Topic: Changing the concentration of the solution – concentration

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Elementary school. Chemistry.

V. Water and aqueous solutions. Pupil:

7) performs calculations using the following notions: solubility, percent concentration (mass percentage), mass of substance, solvent mass, mass of solution, solution density (using solubility table or solubility plot).

General aim of education

The student calculates the percentage concentration of solutions after concentration

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 determine the percentage concentration of the solution after its concentration;

• to determine the percentage concentration of a solution formed after mixing two solutions of the same substance with different concentrations.

Methods/techniques

• activating

  • discussion.

• expository

  • talk.

• exposing

  • film.

• programmed

  • with computer;

  • with e‑textbook.

• practical

  • exercices concerned.

Forms of work

• individual activity;

• activity in pairs;

• activity in groups;

• collective activity.

Teaching aids

• e‑textbook;

• notebook and crayons/felt‑tip pens;

• projector;

• interactive whiteboard, tablets/computers;

• methodician or green, yellow and red cards.

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.

3. Health and safety – before starting the experiments, students familiarise themselves with the safety data sheets of the substances that will be used during the lesson. The teacher points out the need to be careful when working with them.

Realization

1. The teacher gives examples of various situations in everyday life in which the solution is concentrated, for example, by pouring, adding sugar to the soup or tea. It draws students' attention to the fact that these activities often determine the successful course of chemical processes, eg pickling cucumbers. Then he asks the students to follow the Task 1 in the abstract - a talk. He asks the question: „By what means can you concentrate the solution?” Initiating the discussion. At the end of this stage, displays a photo from the abstract showing the evaporator on the multimedia board.

2. The teacher presents illustrations from the abstract - „Model of process concentration of the solution by evaporation of the solvent” and „Model of the process of concentrating the solution by adding another portion of the substance to it”. He discusses the process of concentration of solutions (at the microworld level).

3. The lecturer shows an presentation showing the stepwise solution of the task for calculating the concentration after concentration.

4. The teacher refers students to the abstract, displays subsequent tasks on the board. They are discussed and solved in the forum by the students.

5. The teacher refers to the situations known from everyday life, where it happens that two solutions with different percentage concentrations mixed together give a third, new solution with a different concentration than those in the initial solutions. The teacher displays another abstract task on the board, discussing it and solving it together with the class participants.

6. The teacher, referring to the situation of mixing solutions with different concentrations, refers to the so‑called rules of St. Andrew. This rule is called differently - the following definitions are found in the literature: dilution square, mixing rule, cross rule. It presents an presentation with a stepwise solution of the task for calculating the percentage by the cross‑method.

7. The lecturer gives the students successive tasks to solve, asking those willing to the board (examples of tasks described in the methodical commentary). Other students solve tasks in notebooks - work in pairs.

Summary

1. 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. Listen to the abstract recording at home. Pay attention to pronunciation, accent and intonation. Learn to pronounce the words learned during the lesson.

2. Make at home a note from the lesson using the sketchnoting method.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Changing the concentration of the solution – concentration

Sometimes, when drinking tea or other drink, we find that they are not sweet enough. Then we add another portion of sugar to them. We do the same with soup, which we consider to be quite salty – we also add it. In each case, we deliver the dissolved substance to its solutions. During these treatments, the amount of water does not change. The activity we do is an example of concentration.

In analytical laboratories, sometimes the amount of test substance in the solution is so small that attempts to detect it may fail. Therefore, very often before the analysis, the solvent is evaporated from the solution in order to increase the concentration of the test substance. Evaporation of the solvent is also a common process used in the food industry, eg during the production of sugar, concentrates, jams, concentrated stocks or juices.

Sometimes we mix solutions of the same substance with each other. If the concentration of both solutions is the same, the solution after mixing will also have the same concentration.

If, however, we mix solutions of the same substance with different concentrations, the concentration of the resulting solution will be different from the concentration of both solutions used. You can calculate them.

1. We calculate the mass of sugar in both solutions.
   Calculations for a 5% solution:

Calculations for a 12% solution:

2. We calculate the total weight of sugar:

   total mass of sugar = mass of sugar from a 5% solution + mass of sugar from a 12% solution = 2.5 g + 4.8 g = 7.3 g

3. We calculate the total mass of the solution resulting after mixing:

   total solution weight = 50 g + 40 g = 90 g total solution weight = 50 g + 40 g = 90 g

4. Calculate the percentage concentration of the solution after mixing:

• The concentration of the solution may be varied by dilution or concentration.

• As a result of dilution, the concentration of the solution decreases, and as a result of concentration – increases.

• The solution can be diluted by adding a solvent to it.

• Increasing the concentration of the solution occurs after evaporation of the solvent or the addition of a new portion of solute.

• The concentration of the solution that will result from mixing solutions of the same substance but with different concentrations will differ from the concentrations of the solutions used to mix.