Topic: Water and its properties – molecular structure

Target group

Elementary school student (grades 7. and 8.)

Core curriculum:

Elementary school. Chemistry.

V. Water and aqueous solutions. Pupil:

1) describes the structure of the water molecule and predicts the ability to dissolve various substances in water.

General aim of education

The student discusses the structure of the water molecule and its properties.

Key competences

• communication in foreign languages;

• digital competence;

• learning to learn.

Criteria for success
The student will learn:

• to explain the term “polarity of the water molecule”;

• to present the interactions between water molecules;

• to carry out an experiment, the purpose of which is to investigate whether a given substance is soluble in water or not.

Methods/techniques

• expository

  • talk.

• 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 asks a willing student to write on the board the reaction of water synthesis: 2HIndeks dolny 2₂ + OIndeks dolny 2₂ → 2HIndeks dolny 2₂O, noting that it is a compound of two elements: hydrogen and oxygen.

2. The teacher uses the text of the abstract for individual work or in pairs, according to the following steps: 1) a sketchy review of the text, 2) asking questions, 3) accurate reading, 4) a summary of individual parts of the text, 5) repeating the content or reading the entire text.

3. Students, working in groups, prepare a map of the concepts discussed in the abstract. Representatives of groups report the results of work.

4. The students merge in pairs. Each pair receives a sample of tap water. Students describe its physical properties: physical state, color, smell; they determine the taste (if the water is drinkable). Then they collect the applications together.

5. Students perform the task 1: in the abstract form they explain why the water molecule is polar. Selected people read their answers, the teacher corrects possible mistakes.

6. Students work in pairs. Their task is to indicate discrepancies between the description typical for many substances and the model of packing molecules for water (task 2). While working, they use the information contained in the abstract. Then the volunteers present their observations on the class forum.

7. Participants of the course, referring to the content presented in the interactive diagram, explain what meaning for the water life on Earth has low in relation to the density of liquid water density of ice.

8. Students perform an interactive exercise. The teacher makes sure that the task has been correctly completed and gives feedback.

Summary

1. The teacher plays the recording of the abstract. Every now and then he stops it, asking the students to tell in their own words what they have just heard. In this way, students consolidate information learned during the lesson and practice listening comprehension.

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. Carry out task number nr 3.1.

The following terms and recordings will be used during this lesson

Terms

Texts and recordings

Water and its properties – molecular structure

Water is a substance made of molecules. Each molecule consists of two hydrogen atoms connected with one oxygen atom. Oxygen and hydrogen atoms are connected by polar covalent bonds. Hydrogen and oxygen atoms do not lie along one line, the bonds between them form an angle of about 104.5 °.

As we know, opposite (positive and negative) charges attract each other. This is also the case with water molecules – the hydrogen atom of one molecule can interact electrostatically with an oxygen atom of the other molecule. This phenomenon can be clearly observed in water in liquid and solid state. In water in the liquid phase, apart from free molecules, there are also clusters of molecules that form exactly in the result of electrostatic attraction. These clusters are not permanent. Some molecules are released from them, while others join the system. In the solid state, water molecules, due to electrostatic interactions, form relatively stable structures.

The phenomenon of particles (molecules, ions, atoms) interconnecting to form larger systems composed of two or more particles as a result of electrostatic interactions is called association.

The polar structure of water molecules has its consequences – the physical properties of water. Usually, the particles in solids are closer to each other than in liquids, and a substance in the solid state has a higher density than in the liquid state. In the case of water, solid phase molecules form structures that leave a lot of free space; this is why the distances between particles are larger in ice than in water. For this reason ice has a lower density than water in the liquid state.

Understanding the physical properties of water will be easier if we recall a few facts about the states of matter.

When a substance changes its state from solid to liquid and then gaseous, energy must be supplied so that molecules can move away from each other. The more energy has to be supplied, the higher the melting point and the boiling point of the substance.

As ice passes into liquid, water molecules must free themselves from electrostatic interactions and then obtain energy that will allow them to move. Similarly, during vaporization, water molecules must obtain the energy needed to overcome electrostatic forces and necessary to move away from each other over long distances and to move around. Therefore, water has relatively high melting and boiling point. Much higher than substances with a similar structure, with molecules that do not interact with one another, e.g. hydrogen chloride ($\text{HCl}$) or hydrogen sulfide (${\text{H}}_{\text{2}}\text{S}$).

• Water is a polar substance. There is a partial negative charge on the oxygen atom, while on hydrogen atoms – positive one.

• Water molecules in water in the liquid phase interact with one another: the hydrogen atom of one molecule interacts electrostatically with the oxygen atom of the other molecule, and next to the free molecules there are clusters of molecules that persist due to electrostatic attraction.

• Water molecules in ice form ordered structures in which they are at greater distances from one another than in water in the liquid state. Ice has lower density than water.

• Water dissolves substances that are also polar. It also dissolves most of the ionic compounds.