ChemixaLEARN CHEMISTRY

Endothermic chill with baking soda

Objective: run an endothermic reaction between baking soda and citric acid and measure the temperature drop. You will mix measured amounts, track the temperature, and explain why the cup gets cold using enthalpy.

Safety:

Do not taste or drink the mixture even though the ingredients are food-grade. Keep the powder away from eyes; rinse with water if contact occurs. Adult supervision is required for minors. Wash hands after handling citric acid powder.

Materials

  • Baking soda
  • Citric acid powder (sold as sour salt in grocery stores)
  • Two foam or plastic cups
  • Water
  • Thermometer
  • Measuring spoons
  • Stirring stick

Procedure

1

Measure the starting temperature

Pour half a cup of room-temperature water into a foam cup and record its temperature.

2

Dissolve the baking soda

Stir in two tablespoons of baking soda until mostly dissolved.

3

Add the citric acid

Add one tablespoon of citric acid powder, stir, and start watching the thermometer.

4

Track the temperature drop

Record the temperature every 30 seconds for three minutes.

5

Calculate the change

Compare the lowest reading with your starting temperature and calculate the change.

What to observe

  • The mixture fizzes as carbon dioxide is released.

  • The thermometer reading drops within a minute of mixing.

  • The outside of the cup feels cold to the touch.

  • More citric acid produces a bigger temperature drop.

Why it happens

Baking soda (sodium bicarbonate) reacts with citric acid in water to release carbon dioxide, and this reaction has a positive enthalpy change: it absorbs thermal energy from its surroundings. Because the reacting mixture pulls heat out of the water and the cup, the temperature drops measurably, sometimes by several degrees. This is an endothermic process, the opposite of the exothermic crystallization in a hand warmer. Instant cold packs use the same principle on a larger scale, typically with ammonium nitrate dissolving in water. Feeling the chill is direct evidence that chemical reactions exchange energy with their surroundings, not just matter.

Try variations

  • Double the citric acid and check whether the temperature drops further.

  • Insulate the cup with a towel and compare the minimum temperature.

  • Test vinegar instead of citric acid and note the difference.