stated estimate with uncertainty; an exact 0.344 g claim is not established.

  • The readings can bracket the transition in the stated controlled model.
  • The pH scale, curve shape and indicator resolution do not justify an exact linear result.

Explore further

Return to this tomorrow

Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.

  • Why allow an appropriate reading to stabilise?
  • Does one pH reading identify every dissolved ion?

Test yourself

Answer a few questions, then review anything you missed.

Next step

Keep practising

Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.

Browse practice topics

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