2.4 The particle model can explain the properties of matter

2.4

Learning intentions

By the end of this topic, you will be able to: • define and explain tensile strength, hardness, viscosity, compressibility and density in terms of the particle model.

Key ideas

• Strong bonds between particles can make the material stronger (able to withstand force), harder and more viscous. • Tightly packed particles make the material dense and less able to be compressed.

Video 2.4 Calculating density

tensile strength a measure of the fl exibility of the bonds between particles in a substance compressional strength the ability of a substance to withstand large forces hardness how easily a mineral can be scratched viscosity a measure of how slowly a liquid changes its shape; the thickness of a liquid

compressibility the extent to which a substance can be compressed (squashed); gases can be compressed but solids and liquids cannot

Figure 1 Reinforced concrete combines the tensile strength of steel with the compressional strength of concrete.

The particle model can explain the properties of matter

Strength The idea of strength can be considered in different ways. A rubber band is easily stretched, but what about a piece of wire? Different wires made of different metals will break if stretched. Tensile strength is a measure of the fl exibility of the links or bonds between the particles. The bonds between the particles in steel are stronger than those between tin particles. Another type of strength is compressional strength . Substances that can withstand large forces without being crushed have a high compression strength. Hardness Hardness is the ability of a substance to scratch another substance. An iron nail will scratch a plastic ruler because the iron is harder than plastic (Figure 2). However, the iron nail will not scratch glass because the iron is softer than glass. For these substances, the order of hardness is glass, next iron and then plastic. Hardness is not the same as strength. A very hard substance may shatter easily. If this happens, the material is described as ‘brittle’. The particle model of matter explains hardness in terms of the forces that hold the particles together. The particles in hard substances are held together very strongly and it is diffi cult to separate them. In plastic, the particles are not held together as strongly and can be removed or scraped off. Therefore, plastic is not a hard substance. There is a connection between hardness and melting. Substances that are hard have strong forces (bonds) between their particles. These strong forces mean that for hard substances to melt, a lot of heat energy is needed. These substances usually have a high melting temperature.

Iron nail Plastic Scratch Figure 2 An iron nail will scratch plastic because iron is harder than plastic. Viscosity Viscosity is the thickness, or ‘gooiness’, of a liquid. It describes how easily the particles move around one another. Viscous liquids are hard to pour out of a container. Water has a low viscosity, cooking oil is more viscous and honey is very viscous (Figure 3). Engine oils used in engines have different viscosities. DRAFT Figure 3 Honey has a higher viscosity than water and so can’t be poured as easily as water. Compressibility Compressibility refers to the ability of a substance to be squashed or compressed. You can test for compressibility when substances are in a plastic syringe. If you put your fi nger over the end of a syringe, you can compress the air inside it.

However, if you replace the air with a liquid such as water, you cannot compress the water. Similarly, if you fi lled the syringe with a solid such as sand, you are not be able to compress it. In solids and liquids, there are no empty spaces between the particles, so it is not possible to compress the particles closer together. Solids and liquids are said to be incompressible . Gases, like air, can be compressed. This is because the particles are spread out and there is space between them (Figure 4).

Density

Gas in bike pump Normal Compressed

One way of comparing the ‘heaviness’ of two substances is to compare their densities. Density describes the number of particles and how tightly packed they are. The density of a substance will affect its properties, such as its ability to fl oat. Objects with lower density fl oat on liquid if the liquid has a higher density. For example, a piece of cork will fl oat on water. This is because Figure 4 the water has a higher density than the cork. Compression On the other hand, there are coins will sink to reduces the space between particles. incompressible unable to be compressed; solids and liquids are incompressible

the bottom of water. This is because the coins have a higher density than the water. In Figure 5, we see that 1 L of water is heavier than 1 L of air. We say that water has a greater density than air. Sand has a greater density than water or air, but it has a lower density than lead. The particle model of matter explains density in terms of both the mass and the closeness of the particles. Gases always have low densities because there is a lot of empty space between the particles. Solids normally have high densities because there is no space between the particles. The densities of some common substances are given in Table 1. Table 1 Densities of some common substances Substance Density (g/cm3)

Air 0.001

Foam rubber 0.05

Wood 0.3

Oil 0.75

Water 1.0

Glass 2.6

Steel 7.8

Iron 7.8

Copper 8.9

Lead 11.3

Mercury 13.6

Gold 19.3

1 L air = 0.003 g density Figure 5 Density compares the mass of objects of a measure of mass per unit the same volume. of volume 2.4 Check your learning Retrieve 1 Identify the most and least dense material in Table 1. 2 Name the following in order of compressibility from least to most: solid, liquid, gas. Comprehend

1 L water = 1.0 kg 1 L sand = 2.5 kg

1 L lead = 8.7 kg DRAFT 3 Prepare a table (set up like Table 2) that summarises the following physical properties: strength, hardness, viscosity, compressibility and density. Apply

6 Use Table 1 to predict which of the following would sink or fl oat in water: oil, an iron nail, a balloon fi lled with air, a glass marble.

Table 2 Physical properties and their meanings

Physical property Meaning Quiz me Complete the Quiz me to check how well you’ve mastered the learning intentions and to be assigned a worksheet at your level.

4 Use the particle model of matter to explain why steel is stronger than tin. 5 Describe what would happen if you placed a highly viscous liquid, such as oil, into a water pistol. Explain your reasoning (by comparing the properties of water and oil and how this will affect how the oil will behave in the water pistol).