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Heterogeneous Mixtures - Bricks!Topic Area(s) Time Grade Level Supplies

Heterogeneous mixtures 60-90mins Grade 6 Playdough, clay, dry rice, pebbles/gravel, sand, toothpicks,

straws, beads, scissors

Curriculum Links

Science:● Grade 6 Big Ideas: Everyday materials are often mixtures● Grade 6 and 7 Curricular Competencies: Questioning and predicting, planning and

conducting, applying and innovating● Grade 6 Content: Heterogeneous mixtures

Applied Design, Skills, and Technologies:● Grade 6 and 7 Curricular Competencies: Prototyping, testing, making

Materials (for class of 30, working in pairs)

● Playdough/clay - 2100g

● Dry rice - 4 cups

● sand - 4 cups

● pebbles/gravel - 4 cups

● toothpicks - 150

● straws - 45

● scissors - 15Background Information

Definitions:● Nano - scientific term meaning one billionth (1/1,000,000,000). Comes from the

greek word for “dwarf”. For example, one nanometer is one-billionth of a meter. Hydrogen atoms are about one nanometer in size (1nm). At this size, common

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materials start showing unusual properties, such as lower resistance to electricity and faster chemical reactions.

● Composite - Something made up of different elements/things. Bones are composites made up of minerals, proteins and other substances. Composites are useful because they can combine the advantages of their components. Composite materials often contain a matrix and a reinforcement material.

● Matrix - the majority of continuous material in a composite. The matrix surrounds the reinforcement material and keeps it in place. In concrete, the matrix is cement.

● Reinforcement material - Materials used to improve the mechanical properties of the composite. They often improve the hardness/stiffness by complementing the matrix material. In concrete, the reinforcement material is often gravel.

● Grain-direction / Fiber-direction - In a material with fibers, the grain- or fiber-direction describes the direction of the fibers. It’s important to know the grain-direction, because objects often have different mechanical properties in the grain-direction compared to other directions. For example, the fibre-direction of our bones makes them extremely resistant to be broken along their length, but vulnerable to be broken perpendicular to their length (think of what bone fractures usually look like).

● Hardness - How well a material resists permanent changes when a force is applied. Ceramic plates are very hard since they are difficult to scratch and indent.

● Toughness - How well a material can withstand force without breaking. Ceramic plates are not tough since they break easily when dropped. Toughness can be thought of as how much energy an object absorbs when impacted.

Our world is surrounded by composites. Some of them are man-made, such as concrete, whereas many are naturally occurring, such as bones. Mankind uses composites to create new materials that are stronger and lighter than non-composites. For example, the metals used in airplanes are extremely light, while being both tough and hard. This is achieved by making composites with components that are individually tough or hard. When engineered into a composite, their toughness and hardness combine into a material that is both.Naturally occurring composites such as bones work the same way. The minerals in bones make them very hard, so they don’t bend under weight, while their proteins make them tough so they don’t break easily. The components of bones are nano-sized. Even though bones themselves aren’t nano-sized, the individual materials that make them up are. The secret behind the strength of our bones is that their structure is small to the point that the special properties of their components are able to come together the way they do.

Mud Bricks

Humans have been making composite materials for thousands of years. Even though the bronze age started about 3 thousand years ago, we’ve been making composites since as early as 6000

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BC! Back then, humans used soil/clay, water, and fibrous materials such as straw or sticks to make mud bricks! They would mix up the materials and then leave them out in the sun. While this technology may seem simple, the result would create very cheap, light materials that could be produced very easily and used for all sorts of building. Additionally, the bricks were resistant to wind, rain, and even fire, all while also helping maintain good temperature control and block sound.

(A) Photo of an Ethiopian woman making mud bricks for her house. Image is in the public domain (http://commons.wikimedia.org/wiki/File:Bati_woman_making_mud_bricks.jpg).

(B) Mud brick house in Sudan. Source: http://commons.wikimedia.org/wiki/File:Tuti_Island_(Khartoum,_Sudan)_006.jpg.

To this day there about 30% of people still live in earthen brick buildings. These buildings aren’t restricted to developing nations either, some first world countries are investing in mud brick research to improve their strength.

In this activity, students will take the role of scientists and engineers to design and test their own mud bricks. They will also be introduced to materials engineering and will need to consider what properties they want in their bricks. Finally, they will learn about simple materials testing to use on their own bricks to evaluate their creations.

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Procedure

Set Up:

1. Create 3 stations for students to pick out the materials they want to use.a. General materials

i. Containers to hold matrix and reinforcement materialsii. Paper & writing utensils to record brick design and results

b. Matrix materialsi. playdoughii. clay

c. Reinforcement materialsi. Stawsii. pebbles

iii. cardboard piecesiv. dry ricev. toothpicks

Activity:

1. Introduce students to composite materials using the information above and explain what each material is used for including what function it will have in a composite.

2. Have students brainstorm and discuss important properties of bricks. The key points should be that bricks aren’t too heavy or expensive, while still being tough (can support weight without cracking), hard (don’t compress) and easy to make.

3. Now that the properties have been considered, have students characterize the available materials in whether they are hard (resist changing shape) or tough (resist breaking). This should result in the materials being grouped in the same way as the setup above.

4. Now that you have the materials sorted out, address the students about whether the arrangement of the materials in the brick is important. Ask them how the arrangement will affect the physical characteristics of the bricks. If the reinforcement materials are all close to the middle of the brick, how will the brick respond to weight being added on top of it? What about the opposite? Students should want to have their reinforcement material be evenly spread out in their bricks, and surrounded by the matrix material, much like concrete.

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5. Have students pair up and, for 5-10 minutes, get them to sketch what their brick should look like in relation to the placement and arrangement of reinforcement materials within the matrix material. They also need to record what materials they want to use and how much of each.

6. Have student pairs pick out their materials and start assembling their bricks for 20 minutes.

a. Recommend that they knead their playdough/clay well before they start to avoid cracking. Also suggest that they first roll up their composite and then flatten the sides at the end to achieve the desired shape.

7. Have students test their bricks for 20 minutes. Measure the dimensions of the bricks with rulers.

a. Toughness - start testing by placing a heavy textbook on them and then stacking heavier objects on them. Observe any changes that occur to the bricks as the weight increases. Look for cracks too! As a more extreme toughness test, students can choose to drop their bricks from different heights to see the limit of their toughness - just make sure to have this be the last test since it will likely destroy the bricks!

b. Hardness - Use a pencil or similar object and place it on the bricks with the sharp end facing the brick. Then slowly apply force downwards and see what happens! A hard brick will not give much to the pencil, even if a hole does form, whereas a softer brick will deform very easily.

c. Cohesion: Cohesion describes the ability of something to stick together, or stay stuck together. Test the cohesion of your brick by rolling it end-over-end and observing any changes that occur.

8. As you conclude the activity, have students share what went well and what didn’t, as well as have them consider relationships between the characteristics of their bricks to the materials they used and the structure in which they were arranged.

What Happened?

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Ideally, the bricks will have a good balance of both strength and hardness. This can be achieved by both the orientation of the reinforcement materials, as well as the selected materials themselves. Orientation of the different materials is key to the strength of the brick as each separate material has an ‘ideal’ orientation that will provide the greatest strength. Ultimate strength is achieved by maximizing the ‘ideal’ orientation of the fibre and combining these orientation in the direction of the load. For the case of bricks, forces are applied from each side and need to be able to take the load without breaking.

Were cracks noticed? Once a crack is initiated it significantly drops the strength of the material. The crack will propagate with more applied force and eventually lead to failure of the material.

It is possible for materials to have a high toughness but a very low hardness. Did the pencil penetrate the brick easily? Hardness is highly dependable on the material density - it is easier to penetrate the clay than penetrate a toothpick for example. Materials are higher density have a greater hardness.

Connecting Engineering to Your Classroom

This activity is a perfect example of what a materials engineer does. Obviously, these engineers do much more than just test and design bricks - they are needed in many other situations as well. For example, in nuclear reactors, materials engineers are responsible for designing a

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special kind of concrete that can absorb as much radiation as possible, to reduce the chance of the environment nearby being affected.

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