Science 7–10 · Years 9–10
Which antacid neutralises the most acid
Reactions
The idea
A carbonate in an antacid tablet neutralises acid, so the volume of acid a tablet can neutralise before the indicator changes compares how much base different brands contain.
Safety card
Hazards
- hydrochloric acid 0.4 mol/L is an irritant
- methyl orange is harmful if swallowed
Controls
- eye protection
- tablets are not to be eaten in the laboratory
- solutions diluted to the sink
Note
NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.
What you need
- two or three brands of chalky antacid tablet (calcium carbonate type), one tablet per run, crushed in a mortar
- hydrochloric acid 0.4 mol/L or lower in a 50 mL burette
- conical flask, 25 mL of water, methyl orange indicator, white tile, balance reading to 0.01 g
How to do it
- Weigh a tablet, crush it and transfer it to the flask with 25 mL of water and 3 drops of methyl orange.
- Add acid from the burette 1 to 2 mL at a time with swirling, then 0.5 mL at a time as the colour begins to linger, until a red colour persists for one minute.
- Record the volume of acid added, refilling the burette and adding the readings if a tablet needs more than 50 mL. Repeat with a second tablet of the same brand and then with the other brands.
- Rank the brands by the volume of acid neutralised per tablet and per gram of tablet, and compare the ranking with the amount of calcium carbonate each label lists.
What you should see
The tablet fizzes as it neutralises acid (CaCO3 + 2HCl gives CaCl2 + H2O + CO2). For the teacher: a tablet containing 500 mg of calcium carbonate neutralises 25.0 mL of 0.4 mol/L acid (100 mL of 0.1 mol/L), so at 0.4 mol/L a tablet with up to 1 g of calcium carbonate needs no more than one 50 mL burette-full. The end point drifts because undissolved carbonate keeps reacting after the red first appears, which is why the red must last a full minute; brands that also contain magnesium compounds neutralise more acid than their calcium carbonate alone would, so the full active-ingredient list on the label is used. The learner knows it worked when two tablets of one brand agree within 1 mL and, among brands whose only active ingredient is calcium carbonate, the brand whose label lists more calcium carbonate per tablet neutralises more acid.
What changes
- What you change
- brand of tablet
- What you measure
- volume of acid neutralised per tablet
- What you keep the same
- acid concentration
- indicator
- water volume
- end-point rule (red for one minute)
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A bigger tablet always neutralises more acid.
- Neutralising means the acid is destroyed rather than turned into a salt and water.
- The fizz is the acid boiling.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-RXN-01SC5-WS-03
- Australian Curriculum v9AC9S9U07AC9S10U07AC9S10I02AC9S9I02
Sources
The pages the author read to write this activity.