Melting and dissolving
Jill and Toby wanted to see who could boil water the quickest. The diagram above shows how they each set up their equipment.
Melting and dissolving are two processes that all students will have experienced frequently during every day activities. They are easy to replicate in the classroom, and are suitable for students to carry out their own investigations at all levels of the curriculum, building on their existing knowledge. For the above reasons, they provide a rich context for students to start developing a key science idea, particle theory.
This task has a Communicating in science focus - descriptions and explanations - within the context of melting and dissolving. The key ideas that are covered in the task are shown in the table below.
| Nature of science ideas | Science knowledge and skills |
Communicating in science
|
Melting and dissolving
|
| Y6 (08/2009) | Y8 (08/2009) | Y10 (08/2009) | |||
| a) |
i) ii) |
A description that mentions any one or more of the following observations:
An explanation that links heat and melting, e.g., heat causes the butter to change into a liquid/melt |
very easy
easy NA |
very easy
easy very difficult |
very easy
easy very difficult |
| b) |
i) ii) |
Descriptions that link what can be seen happening to the dissolving sugar, e.g., sugar disappears into the water. Explanations that demonstrate an understanding that the sugar doesn't really vanish but becomes mixed with the water, i.e., it is a solution or Students also include that the sugar breaks up into smaller particles which disperse between the water particles. |
very easy
very difficult NA |
very easy
very difficult very difficult |
very easy
very difficult very difficult |
|
Levels 3 & 4 |
Communicating in science | Relevance to this task |
| Begin to use a range of scientific symbols, conventions and vocabulary |
We want students to:
|
|
| Level 5 | Use a wider range of science vocabulary, symbols and conventions |
We want students to:
|
Communicating in science is the expression of the above competency in the context of science.
The resource was structured to support students to differentiate between describing and explaining.
In the data from the trial students show that:
- writing a description was easier than writing an explanation
- most students were able to describe what they would observe, and keep explanations separate
- in the melting example most students were also able to write some sort of explanation
- far fewer students were able to write an explanation about dissolving. This is likely to be linked in many cases to their lack of knowledge about the science involved in dissolving, which is a much more difficult phenomenon to explain than melting.
The butter melts.
I see that the sugar dissolves.
Interpreting visual prompts
Some students used the drawing prompts to help with their descriptions of the process.
- Sometimes this was helpful. For example, some students interpreted the wavy lines above the pan of butter to be steam or heat. Either of these interpretations could help students to think about the process of melting.
- Other times it was not helpful. Some students thought that the grains of sugar were bubbles, and this could lead to explanations about chemical reactions (e.g., as you get when mixing baking soda and vinegar).
There is acid in the water causing it.
Use of scientific vocabulary
Some students used the words melting and dissolving interchangeably, most often referring to dissolving as melting.
Most students at all levels were able to describe the process of melting and explain it by linking it to temperature.
Most students at all levels were able to describe the process of dissolving, but few could explain this process. Research shows that dissolving is a difficult concept to understand. Go to Why "dissolving" is a difficult idea
Particle theory
Few students used particle theory in their explanations for melting and dissolving. (There is no curriculum expectation for Level 3 students to be able to do this, and none did).
| Year | Use of particle theory to explain melting | Use of particle theory to explain dissolving |
|
Yr 8 |
Correctly: 4 students |
Correctly: 4 students |
|
Yr 10 |
Correctly: 15 students |
Correctly: 2 students |
The following are examples of students beginning to use particle theory to explain melting or dissolving. These partial explanations provide a possible entry point to begin developing a more accurate explanation.
Year 10 examples of melting:
- Because the particles are no longer together.
- Because the heat is breaking up the particles that make up the object.
- Because the heat breaks it down.
- Heat is applied to the solid which causes its particles to shift and change state.
Year 8 examples of dissolving:
- This happens because the thing that's dissolving can be broken down
- Because the substance is broken down into even smaller pieces we cannot see
- I think it dissolves because it's breaking down into little particles until it disappears.
If students did not separate descriptions and explanations, or could not accurately write either, attention may need to be paid to unpacking the purpose of each. This could be followed by practice in describing what they can see and explaining what they are thinking. Using some sort of graphic organiser may help some students.
While the focus of the assessment strategy Predict, observe, explain is primarily to uncover students' thinking, it could also be useful for supporting students to write descriptions and explanations.
Try getting students who were able to complete this task successfully to write about a science event without the supports of the two questions used in this resource. Do they still organise their descriptions and explanations coherently?
To help students look more critically at visual prompts:
- Look at and discuss other examples as they arise where students may interpret visual cues differently.
- Ask students to think about the author's/artist's likely intention.
Scientific vocabulary
- The words melting and dissolving should be modelled correctly during teaching, and similarities and differences made explicit (see the section below on melting and dissolving).
- After learning experiences about melting and dissolving, ask students or groups of students to write their own definitions. Critique and discuss, and agree on a class definition.
Curriculum expectations
Level 3 Communicating in science Relevance to this task
Group materials in different ways, based on the observations and measurements of the characteristic chemical and physical properties of a range of materials.
We want students to:
describe the processes of melting and dissolving, as a precursor to being able to group materials, e.g., those that dissolve and those that don't.
Level 4 Begin to develop an understanding of the particle nature of matter and use this to explain observed changes.
We want students to:
describe the processes of melting and dissolving, making some reference to beginning understandings of particle theory.
Level 5 Distinguish between a pure substance and a mixture at particle level.
We want students to:
use particle theory to explain melting and dissolving.
Students should carry out their own investigations of melting and/or dissolving, and be encouraged to observe closely. In groups, get students to write down as many observations as they can and share with other groups. They then can be asked to explain their observations.
After students have closely observed instances of melting and dissolving actually happening, and investigated conditions that affect the rate of melting and dissolving.
In groups fill in a table of similarities and differences. For example:
Ways melting and dissolving are different Ways melting and dissolving are the same
Melting Dissolving
1 substance involved
Solid turns to liquid
Must have heat
When a substance melts you can see the original substance in another state
More than 1 substance (a solute and a solvent)
involved
Liquid + solid becomes a liquid
Heat not necessary
When a substance dissolves you can't see the solid (may still be some evidence e.g. colour).
Finish up with a liquid
Hotter = more quickly
It happens more quickly
when solids are in small
pieces.
It happens more quickly
when you stir.
All the original substances are still there (physical change).
Role play is a good way of introducing particle theory. This can be led by the teacher at lower levels, but older students can be challenged to choreograph their own role play. For example:
- Melting: The whole class (or group) represents a lump of butter. Each person is a molecule of butter. Show how the molecules behave as the butter gets warmer.
- Dissolving: Most of the class represents water molecules. A group of students represents a grain of sugar. Demonstrate how the grain of sugar breaks into smaller particles as it mixes with the water and slides between the water molecules. Challenge students to also demonstrate what happens at a particle level when variables change, e.g., the temperature of the water, water to sugar ratio.
- Peer assessment: Develop criteria together for judging effective role plays. Critique and discuss each role play.
Misconceptions
Refer to Why "Dissolving" is a Difficult Idea
- Book 16, Sand, Salt and Jelly Crystals (2001) – dissolving
- Book 58 (2004), Ice – melting
- Book 64, Candles (2004) – melting
- Water
- Drinks