Topic: Factors affecting the dissolution rate of a substance in water

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Primary school. Chemistry.

V. Water and aqueous solutions. Pupil:

4. designs and conducts experiments showing the influence of various factors on the dissolution rate of solid substances in water.

General aim of education

The student discusses the types of solutions due to the particle size of the substance dissolved in water.

Key competences

• communication in the mother tongue;

• communication in foreign languages;

• mathematical competence and basic competences in science and technology;

• digital competence;

• learning to learn.

Criteria for success
The student will learn:

• to name factors that affect the dissolution rate of a solid in water;

• to conduct an experiment that investigates how mixing, substance fragmentation and temperature affect the rate of dissolution;

• to explain why mixing, substance fragmentation and rise of temperature increase the rate of dissolution of the substance in water.

Methods/techniques

• expository

  • talk.

• exposing

  • film.

• programmed

  • with computer;

  • with e‑textbook.

• practical

  • exercices concerned.

Forms of work

• individual activity;

• activity in groups;

• collective activity.

Teaching aids

• e‑textbook;

• notebook and crayons/felt‑tip pens;

• interactive whiteboard, tablets/computers.

Lesson plan overview

Introduction

1. The teacher hands out Methodology Guide or green, yellow and red sheets of paper to the students to be used during the work based on a traffic light technique. He presents the aims of the lesson in the student's language on a multimedia presentation and discusses the criteria of success (aims of the lesson and success criteria can be send to students via e‑mail or posted on Facebook, so that students will be able to manage their portfolio).

2. The teacher together with the students determines the topic – based on the previously presented lesson aims – and then writes it on the interactive whiteboard/blackboard. Students write the topic in the notebook.

Realization

1. The teacher introduces the students to the issue implemented during the lesson. He asks his charges to read the passage titled „Dissolution as a physical or chemical phenomenon”. Then the participants, working in pairs, ask each other about the knowledge of the fragment.

2. The teacher divides the class into groups. He brings to the students the content of the films in the abstract. The task of each team is to formulate a research question and hypothesis before watching the assigned film, and then recording observations and conclusions. After completion of activities, the group leaders present the effects of work.

3. The teacher summarizes the most important information. The teacher points the person who explains why mixing, substance fragmentation and rise of temperature increase the rate of dissolution of the substance in water.

4. The students check their knowledge about factors affecting the dissolution rate of a substance in water, working with the tabs in the abstract.

5. Students, working individually or in pairs, carry out interactive exercises to check and consolidate knowledge learned during the lesson. Selected people discuss the correct solutions for interactive exercises. The teacher completes or straightens the statements of the proteges.

Summary

1. The students consolidate the acquired information, discussing it with their nearest neighbors („tell your neighbor” method.

2. The teacher asks the students to finish the following sentences:

   • Today I learned ...

   • I understood that …

   • It surprised me …

   • I found out ...

   The teacher can use the interactive whiteboard in the abstract or instruct students to work with it

Homework

1. Imagine that you have the opportunity to interview an academic - a specialist in the field of today's lesson. What questions would you like to ask him? Write them down.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Factors affecting the dissolution rate of a substance in water

We have already noticed in everyday life that after adding common salt or sugar to the water these substances disappear after some time. We already know that mixtures are created then and these are called solutions. How can you name the process of their formation?

Dissolution of substances with a covalent structure is not accompanied by the transformation of one particle into another, it is a physical phenomenon. The solutions obtained as a result of dissolution can be separated by means of physical methods into the components from which they were formed. The physical dissolution process is not considered a chemical reaction, as no permanent chemical bonds are formed as a result of the interaction between the solute and the solvent.

However, dissolution can be accompanied by processes of disintegration and formation of unstable hydrogen bonds, generation of ions and supramolecular structures. These processes are called solvolysis, solvation and electrolytic dissociation. If the dissolution is accompanied by a chemical or electrochemical reaction that results in a permanent change in the chemical composition, this process is called chemical dissolution or electrochemical dissolution, respectively.

For example, when the potassium chloride crystal dissolves in water, the ionic bonds between the chloride (ClIndeks górny -^-) and potassium (KIndeks górny +⁺) ions occur in the first stage. In the second stage, the ions taken out of the crystal are solvated, i.e. the water molecules around the two ions are formed due to the ion‑stable dipole interactions.

Intuition tells us that mixing has a positive effect on the dissolution of solids in water, so when sweetening the tea, we mix it with a teaspoon. During dissolution, the solvent particles rupture the solid particles from the crystal and slowly transfer them into the solution. The removed particles stay in the vicinity of the crystal for a relatively long time and in a way block the access of the particles of the solvent. Mixing removes these from around the crystal and accelerates their spreading in the entire volume of the solution. For this reason, this treatment significantly accelerates the dissolution of solids.

One of the dissolution steps is the detachment of solid particles from the crystal by the solvent particles. The fragmentation increases the area of the crystal available for the solvent particles. The larger the surface area of the crystal, the greater the number of particles of the dissolved substance can be simultaneously detached and can go into solution.

The increase in temperature accelerates the movement of particles. As a result, the solvent particles more often collide with the crystal and thus detach a greater number of solid particles from it, and the removed particles are transferred faster into the solution.

• Dissolution is a physical process and consists in mixing particles of solute with solvent particles.

• Mixing accelerates the dissolution of the substance in water.

• The more fragmented the substance, the faster it dissolves.

• The temperature has an effect on the rate of dissolution of the substance in water – the rate of dissolution increases with its increase.