The Disappearing Cross: A Rate of Reaction Classic – Now with PASCO Power

 

The Disappearing Cross: A Rate of Reaction Classic – Now with PASCO Power

Hemel Private Tuition | GCSE & A-Level Chemistry
#RatesOfReaction #GCSEChemistry #PASCOScientific #ALevelChemistry #HemelPrivateTuition

At some point in every chemist’s journey, they encounter the famous “disappearing cross” experiment. In this simple, eye-catching demo, a black cross vanishes beneath a conical flask as the solution turns cloudy.

It’s a brilliant classroom classic for teaching the rate of reaction, and students love it… the first time.

But what happens after the "Wow!" fades and we want to actually measure the science?

That’s where @pascoscientific comes in—and the learning goes from qualitative to quantitative.

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🔍 From Eyeballing to Evidence: Enter the PASCO Colorimeter

After we've done the old-school visual demo with a cross and conical flask, we crank up the rigour using PASCO’s Wireless Colorimeter and Turbidity Sensor.

This clever bit of kit helps students:

• Measure the cloudiness (turbidity) of the solution accurately

• Record data continuously over time using PASCO Capstone or SparkVue software

• Visualise and compare rates of reaction instead of just guessing when the cross disappears

In short, we move from “when can you not see the cross?” to “how fast is the reaction actually proceeding?”

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🧪 The Reaction: Sodium Thiosulfate + Hydrochloric Acid

This classic reaction forms sulphur, making the solution turn cloudy. We prepare it in cuvettes with 6 ml total volume:

• 5 ml total thiosulfate + water mixture

• 1 ml hydrochloric acid added to start the reaction

We vary thiosulfate concentration while keeping acid constant:

Thiosulfate (ml)	Water (ml)	Acid (ml)
1	4	1
2	3	1
3	2	1
4	1	1
5	0	1

Then we run the same experiment using just one concentration, usually 2ml of Thiosulfate and 3 ml of water with 1 ml of Acid, at various temperatures (1°C to 60°C in 5°C intervals) using a water bath, to explore how heat speeds up reactions. The reactant must be placed in the same water bath to reach the required temperature before the reaction can be initiated, ensuring that there is no temperature change. At the higher temperatures, the reaction is so fast that there is little to no time for the reaction mixture to cool down. At the colder temperatures, precautions need to be taken to ensure that the mixture doesn't warm too much. Doing this experiment in the fridge often amuses the students.

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📈 What the Students Learn:

• How concentration and temperature affect reaction rate

• How to collect clean, numerical data using sensors

• How to plot turbidity vs time and identify trends

• At A-Level: How to calculate the rate of reaction from gradients

• How to build a rate–concentration graph and use it to determine order

Everyone leaves the lesson with:
✅ A full, student-generated graph
✅ A strong grasp of experimental design
✅ A sense of scientific satisfaction

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🎓 Teaching That Moves Beyond the Demo

We love a bit of drama in the lab—but once the fizz and cloudiness settle, it’s the hard data that gets students thinking like scientists.

With PASCO sensors, we can:

• Repeat the experiment for accuracy

• Eliminate the guesswork

• Analyse deeper at both GCSE and A-Level

And yes, the students still enjoy watching the cross disappear. But now they understand why, and they can explain it—backed by graphs.

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🔬 Want your child to learn science this way?
We offer 1:1 and small-group GCSE & A-Level tuition in Chemistry, Physics, Biology and Maths—with real experiments, filmed lessons, and sensor-supported investigations.

📍 Visit: www.philipmrussell.co.uk
📞 Book a trial: 01442234892 or email philip@philipmrussell.education

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#ChemistryEducation #PASCOScientific #TurbiditySensor #GCSEChemistry #RatesOfReaction #HandsOnLearning #STEMTeaching #ALevelScience #HemelPrivateTuition #ScienceMadeSimple