UNEB Practical Chemistry 2025: Comparing Lime Water Solutions

Vocabulary flashcards covering the experimental design, titration procedures, data collection, safety precautions, and scoring rubric for the UNEB 2025 practical chemistry investigation comparing lime water solutions from suppliers X and Y.

Koile

A progressive vegetable farmer in Eastern Uganda who uses lime water to lower soil acidity in his garden and seeks to determine whether supplier X or supplier Y provides the more effective product.

$$DA_1$$

The laboratory code representing the dilute hydrochloric acid ($HCl$) solution used as the titrant in the burette.

$$DA_2$$

The laboratory code designating the lime water solution obtained from supplier X.

$$DA_3$$

The laboratory code designating the lime water solution obtained from supplier Y.

Aim (UNEB Practical Investigation)

To investigate which of the two lime water solutions ($DA_2$ and $DA_3$) is more alkaline and requires a larger volume of $DA_1$ to reach neutralisation, hence determining which is effective in lowering soil acidity.

Independent variable

The type of lime water solution tested, namely lime water solutions $DA_2$ (supplier X) and $DA_3$ (supplier Y).

Dependent variable

The volume of dilute hydrochloric acid solution ($DA_1$) required to reach the neutralisation point.

Controlled variables

The parameters kept constant across all trials, including the volume of lime water solution pipetted, the concentration of $HCl$, the indicator (phenolphthalein), the temperature, and the experimental procedure.

Phenolphthalein indicator

An acid-base indicator added ($2\text{--}3$ drops) to the conical flask that turns the alkaline lime water pink and becomes colourless at the neutralisation point.

Neutralisation end point

The stage in the titration when the pink colour of the phenolphthalein indicator in the lime water solution just disappears.

Average volume of $DA_1$ used for $DA_2$

The mean volume of hydrochloric acid needed to neutralise lime water $DA_2$, calculated as $\frac{19.80 + 19.70}{2} = 19.75\,cm^3$.

Average volume of $DA_1$ used for $DA_3$

The mean volume of hydrochloric acid needed to neutralise lime water $DA_3$, calculated as $\frac{8.60 + 8.40}{2} = 8.50\,cm^3$.

Conclusion and recommendation

Lime water $DA_2$ is more alkaline because it requires a larger volume of $DA_1$ ($19.75\,cm^3$ compared to $8.50\,cm^3$) for neutralisation; therefore, Koile should use $DA_2$ to lower the acidity of his soil.

Safety precautions for hydrochloric acid

Wearing a lab coat and safety goggles, handling the acid with care because it is corrosive, never pipetting by mouth, and immediately wiping up spillages.

Data presentation ($D$)

A practical chemistry scoring category that evaluates the use of the correct table format, the recording of all required data sets, and the inclusion of correct units ($cm^3$).

Data recording ($Dr$)

A practical chemistry scoring category that checks for the correct number of decimal places, adherence to error margins (ensuring values for $DA_2$ are larger), and correct subtractions between burette readings.