# Property of matter gas liquid gas particles atoms or

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Property of matter Gas Liquid Gas Particles Atoms or molecules Atoms or molecules Atoms or molecules Energy and move- ment of particles Particles have high energy and are con- stantly moving Particles have less energy than in the gas phase Low energy - parti- cles vibrate around a fixed point Spaces between par- ticles Large spaces be- cause of high energy Smaller spaces than in gases Very little space between particles. Particles are tightly packed together Attractive forces be- tween particles Weak forces because of the large distance between particles Stronger forces than in gas. Liquids can be poured. Very strong forces. Solids have a fixed volume. Changes in phase In general a gas becomes a liquid or solid when it is cooled. Particles have less energy and therefore move closer together so that the attrac- tive forces become stronger, and the gas becomes a liquid or a solid A liquid becomes a gas if its tempera- ture is increased. It becomes a solid if its temperature de- creases. Solids become liq- uids or gases if their temperature is in- creased. Let’s look at an example that involves the three phases of water: ice (solid), water (liquid) and water vapour (gas). solid liquid gas Figure 2.7: The three phases of matter In a solid (e.g. ice), the water molecules have very little energy and can’t move away from each other. The molecules are held close together in a regular pattern called a lattice . If the ice is 27

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2.5 CHAPTER 2. WHAT ARE THE OBJECTS AROUND US MADE OF? - GRADE 10 heated, the energy of the molecules increases. This means that some of the water molecules are able to overcome the intermolecular forces that are holding them together, and the molecules move further apart to form liquid water . This is why liquid water is able to flow, because the molecules are more free to move than they were in the solid lattice. If the molecules are heated further, the liquid water will become water vapour, which is a gas. Gas particles have lots of energy and are far away from each other. That is why it is difficult to keep a gas in a specific area! The attractive forces between the particles are very weak and they are only loosely held together. Figure 2.8 shows the changes in phase that may occur in matter, and the names that describe these processes. freezing melting evaporation condensation sublimation re-sublimation Liquid Solid Gas Figure 2.8: Changes in phase 2.5 The Properties of Matter Let us now look at what we have learned about chemical bonds, intermolecular forces and the kinetic theory of matter, and see whether this can help us to understand some of the macroscopic properties of materials. 1. Melting point Definition: Melting point The temperature at which a solid changes its phase or state to become a liquid . The reverse process (change in phase from liquid to solid) is called freezing .
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