Core practical

Investigating neutralisation

There are a number of ways that you could investigate neutralisation in Chemistry. This is an outline of the required steps to undertake one of these methods. Calcium oxide or calcium hydroxide can be used in this practical.

Aims

To investigate the change in pH when adding powdered calcium hydroxide to a fixed volume of dilute hydrochloric acid.

Apparatus

The apparatus required for the core practice

Method

  1. Use a measuring cylinder to add dilute hydrochloric acid to a beaker.
  2. Dip a clean glass rod into the contents of the beaker. Use it to transfer a drop of liquid to a piece of universal indicator paper on a white tile. Wait 30 seconds, then match the colour to a pH colour chart. Record the estimated pH.
  3. Add a level spatula of calcium hydroxide powder to the beaker.
  4. Stir thoroughly, then estimate and record the pH of the mixture (as in step 2).
  5. Repeat steps 3 and 4 until there are no more changes in pH.

Results

Record the results in a suitable table. For example:

Number of spatulas addedpH of reaction mixture
0
1

Analysis

Plot a graph to show:

  • pH on the vertical axis
  • number of spatulas added on the horizontal axis

Make sure you choose suitable scales so that at least 50% of the graph area includes plotted points. Draw a curve of best fit through these points.

The diagram shows an example:

Graph change in pH on adding powdered calcium hydroxide to a fixed volume of dilute hydrochloric acid.Plot the points accurately when drawing a graph
Question

Use the graph to estimate the number of spatulas of calcium hydroxide needed to produce a neutral solution.

Six spatulas of calcium hydroxide were needed to produce a neutral solution (pH 7).

Evaluation

Question

Suggest two ways in which more accurate results could be obtained.

You could measure the mass of calcium hydroxide powder using a balance. You could also use a pH probe and meter to measure the pH, rather than estimating it using universal indicator.

Hazards, risks and precautions

It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

HazardPossible harmPrecaution
Hydrochloric acidConcentrated acid is corrosive and damages skin and clothesUse dilute hydrochloric acid
Calcium hydroxide powderCauses skin irritationWear gloves
Calcium hydroxide powderCauses serious eye damageWear eye protection
Calcium hydroxide powderMay cause respiratory irritationAvoid making a dust