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Classification of matter:
Properties
Free High School Science Texts Project
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1 Electrical conductors, semi-conductors and insulators
An electrical conductor is a substance that allows an electrical current to pass through it. Electrical conductors are usually metals. Copper is one of the best electrical conductors, and this is why it is used to make conducting wire. In reality, silver actually has an even higher electrical conductivity than copper, but because silver is so expensive, it is not practical to use it for electrical wiring because such large amounts are needed. In the overhead power lines that we see above us, aluminium is used. The aluminium usually surrounds a steel core which adds tensile strength to the metal so that it doesn't break when it is stretched across distances. Occasionally gold is used to make wire, not because it is a particularly good conductor, but because it is very resistant to surface corrosion. Corrosion is when a material starts to deteriorate at the surface because of its reactions with the surroundings, for example oxygen and water in the air. An insulator is a non-conducting material that does not carry any charge. Examples of insulators would be plastic and wood. Do you understand now why electrical wires are normally covered with plastic insulation? Semi-conductors behave like insulators when they are cold, and like conductors when they are hot. The elements silicon and germanium are examples of semi-conductors.
De nition 1: Conductors and insulators A conductor allows the easy movement or ow of something such as heat or electrical charge through it. Insulators are the opposite to conductors because they inhibit or reduce the ow of heat, electrical charge, sound etc through them. Think about the materials around you. Are they electrical conductors or not? Why are dierent materials used? Think about the use of semiconductors in electronics? Can you think of why they are used there?
1.1 Experiment : Electrical conductivity
Aim:
To investigate the electrical conductivity of a number of substances Apparatus:
- two or three cells
- light bulb ∗Version 1.3: Feb 23, 2012 1:06 am US/Central †http://creativecommons.org/licenses/by/3.0/
- crocodile clips
- wire leads
- a selection of test substances (e.g. a piece of plastic, aluminium can, metal pencil sharpener, magnet, wood, chalk).
Figure 1
Method:
- Set up the circuit as shown above, so that the test substance is held between the two crocodile clips. The wire leads should be connected to the cells and the light bulb should also be connected into the circuit.
- Place the test substances one by one between the crocodile clips and see what happens to the light bulb.
Results:
Record your results in the table below:
Test substance Metal/non-metal Does the light bulb glow? Conductor or insulator
Table 1
Conclusions:
In the substances that were tested, the metals were able to conduct electricity and the non-metals were not. Metals are good electrical conductors and non-metals are not. The following simulation allows you to work through the above activity. For this simulation use the grab bag option to get materials to test. Set up the circuit as described in the activity.
Figure 2
run demo^1 (^1) http://phet.colorado.edu/sims/circuit-construction-kit/circuit-construction-kit-dc_en.jnlp
Material Thermal Conductivity (W · m−^1 · K−^1 ) Silver 429 Stainless steel 16 Standard glass 1. Concrete 0.9 - 2 Red brick 0. Water 0. Snow 0.25 - 0. Wood 0.04 - 0. Polystyrene 0. Air 0.
Table 2
Use this information to answer the following questions:
- Name two materials that are good thermal conductors.
- Name two materials that are good insulators.
- Explain why: a. cooler boxes are often made of polystyrene b. homes that are made from wood need less internal heating during the winter months. c. igloos (homes made from snow) are so good at maintaining warm temperatures, even in freezing conditions.
note: It is a known fact that well-insulated buildings need less energy for heating than do buildings that have no insulation. Two building materials that are being used more and more worldwide, are mineral wool and polystyrene. Mineral wool is a good insulator because it holds air still in the matrix of the wool so that heat is not lost. Since air is a poor conductor and a good insulator, this helps to keep energy within the building. Polystyrene is also a good insulator and is able to keep cool things cool and hot things hot. It has the added advantage of being resistant to moisture, mould and mildew.
Remember that concepts such as conductivity and insulation are not only relevant in the building, industrial and home environments. Think for example of the layer of blubber or fat that is found in some animals. In very cold environments, fat and blubber not only provide protection, but also act as an insulator to help the animal keep its body temperature at the right level. This is known as thermoregulation.
3 Magnetic and Non-magnetic Materials
We have now looked at a number of ways in which matter can be grouped, such as into metals, semi-metals and non-metals; electrical conductors and insulators, and thermal conductors and insulators. One way in which we can further group metals, is to divide them into those that are magnetic and those that are non-magnetic.
De nition 2: Magnetism Magnetism is one of the phenomena by which materials exert attractive or repulsive forces on other materials.
A metal is said to be ferromagnetic if it can be magnetised (i.e. made into a magnet). If you hold a magnet very close to a metal object, it may happen that its own electrical eld will be induced and the object becomes magnetic. Some metals keep their magnetism for longer than others. Look at iron and steel for example. Iron loses its magnetism quite quickly if it is taken away from the magnet. Steel on the other hand will stay magnetic for a longer time. Steel is often used to make permanent magnets that can be used for a variety of purposes. Magnets are used to sort the metals in a scrap yard, in compasses to nd direction, in the magnetic strips of video tapes and ATM cards where information must be stored, in computers and TV's, as well as in generators and electric motors.
3.1 Investigation : Magnetism
You can test whether an object is magnetic or not by holding another magnet close to it. If the object is attracted to the magnet, then it too is magnetic. Find some objects in your classroom or your home and test whether they are magnetic or not. Then complete the table below:
Object Magnetic or non-magnetic
Table 3
3.2 Group Discussion : Properties of materials
In groups of 4-5, discuss how our knowledge of the properties of materials has allowed society to:
- develop advanced computer technology
- provide homes with electricity
- nd ways to conserve energy
4 Summary
- All the objects and substances that we see in the world are made of matter.
- This matter can be classi ed according to whether it is a mixture or a pure substance.
- A mixture is a combination of one or more substances that are not chemically bonded to each other. Examples of mixtures are air (a mixture of dierent gases) and blood (a mixture of cells, platelets and plasma).
- The main characteristics of mixtures are that the substances that make them up are not in a xed ratio, they keep their individual properties and they can be separated from each other using mechanical means.
- A heterogeneous mixture is non-uniform and the dierent parts of the mixture can be seen. An example would be a mixture of sand and water.
c. A substance that contains two or more substances, in de nite proportion by weight d. A uniform substance Click here for the solution^3
- Classify each of the following substances as an element, a compound, a solution (homogeneous mixture), or a heterogeneous mixture: salt, pure water, soil, salt water, pure air, carbon dioxide, gold and bronze. Click here for the solution^4
- Look at the table below. In the rst column (A) is a list of substances. In the second column (B) is a description of the group that each of these substances belongs in. Match up the substance in Column A with the description in Column B.
Column A Column B iron a compound containing 2 elements H 2 S a heterogeneous mixture sugar solution a metal alloy sand and stones an element steel a homogeneous mixture
Table 4
Click here for the solution^5
- You are given a test tube that contains a mixture of iron lings and sulphur. You are asked to weigh the amount of iron in the sample. a. Suggest one method that you could use to separate the iron lings from the sulphur. b. What property of metals allows you to do this? Click here for the solution^6
- Given the following descriptions, write the chemical formula for each of the following substances: a. silver metal b. a compound that contains only potassium and bromine c. a gas that contains the elements carbon and oxygen in a ratio of 1: Click here for the solution^7
- Give the names of each of the following compounds: a. N aBr b. BaSO 4 c. SO 2 Click here for the solution^8
- For each of the following materials, say what properties of the material make it important in carrying out its particular function. a. tar on roads b. iron burglar bars c. plastic furniture d. metal jewellery
(^3) http://www.fhsst.org/llF (^4) http://www.fhsst.org/llG (^5) http://www.fhsst.org/ll (^6) http://www.fhsst.org/llA (^7) http://www.fhsst.org/llo (^8) http://www.fhsst.org/lls
e. clay for building f. cotton clothing Click here for the solution^9
(^9) http://www.fhsst.org/llH
