What happens to particles in chemical reactions? | 11-14 years

In this practical activity students explore their ideas about what happens to atoms and molecules when new materials are made during a chemical reaction. They follow this up by using ball and stick models to explain their observations. These models are the ‘prop’ to focus student thinking and discussion about chemical reactions.

Learning objectives

Students will be able to:

• Describe what happens to particles during a chemical reaction.

Sequence of activities

Demonstration and introduction

1. As an opening demonstration, mix solutions of lead nitrate and potassium iodide in a large beaker or conical flask, to produce a precipitate of lead iodide.
2. Explain to the students that they are going to explore what happens to particles during a chemical reaction.

Practical 1

Organise students into groups of three and give each student an ‘Activity sheet’. Support and supervise as students:

1. Mix and shake together small amounts of finely ground lead nitrate and potassium iodide in a test tube.
2. Record what they see on their ‘Activity sheet’.
3. Place a little distilled water in a Petri dish and add a few drops of silver nitrate solution to one edge of the dish, followed very quickly by a few drops of potassium iodide added to the opposite edge of the dish.
4. Record what they see.
5. Wash out the Petri dish.
6. Add some more distilled water to the dish then add a few drops of silver nitrate solution to the middle of the dish, followed a minute later by a few drops of potassium iodide also added to the middle of the dish.
7. Record what they see.

Ask students to explain to their group what they have seen in terms of particles and write this on their ‘Activity sheet’.

Plenary 1

1. Call a short plenary. Ask students to use ‘traffic lights’ to indicate what they think happens to the mass of the Petri dish when the two solutions mix:
   • Red – mass increases.
   • Yellow – mass stays the same.
   • Green – mass decreases.
2. Tell the class that they are going to devise a way of finding out who is right about the mass question. Give them an equipment list:
   • Lead nitrate solution
   • Potassium iodide solution
   • Conical flask
   • Bung
   • Ignition tube
   • Length of cotton
   • Balance

Practical 2: planning

 Support the students as they:

1. Work in their groups.
2. Agree and draw on a mini whiteboard how the equipment should be arranged at the start of the experiment.
3. Show their diagrams to the class.

Be sure to check on safety issues.

Practical 2

Supervise students as they carry out their experiment:

1. Weigh the conical flask before and after the solutions are mixed.
2. Record the measurements they have taken.
3. Draw a diagram of their apparatus at the start and the end of the experiment.

Plenary 2

1. Call a short plenary. Using ’traffic lights’ as before, check that there is agreement that the mass does not change on mixing.
2. Give students time to check and add to their explanations of what happens when they mix the solutions using ideas about particles.

Modelling

Introduce ball and spoke models. Circulate and support with prompts while student groups:

1. Construct a sequence that will illustrate what happens during the chemical reaction, using four different coloured balls and spokes to connect them.
2. Choose one person in the group as spokesperson.
3. Demonstrate the ideas they have developed to the whole class.

Before finishing

Provide a final opportunity for students to add to and modify their explanations about what happens when the solutions mix.

Feedback

Collect in student sheets and add comments to identify achievement. Ask questions to help students focus on the areas they need to develop.

Commentary

The initial demonstration gives substance to the purpose of the session.

Although simple, using ‘traffic lights’ forces students to think for themselves. The ideas they develop are then extended as they listen to the ideas of other students. The group work, both in the experiment and in the modelling exercise, is creative and evaluative; it encourages thinking, which leads to learning.

By reviewing the worksheet, the teacher can see how much each student has moved forward and so address their individual needs in the feedback.

Practical notes

Equipment

For experimental work:

• Safety equipment
• Finely ground potassium iodide with spatula
• Finely ground lead nitrate (TOXIC and OXIDISING) with spatula
• Test tubes
• Petri dishes
• Solution of potassium iodide with teat pipette
• Solution of silver nitrate (CORROSIVE) with teat pipette
• Distilled water
• Conical flask, 250 cm³, with bung
• 10 cm length of cotton
• Access to a balance
• Ball models (four different colours) with spokes

Other equipment:

• Set of ‘traffic light’ cards
• Mini whiteboard

Health, safety and practical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• It is the responsibility of the teacher to carry out an appropriate risk assessment.

Answers

1. When we mixed together solid potassium iodide and solid lead nitrate:
   • Yellow solid is produced.
2. When we added solutions of potassium iodide and lead nitrate at the edge of the Petri dish:
   • Yellow solid (precipitate) is produced when solutions mix.
3. When we added solutions of potassium iodide and lead nitrate to the centre of the Petri dish:
   • Yellow solid (precipitate) is produced when lead nitrate is added to the potassium iodide solution.
4. My explanation of what happened when we mixed solutions of potassium iodide and lead nitrate is:
   • Lead nitrate solution contains particles (called ions) of lead, potassium iodide contains particles (called ions) of iodide. When the solutions mix, the lead particles and iodide particles combine to form a new substance, lead iodide, which is a yellow solid.

A possible experimental method is to put one solution in an ignition tube and the other solution in the conical flask. The ignition tube is held by a cotton thread trapped by a cork in the mouth of the conical flask so that it does not become horizontal. This arrangement is weighed. The thread is released by loosening the cork to allow the ignition tube to become horizontal and for the two solutions to mix. The equipment is weighed again.

The two masses are the same showing that there is no change of mass when new substances are formed in a chemical reaction.