Investigating acids and alkalis

In a nutshell

By adding powdered calcium hydroxide $(CaOH)$ or calcium oxide $(CaO)$ to a fixed volume of dilute hydrochloric acid $(HCl)$ , the aim is to see how the $pH$ changes as powdered alkali is added to the acid.

Equations

Word equation	Symbol equation
$hydrochloric\ acid\ +\ calcium\ hydroxide\ \rightarrow\ calcium\ chloride\ +\ water$	$2HCl\ +\ Ca(OH)_2\ \rightarrow\ CaCl_2\ +\ 2 H_2O$
$hydrochloric\ acid\ +\ calcium\ oxide\ \rightarrow\ calcium\ chloride\ +\ water$	$2HCl\ +\ CaO\ \rightarrow\ CaCl_2\ +\ H_2O$

Equipment list

The following equipment list is used to conduct an experiment to evaluate the relationship between adding alkali to an acidic solution and pH.

Equipment	Use
Measuring cylinder	To measure the amount of dilute hydrochloric acid
Beaker	Where the solution is contained
Glass rod	Stirrer when adding powdered alkali and dabbing solution onto universal indicator paper
Universal indicator paper	Measure pH of solution
Spatula	Transfer of powdered alkali to the beaker
Hydrochloric acid solution	Solution to which base is added
Calcium oxide / hydroxide powder	Added to solution to elicit a change in pH

Experiment: Adding powdered calcium hydroxide to hydrochloric acid solution and investigating the changes of pH

Experimental variables

Independent variables are variables which can be changed. Dependent variables are changed and are measured. Control variables are kept constant and do not change.

Independent variable	$Amount\ of\ calcium\ hydroxide$
Dependent variable	$pH$
Control variable	$Amount\ of\ hydrochloric\ acid$

Safety precautions

When performing experiments, it is very important you consider safety precautions. This is so you and others do not get hurt.

Hazard	RISK	SAFETY MEASURE
Hydrochloric acid	Causes serious eye damage	Wear gloves
Hydrochloric acid	Causes skin irritation	Wear safety gloves
Hydrochloric acid	Acid is corrosive and damages the skin and clothes	Use dilute acid
Calcium hydroxide powder	Causes skin irritation	Wear gloves
Calcium hydroxide powder	Causes serious eye damage	Wear safety goggles
Calcium hydroxide powder	Can cause respiratory irritation	Avoid making dust, make sure room is well ventilated

Method

These are the instructions on how to complete the experiment.

1.	Use a measuring cylinder to add a specified amount of dilute hydrochloric acid to a beaker.
2.	Dip a clean glass rod into the solution, transfer a drop of solution from the glass rod to a piece of universal indicator paper and place it on a white tile. Wait $30\ seconds$ , then estimate the pH on the indicator paper by comparing it to the pH scale. Record the estimated pH.
3.	Add a level spatula of calcium hydroxide powder to the solution in the beaker.
4.	Stir thoroughly using a glass rod and record the pH of the mixture as done in step 2.
5.	Repeat steps 3 and 4 until the pH no longer changes.

Chemistry; Chemistry practicals; KS4 Year 10; Investigating acids and alkalis

1.	Litmus paper
2.	White tile

Analysis

Record the results in a table as follows:

Number of spatulas	pH
$0$	-
$1$	-

Plot a graph to show the pH on the vertical ( $Y$ ) axis and the number of spatulas on the horizontal ( $X$ ) axis. Make sure to use the appropriate scale so that the plotted area takes up at least $50\%$ .

Conclusion

Your graph should show an increase in pH until a certain point where adding is powdered alkali does not affect the pH.

Evaluation

Once you have completed the experiment it will be important to understand the shape of the graph and why the pH increase with the addition of alkali.