# 2.3 Crystals and solutions

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## Crystals and solutions

• In the previous unit, mention was made of the formation of snowflakes.

Snow Crystals

• Each snowflake is unique and is formed when drops of water vapour in the atmosphere condense as snow crystals.
• Snow crystals develop six “arms” from a six-sided prism. Each “arm” grows differently as it is affected by continual minute temperature variations
• Snow crystals combine to form snowflakes.
• Some of the most valuable stones in the world consist of solid matter in the form of crystals.
• Diamonds, rubies and sapphires are examples of precious stones. The atoms of these crystals form specific patterns.
• Crystals have flat sides known as facets – they can take the form of triangles, rectangles, or many other shapes.
• Minerals can be identified according to the crystalline form.
• The basic form of crystals vary – salt, for instance, is cubical.
• Most crystals have to be polished to reveal their beauty.

 Do you know the following? 

Sand is composed of quartz crystals. These crystals are shaped by constantly being knocked or rubbed against each other.

Rochhounding ar.com

Class Activity: Making a Solution

• Fill a glass beaker with cold water.
• Add a teaspoon of salt or sugar to the water and stir.
• Continue stirring until the substance stops dissolving.

1. How many spoonfuls of the substance did you add?

• Repeat the experiment with an equal amount of hot water.

2. What do you observe?

3. What deduction can be made?

4. Explain why warm water is more effective as a solvent:

• Pour the solutes into watch-glasses and leave these on the classroom window sill for some days.

5. What do you observe after a few days?

6. What happened to the water?

• Examine the crystals through a magnifying glass or a microscope.

## Assignment: draw a couple of crystals to show their form.

Assessment for demonstration

Are you able to make correct deductions and communicate your findings?

[LO 1.3; LO 2.3]

## Assignment:

Compile a scientific report on your investigation.

Grow your own crystals at home or in the classroom

You will need:

• alum powder (obtainable from a chemist)
• glass jars
• cotton thread and a pair of scissors
• a drinking straw
• elastic band
• Fill the jar with hot water.
• Add alum powder by the method explained in the previous experiment – you will obtain a saturated solution. Use a watch-glass for crystals to form.
• Use the cotton thread to attach the crystals to the drinking straw and suspend them in the jar to a depth of three-quarters down the jar.
• Bend the straw and firmly attach it to the jar with the elastic band to hold it in position.

You will see crystals developing within a few days.

N.B.: Your educator could also let you grow copper sulphate crystals in the classroom.

Assessment of class project

Were you able to plan and execute the project, evaluate the data and apply your knowledge by handing in a properly compiled scientific report?

[LO 1.1; LO 1.2; LO 1.3; LO 2.4]

Problem Solving

Suppose you have a saturated solution, with excess crystals lying at the bottom of the beaker: Why would the crystals disappear if you began to heat the saturated solution slowly?

Assessment of problem solving

Were you able to provide an acceptable explanation for the problem?

[LO 2.4]

 Do you know the following? 

The gigantic rocks that form the Giant’s Causeway in Northern Ireland are hexagonal crystals that were formed when molten rock cooled down rapidly.

www.geocities.com/amegman_uk/symmetry.html

## Assessment

Learning outcome 1: Scientific investigations

The learner will be able to act confidently on curiosity about natural phenomena, and to investigate relationships and solve problems in scientific, technological and environmental contexts.

ASSESSMENT STANDARD: We know this when the learner

1.1 is able to plan investigations;

1.2 is able to execute an investigation and collect data;

• is able to evaluate data and communicate findings

Learning outcome 2: Constructing science knowledge

The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.

ASSESSMENT STANDARD: We know this when the learner

2.3 is able to interpret information.

2.4 is able to apply knowledge.

## Memorandum

CLASS ACTIVITY: MAKING SOLUTIONS

• Warm water contains more energy and crystals dissolve much faster in this

ASSIGNMENT: CLASS PROJECT

• The scientific report must include the following:

Purpose

Method

Materials

Results

Deduction

PROBLEM SOLVING:

• The particles of warm water have more kinetic energy and are therefore further apart – more salt particles can fit into the spaces.

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