Secondary 1 - 3
20 minutes
This programme is a teaching resource produced especially for enriching the classroom teaching of the Syllabus for Science (Secondary 1 – 3) prepared by the
Curriculum Development Council (1998).
The content of the programme serves to illustrate the following parts of the
Syllabus :
“Unit 5 : The Wonderful Solvent – Water
Topic 5.5 Dissolving, Topic 5.6 Growing crystals, and Topic 5.7
Solvents other than water : - All the key points and contents.”
1.
An explanation of the concept of solutes, solvents, solutions and dissolving.
2.
An introduction to crystals and their applications.
3.
An introduction to the safety precautions in using organic solvents.
This programme is divisible into 3 parts as follows:
1.
Solutes, Solvents, Solutions
The concepts of solutes, solvents, solutions and dissolving are clarified with reference to everyday examples.
An explanation is given in terms of the particle model of matter. Computer animated graphic technique is utilized to show that matter is composed of particles and that gaps exist among the particles. In ascending order of size, the gaps among particles of the solid are the smallest, those of the liquid are larger and those of the gas are the largest. Taking a solid solute for example, the solute is originally composed of particles tightly packed together. When the particles of the solute slowly come off and go into the solvent to mix with the solvent particles, the process is called dissolving. If the particles of the solute come off quickly from the solute and mix into the solvent, then the process of dissolving is faster. The speed of dissolving can be increased by stirring, heating, and breaking down the solute into many tiny pieces. A solute can be a solid, a liquid or a gas. scjs022stn
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The process of soft-drink production is used to illustrate this concept.
2.
Crystals

A fixed amount of solvent can dissolve a limited amount of solute. This limited amount is called its solubility. When a solvent has dissolved a large amount of a certain solute, and cannot dissolve any more, we say that the solvent has reached its saturated state. This solution is called a saturated solution.

Cooling down a saturated solution may result in crystal formation. In fast crystallization process, crystals are formed at the same time and usually are very small in size in the form of a powder. While in slow crystallization process, crystals are generally larger and have a clear geometrical shape.
 Crystals may become precious stones if they meet the following criteria: being beautiful, durable and rare. A precious stone generally originates in the magma deep underneath the earth’s crust. After the stone is subjected to a long period of high temperature and high pressure, it crystallizes and finally comes to the earth’s surface in the form of lava. When the lava cools, a precious stone is formed slowly. Precious stones that are crystals are called crystal stones.
Crystal stones are classified into seven systems:
1.
cubic system, e.g. diamond, garnet.
2.
tetragonal system, e.g. zircon
3.
hexagonal system, e.g. green sapphires
4.
trigonal system, e.g. rubies, sapphires, quartz
5.
orthorhombic system, e.g. topaz, peridot
6.
monoclinic system, e.g. hard jade, soft jade
7.
triclinic system, e.g. turquoise

Crystals have multiple uses. Taking the quartz clock for example, when an electric current flows through a quartz crystal, a vibration at a specified frequency is produced. This vibration is used for calibration of time. Crystals are also used in scientific research and hi-tech production.
3.
Solvents other than water
Water can dissolve most materials and is known as the “universal solvent ”
.
However, there are quite a lot of materials that do not dissolve in water. To dissolve these materials, we have to use solvents other than water. The solution especially used for removing nail polish contains a solvent called acetone. Acetone and alcohol are chemicals called organic solvents. Organic solvents are usually toxic and highly volatile, hence posing a health hazard. The safety precautions in using organic solvents are then introduced.
The teacher is advised to show the programme in one lesson. The teacher may spend
5 to 10 minutes at the beginning of the lesson to lead students to discuss with reference to the Preparation before viewing the programme part of the Suggested
Activities.
The teacher may then show the programme. After the show, the teacher may spend another 10 to 15 minutes to discuss with students with a view to consolidating the concepts and methods illustrated in the programme. The teacher is advised to refer to the Activities after viewing the programme part of Suggested
Activities.
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(The following activities are suggested for teacher’s reference only. The teacher may wish to use the activities according to students’ abilities, the learning environment of the class, and the teaching time available.)
Preparation before viewing the programme
(1) The teacher may ask students to name everyday examples of dissolving substances. The teacher may then point out the incorrect examples and guide students to pay attention to view the programme.
(2) The teacher may also invite students to discuss on examples and applications of crystals prior to viewing the programme.
Activities after viewing the programme
(1) The teacher may demonstrate experiments on the different ways to affect the rate of dissolving and different methods of crystal formation.
(2) The teacher could also assign a project work in collecting items in everyday life bearing labels with descriptions of the solutes and solvents in the content. scjs022stn
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