Agriculture and Food 7–12 · Years 7–8

Bicarbonate of soda as a raising agent: gas from heat and from acid

Food and agricultural practices

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Sodium hydrogencarbonate (bicarbonate of soda) works as a raising agent because it produces carbon dioxide gas when it is heated or mixed with an acid; the gas turns limewater milky, a sign that a new substance has formed, and the mixture loses mass as the gas escapes.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

hot glassware and Bunsen flame
limewater sucked back into a hot tube can crack it
limewater irritates skin and eyes

Controls

eye protection throughout
heat gently with the tube angled away from people
remove the delivery tube from the limewater before removing the heat
nothing made in the laboratory is tasted

Note

Heat and limewater: NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

What you need

sodium hydrogencarbonate (bicarbonate of soda, plain, with no other ingredients), 2 x 1.00 g
white vinegar, 50 mL (an excess of acid)
250 mL conical flask with a loose cotton wool plug
balance reading to 0.01 g
boiling tube with a stopper and delivery tube, test tube holder, Bunsen burner and heatproof mat
limewater, 5 mL in a test tube
eye protection

How to do it

Heat test: weigh an empty boiling tube, add 1.00 g of sodium hydrogencarbonate and weigh again.
Fit the delivery tube, dip its end into the limewater and heat the solid gently; watch the limewater.
Lift the delivery tube out of the limewater before removing the flame, so the limewater cannot be sucked back into the hot tube.
Keep heating the solid for 5 minutes, note any droplets on the cooler part of the tube, let it cool, reweigh, then heat and reweigh again until two masses agree.
Acid test: stand the flask with 50 mL of vinegar and its cotton wool plug on the balance and record the mass.
Tip in 1.00 g of sodium hydrogencarbonate, replace the plug at once and record the mass every 30 seconds until it stops changing.
Record what was seen in each test (bubbles, the limewater, droplets, the white solid left in the tube, the falling balance reading) and explain which observations show that a new substance formed.

What you should see

The limewater turns milky, showing that the gas is carbon dioxide. In the heat test the solid loses mass, droplets of water collect on the cooler part of the tube and a white solid (sodium carbonate) stays behind: 2NaHCO3 -> Na2CO3 + H2O + CO2 takes 1.00 g of sodium hydrogencarbonate down by 0.369 g of water and carbon dioxide, about 37% of its mass, leaving 0.631 g of sodium carbonate. In the acid test the mixture fizzes at once and the balance reading falls while the fizzing lasts, then stops changing once the fizzing ends, because the carbon dioxide leaves through the loose plug; with an excess of acid, 1.00 g can lose at most 0.524 g of carbon dioxide. Both losses are reached only if every gram reacts and no powder is lost, so a measured loss a little under them is the usual result. This is why recipes pair bicarbonate of soda with an acid ingredient, and why heat alone leaves alkaline sodium carbonate in the food. The learner knows it worked when the limewater turns milky and the acid-test reading stops falling once the fizzing ends.

What changes

What you change: route (heating or adding acid)
What you measure: mass lost (g) and the limewater result
What you keep the same: 1.00 g of sodium hydrogencarbonate
same balance
time to constant mass

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

All the bubbles in a cake come from air beaten in during mixing.
Fizzing only means the powder is dissolving, not that a new substance is forming.
Baking powder and bicarbonate of soda are the same substance.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Food Technology Years 7–10 Syllabus (2019)FT4-4 FT4-5
Technology 7–8 Syllabus (2023), focus area Food and agricultural practicesTE4-SAF-01
Australian Curriculum v9AC9TDE8K05AC9S8U07

Sources

The pages the author read to write this activity.

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