Topic: pH of the solutions

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Primary school. Chemistry.

VI. Hydroxides and acids. The student:

6) lists the types of reaction of the solution; determines and justifies the pH of the solution (acid, alkaline, neutral);

7) uses the pH scale; interprets the pH value in terms of quality (acidic, alkaline, neutral); performs an experiment that will allow to examine the pH of products occurring in everyday human life (e.g. food, cleaning products).

General aim of education

The student lists the types of reactions and interprets the pH value in qualitative terms

Key competences

• communication in foreign languages;

• digital competence;

• learning to learn.

Criteria for success
The student will learn:

• to explain what the solution's reaction and the pH‑scale is;

• determine the pH values of the tested samples.

Methods/techniques

• expository

  • talk.

• activating

  • discussion.

• 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;

• 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.

3. Health and safety - before starting experiments, the teacher acquaints students with the characteristics of the substances that will be used in the lesson. He indicates the need to be careful when working with them.

Realization

1. The teacher introduces students to the problem of the pH of the solution. It presents the packaging of various cosmetics, leaflets of some medicines and other products on which the term pH appears, encouraging students to discuss. Finally, he explains what the term pH means and displays on the multimedia presentation (or presents in a different form) the general equations for the dissociation of bases and acids. Indicates acidic and basic ions; he explains when we talk about a solution that is alkaline, acidic or neutral.

2. The teacher asks pupils for an independent analysis of the infographics presenting the pH of various compounds, e.g. contained in food and household products.

3. The lecturer discusses the response indicators. Using the illustrations in the abstract, it presents examples of various indicators, discusses the pH scale, based, for example, on illustrative resources from the abstract and e‑textbook.

4. The lecturer asks pupils to watch the film „Color change test” (phenolphthalein, methyl orange, a universal indicator paper) and distribute work cards to students. Students observe changes, write notes in the cards, and then discuss the conclusions from the experiment together. They write them in work cards.

5. The teacher informs students that they will work in pairs by conducting another experiment „Testing the reaction of selected substances with the use of natural indicators”. Distributes instructions and necessary aids and reagents. He asks students to formulate a research question and instructs them to write them in the work sheet. The participants of the classes carry out the experiment in accordance with the received instruction; they record observations and conclusions in the work sheet, and then present them on the class forum.

6. The teacher asks students to do interactive exercises in the abstract.

Summary

1. The student indicated by the teacher sums up the lesson, telling what he has learned and what skills he/she has been practicing.

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

pH of the solutions

Universal indicators in acidic environment take red color and in alkaline - green or blue color.

The measure of acidity and alkalinity of the solution is pH‑scale. It takes the values 0–14.

• pH < 7 acidic pH, concentration HIndeks dolny 3₃OIndeks górny +⁺ > concentration OHIndeks górny -^-;

• pH = 7 neutral pH, concentration HIndeks dolny 3₃OIndeks górny +⁺ = concentration OHIndeks górny -^-;

• pH > 7 alkaline pH, concentration HIndeks dolny 3₃OIndeks górny +⁺ < concentration OHIndeks górny -^-.

A visual comparison of the color of the indicators with a standard, e. g. a color pH scale on the packaging, only allows for an approximate determination of the solution's acidity. The pH value of the solution can be determined precisely by, among others, a device called a pH meter. It is used in chemical, medical, agricultural and cosmetic laboratories.

The pH meter consists of a sensor called an electrode, which is immersed in the tested solution, and a display from which the pH value is read out. Some pH meters are equipped with thermometers because the temperature influences the measurement.

The so‑called universal indicator, which is a mixture of several indicators, can also be used for pH‑testing. It changes its color at different pH values. It takes on a red color in acids and in bases – green to blue color.

Knowing the pH value is of great practical importance, e. g. in the production of medicines (apart from the active substance, drugs contain a protective layer that protects e.g. the esophagus against unfavorable pH) or cosmetics (e.g. nowadays there are liquid soaps with a pH similar to natural pH of human skin – a slightly acidic pH, which has a bactericidal effect).
The pH value of the soil determines its suitability for growing certain types of plants. The quality and height of the yield depend on the soil pH. For most plants pH is optimal in the range of 5.6–7.8, but it is an individual value for each of the species.

Organic fluids in the human body have different pH values, e.g. gastric juice has pH = 1.0–1.5, pancreatic secretion – 7.5–8.8, and the pH of the blood remains within the range 7.35–7.45. Both too low and too high a pH is the cause of many diseases. The decrease of the pH value below 6.8 results in so called acidosis, and the increase above pH 7.45 – alkalosis. This problem may occur during damage to any of the organs responsible for the excretion of toxins, e. g. kidneys or lungs, which makes them work less well.

Most fish species live in water with pH value = 6–8. Lowering the pH of the water to a level below 5 causes an abnormal behavior of the fish, consisting in rapid movements and the appearance of a blue color on the skin. At a pH higher than 8, the skin of the fish is covered with mucus, the scales become turbid and the fins become frayed and fall off.

Red geranium flake indicator
Red geranium, like red cabbage, can be a convenient indicator to use. To this end, pour hot water over the chopped petals and wait 20 minutes. Then filter the decoction through e.g. a coffee filter, and retain for testing.

A good indicator is red onion, which is pale red in the acidic environment and in alkaline environment – green.
Onion and vanillin oil, which do not emit characteristic odors in a strongly alkaline environment, are oflactory indicators.

• The measure of the acidity and alkalinity of the solution is the pH value, which is quantified in numbers 0–14.

• The solution is acidic at pH 

• The solution is neutral at pH = 7.

• The solution is alkaline at pH > 7.

• The higher the concentration of hydrogen ions, HIndeks dolny 3₃OIndeks górny +⁺ (or simplified notation: HIndeks górny +⁺), the lower the pH‑value of the solution.

• The higher the concentration of hydroxide ions, OHIndeks górny -^-, the higher the pH‑value of the solution.