2 MATTER. 2.1 Physical and Chemical Properties and Changes

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1 2 MATTER Matter is the material of which the universe is composed. It has two characteristics: It has mass; and It occupies space (i.e., it has a volume). Matter can be found in three generic states: Solid; Liquid; and Gas. Note however, these three states are not necessarily sufficient to describe the state of a system. For example, at room temperature and pressure, pure carbon may be found in two forms, graphite and diamond. Both are composed of carbon atoms alone, but they have completely different physical appearances. The difference is the organisation of the carbon atoms in the solid. Both solid forms are different states of carbon. Solids: Solids are generally rigid and maintain their shape and size. Liquids: Liquids take on the shape of their container but do not change their volume. For example, under the same conditions of temperature and pressure, the same mass of liquid water will occupy the same volume regardless of whether the water sample occupies a glass or a bathtub (even though the water sample has a different shape when it is placed in the two different containers. Gases: Gases take on the shape and size of whatever container it occupies. A sample of gas will expand or contract to fill the container uniformly. At the sub-microscopic level, the difference between these phases of matter is really due to the strength of the attraction between the smallest particles that make up the material. In a solid, strong attractions between the particles hold them rigidly in place in a three-dimensional arrangement. The particles can vibrate in their position, but cannot easily move in relation to each other. If the solid is heated, there is an increase in the vibrations of the particles. At a certain temperature (the melting temperature), these vibrations are rapid enough to disrupt the three-dimensional array of particles and the particles begin to slide past each other, tumbling around like a bunch of marbles shaken up in a bag. This is the liquid state. Further heating can transform the liquid into a gas (at or above the boiling temperature). In the gas phase, the particles of material are widely separated so that a gas sample occupies significantly more volume than the same sample of material in its liquid or solid phases. Thus, a gas is easily compressed compression simply reduces the average distance between gas particles. 2.1 Physical and Chemical Properties and Changes The physical properties of a substance are the physical characteristics by which a substance may be recognised. These properties include, for example, colour, odour, state (solid, liquid, gas etc.), volume, density, melting point and boiling point (substances have many other physical characteristics). The chemical properties of a substance tell us how the substance combines with other substances to form new substances (having different physical properties). For example, a chemical property of hydrogen is that under certain well-defined conditions, it combines with oxygen to from water. Water has completely different properties from the substances of which it is formed (hydrogen and oxygen). Exercise: Classify the following properties as physical or chemical. 1. Gold is yellow. 2. Gold resists a change in identity. 3. Iron rusts when exposed to air and water. 4. One lb. of gold occupies less space than one lb. of water. 5. Ammonia smells differently than hydrogen sulphide. Physical changes result when a physical stress is placed on the system. Physical changes in a system may be recognised because they are often reversible. For example, when ice is warmed, it eventually melts to form liquid water. If the liquid water is sufficiently cooled, it becomes ice once more. page 20

2 Chemical changes also result when physical stresses are placed on a system. The difference between physical changes and chemical changes is that in chemical changes, new substances are formed. Chemical changes are not often easily reversible. For example, if a mixture of hydrogen gas and oxygen gas is heated strongly, water will be produced. This will be recognised as a chemical change because cooling back the water produced will not regenerate the original hydrogen gas and oxygen gas. Chemical changes can usually be identified by observing one or more of the following changes: a spontaneous change of colour spontaneous formation of a gas spontaneous formation of a solid spontaneous change in temperature. Exercise: Classify the following changes as physical or chemical. 1. Iron rusts. 2. Grape juice turns to wine. 3. Photographic film is exposed to light. 4. Water begins to boil. 5. Grass grows. 6. An infant gains 10 lbs. 7. A broken leg mends itself. 8. Wood burns to ashes. 9. Rock is crushed to powder. 10. Salt dissolves in water. 2.2 Elements and Compounds ELEMENTS Elements are substances that cannot be chemically broken down into other substances. There are now about 110 known elements of which 92 are naturally occurring. The remainder of the elements have been produced artificially. Elements are composed of identical units called atoms (i.e., elements consist of only one type of atom) COMPOUNDS Compounds (molecules) are chemical combination of atoms of different elements. A compound always consists of a chemical association of atoms of different elements. Their physical and chemical properties are different from their constituent elements. Compounds can be broken down through chemical changes into their elements. A compound always has the same chemical combination of atoms. For example, the compound carbon dioxide, one of the so-called greenhouse gases and a product of respiration, consists of one atom of carbon chemically combined with two atoms of oxygen. It is written as CO 2. All samples of carbon dioxide, regardless of their origin, consist of exactly one atom of carbon and two atoms of oxygen. page 21

3 2.3 Mixtures and Pure Substances PURE SUBSTANCES A pure substance always has the same composition. There are two classes of pure substances: elements compounds A sample of a pure substance contains only one type of compound or element. Pure substances are rarely found in nature MIXTURES Mixtures are by far the most common way that chemicals are found. They are a physical combination of different compounds or elements (i.e., physically combining different pure substances). Mixtures may be either homogeneous or heterogeneous. Homogeneous mixtures (referred to as solutions) are uniform in composition throughout. For example, when table salt (sodium chloride) is dissolved in pure water, the resulting salt water has a uniform appearance. No part of this mixture differs from any other part, so salt water is a homogeneous mixture. Heterogeneous mixtures do not have a uniform appearance. Parts of such mixtures appear as physically different. For example, if sand is poured into water, it settles to the bottom. The mixture of sand and water is therefore a heterogeneous mixture. Exercise: Classify the following substances as a pure substance, a homogeneous mixture or a heterogeneous mixture: 1. Wine 2. Air 3. A salt-sand mixture 4. Zinc 5. Oxygen 6. Tap water 7. Steel 8. Wood 9. Blood 10. A page in the textbook 2.4 Separation of Mixtures Mixtures, being a physical combination of different pure substances, can be separated by physical changes. Distillation is one example of a physical separation. If a sample of seawater, which is a mixture of water and dissolved minerals, is heated to boiling, the liquid water evaporates (steam is liquid water), leaving the solid mineral salts behind. If the steam is collected and cooled, it condenses to pure water. The water has been changed from a liquid state to a gaseous state (steam) then back to a liquid state all physical changes. page 22

4 Figure 1: Distillation Apparatus Thermometer Condenser Used to separate a homogeneous mixture of: 1. liquids 2. solid(s) dissolved in a liquid. The principle of this separation is based on a large difference in boiling temperatures of the components of the mixture. Saltwater Heat Source Cooling Water Out Cooling Water In Pure water (Condensate) Figure 2: Separatory Funnel Separatory funnel is used to separate a heterogeneous mixture of liquids. The principle of this separation is based on a large difference in densities of the immiscible components of the mixture. page 23

5 Filtration is another example of physical separation. In this case a heterogeneous mixture of solid and liquid is being separated. Figure 2: Filtration Apparatus Stirring rod Funnel Solid-Liquid mixture Filter Paper Filtrate Figure 3: Schematic Description of the Preparation of Pure Substances Matter Pure Substances Mixtures Elements Chemical Methods of Separation Compounds Physical Methods of Separation Pure Substances page 24