Chemistry 11. Unit 3 The Physical Properties and Physical Changes of Substances

Transcription

1 Chemistry 11 1 Unit 3 The Physical Properties and Physical Changes of Substances

2 2 1. Definitions in science Science is the observation, identification, description, experimental investigation, and theoretical explanation of natural phenomena. We gather information about our surrounding through the use of our senses: sight, smell, hearing, taste, and touch. Information usually takes one of two forms: QUALITATIVE information is NON-NUMERICAL information (Descriptions) QUANTITATIVE information is NUMERICAL information (Measurements)

3 3 Common terms used in chemistry An OBSERVATION is qualitative information collected though the direct use of our senses. An INTERPRETATION (or inference ) is an attempt to put meaning into an observation. A DESCRIPTION is a list of the properties of something. DATA is quantitative information which is experimentally determined or obtained from references. An EXPERIMENT is a test or a procedure that is carried out in order to discover a result.

4 4 Examples of biased interpretation Which cylinder is largest? Young lady or old woman?

5 5 Hypothesis versus theories versus laws A HYPOTHESIS is a SINGLE, UNPROVEN assumption or idea which attempts to explain why nature behaves in a specific manner. When put forward, hypotheses are tentative but, if they survive testing, eventually gain general acceptance. A THEORY is a set of hypothesis that ties together a large number of observations of the real world into a logically consistent and understandable pattern. In other words, a theory is a TESTED, REFINED, and EXPANDED explanation of why nature behaves in a given way. A LAW is a broad generalization or summary statement which describes a large amount of experimental evidence stating how nature behaves when a particular situation occurs.

6 6 Hypothesis The following are general characteristics of HYPOTHESES: Hypotheses are normally single assumptions. Hypotheses are narrow in their scope of explanation. When originally proposed, hypotheses are tentative (being based on very incomplete evidence) but may become generally accepted after more complete testing. Hypothesis is the connection between experimental observations and theory in scientific methods: Observations Hypotheses Theories

7 7 Theories The following are general characteristics of THEORIES: Theories are composed of one or more underlying hypotheses. Theories are broad in scope and may have subtle implications which are not foreseen when they are proposed because they provide explanations for entire fields of related behaviour. Theories are sometimes called models because they often provide a concrete way to examine, predict, and test the workings of nature. Theories must be falsifiable ; that is, they must make testable predictions about the behaviour of the system under NEW conditions.

8 8 Laws The following are general characteristics of LAWS: Laws summarize the results of many experiments or observations and state what will happen when a specific situation occurs. Laws do not try to explain why something occurs. Laws are not proven theories. Laws are often stated before any theory exists to explain why the law is true. Laws do not posit a mechanism or explanation of phenomena. As such, they are limited in applicability to circumstances resembling those already observed.

9 9 Example: What are the following statements about gas? All gases are made up of tiny, fast moving particles. The tiny particles of a gas transfer some of their energy when they collide with other particles or with the container. Gases behave the way they do because they are made up of point-like particles which are constantly moving, colliding, and exchanging energy. If the applied pressure is unchanged, the greater the temperature of a sample of gas, the greater its volume. If the temperature is unchanged, the greater the pressure applied to a sample of gas, the smaller its volume.

10 10 Example: What are the following statements about gas? All gases are made up of tiny, fast moving particles. (Hypothesis) The tiny particles of a gas transfer some of their energy when they collide with other particles or with the container. (Hypothesis) Gases behave the way they do because they are made up of point-like particles which are constantly moving, colliding, and exchanging energy. (Theory) If the applied pressure is unchanged, the greater the temperature of a sample of gas, the greater its volume. (Charles law) If the temperature is unchanged, the greater the pressure applied to a sample of gas, the smaller its volume. (Boyle s law)

11 11 2. Physical properties of matters Chemistry is the study of matter and its interactions. But what is matter? MATTER is defined as anything that has mass and occupies space. (Matter is what makes up everything other than energy.) Hence, chemistry may be better described as the science concerned with the properties, composition, and behaviour of matter.

12 12 Physical properties of substances SUBSTANCES are things that have a unique and identifiable set of properties. A PHYSICAL PROPERTY of a substance is a property that can be found without creating a new substance. Some physical properties: Color Size Odor Density Luster Hardness Melting point Boiling point Conductivity

13 13 Example: Physical properties of silver Color Luster Ductility Malleability Conductivity Solubility Hardness Density White Lustrous shine and is capable of a high degree of polish It can be beaten into extremely thin sheets Capable of being shaped or bent Excellent electrical and heat conductor. It has the highest electrical and thermal conductivity of any metal Solubility (ability to be dissolved) A relatively soft metal It is a dense metal

14 14 Physical properties can be either intensive or extensive. INTENSIVE PROPERTY: a physical property that does not depend on how much of the substance is present. EXTENSIVE PROPERTY: a physical property that does depend on the amount of substance present.

15 15 Example: Sulfur

16 16 Three phases of substances Matter can exist in three common states or PHASES : solid, liquid, and gas. Each of the three common phases of matter has a unique set of properties which allow a given substance to be classified.

17 17 (1) Solids Solids are rigid and DO NOT readily change their shape. Solids experience very small changes in volume (expand) when heated or subjected to pressure. In a solid, the particles are packed closely together into a given volume. The particles are highly organized and rigid. In general solids are NOT compressible.

18 18 (2) Liquids Liquids conform to the shape of their container and experience only slight changes in volume when heated or subjected to pressure. In a liquid, the particles remain in close contact with each other but have enough room to slide past one another easily and prevent an organized packing. Because the particles of a liquid are in close contact, they are also NOT compressible.

19 19 (3) Gases Gases conform to the shape of their container but unlike solids or liquids they occupy the entire volume of the container. Gases experience drastic changes in volume when heated or subjected to pressure. In a gas, the particles are widely separated and only contact each other during collisions. Most of the volume of a gas is EMPTY SPACE. Because the large separation of particles can be decreased, gases are compressible.

20 20 3. Classification of matter Substances can be put into any of the following categories: ELEMENT is a substance which cannot be separated into simpler substances as a result of any chemical process. ATOM is the smallest possible unit of an element which retains the properties of the element. MOLECULE is a group of two or more atoms joined together by chemical bonds. ION is an atom or molecule which possesses an electrical charge (positive or negative). PARTICLE is the general term used to describe a small bit of matter such as an atom, molecule, or ion.

21 21 Flowchart of classification

22 22 Pure substance versus mixture Depending on the composition, a substance can be classified as a pure substance or a mixture. A PURE SUBSTANCE is a substance that has an unchangeable composition. A MIXTURE is a system made up of two or more substances with variable composition.

23 23 Elements ELEMENTS are substances that cannot be separated into simpler substances by chemical processes. A chemical element is a species of atoms having the same number of protons in their atomic nuclei.

24 24

25 25 Compounds COMPOUNDS are substances composed of two or more types of atoms, in a fixed proportion, which associate through chemical bonds. All compounds are molecules but not all molecules are compounds.

26 26 Compounds Compounds have many possible sizes, ranging from very small to extremely large! Carbon monoxide Fructose Human fibronectin

27 27 Homogeneous versus heterogeneous mixtures There are two types of mixtures depending on their appearance. A HOMOGENEOUS substance is a substance consisting of only one phase. A HETEROGENEOUS substance is a substance consisting of more than one phase.

28 28 Solutions A SOLUTION is a homogenous mixture of two or more substances. That solutions are in liquid form is a common mistake. Solutions indeed can be in solid, liquid or gas state. Solvent: the major component in quantity; solute: minor component in quantity. Gas-in-gas Gas-in-liquid Liquid-in-liquid Solid-in-liquid Solid-in-solid

29 29 4. Separation methods The purpose of separation is to differentiate different components in a mixture. Separation techniques are based on the difference in the physical properties of components. The more similar the properties of components, the harder they can be separated. Depending on the nature of the mixtures (either solid in solid, solid in liquid or liquid in liquid), many practical separation strategies have been developed.

30 30 (1) Filtration (For undissolved solid in liquid) Pass the mixture through a porous filter paper. Liquid (called filtrate) will be collected in a beaker, while solid (called residues) will remain on the filter paper.

31 31 (2) Gravity separation (for solid in solid or solid in liquid) This method is based on difference in density. Sample is spun by a centrifuge, and the more dense materials sink to the bottom. Works well for small volume of sample

32 32 (3) Crystallization (for dissolved solid in liquid) The solution containing the dissolved solid is heated to evaporate most of the solvent. The remaining hot and concentrated solution is allowed to cool down slowly. The solid will appear as pure crystals. Hot saturated sugar solution The same solution after a week

33 33 (4) Solvent extraction (for all cases) This method relies on the preferential solubility of one substance over the other to the added solvent which is immiscible to the original solution. By separating the added solvent and the original solution, the substances in the mixture can be partitioned. Usually used for liquid-liquid extraction. Youtube video clip demonstrating the use of separation funnel:

34 34 (5) Distillation (for solid in liquid or liquid in liquid) When a mixture is heated, the component with lower boiling point will evaporate first; the other components will be left behind. Usually the one that gets distilled first (called distillate) is the species wanted.

35 35 (6) Chromatography (for solid in liquid) The basic idea is to flow a mixture through a medium which is more adhesive to one component than the others. The less retained one will flow faster than the more retained one. There are a variety of chromatography depending on the medium used. In Chemistry 11, we will learn (i) paper chromatography; (ii) thin-layer chromatography; (iii) column chromatography. All types of chromatography consist of a stationary phase (solid or liquid) and a mobile phase (liquid or gas).

36 36 (i) Paper chromatography Stationary phase: a thin absorbent paper Mobile phase: a liquid solvent or a mixture of solvent Different components appear as separate spots spread out on the paper after drying or developing

37 37 (ii) Thin-layer chromatography (TLC) Similar to paper chromatography except that the stationary phase is silica gel or aluminum oxide coated on a piece of glass, plastic or aluminum foil. Analysis: determine the retention factor (R f ) by finding the ratio of the distance of the spot to the distance of the solvent front.

38 38 (iii) Column chromatography Stationary phase is packed in a glass column, and the mobile phase flows slowly through the packed column. Components will be eluted out of the column at different time. They can be collected separately for further analysis.

39 39 5. Phase change Substance can undergo two kinds of changes: chemical change or physical change. Chemical change: a change in which a new substance is formed. Physical change: a change in the phase of the substance yet no new substance is formed. Usually chemical changes are accompanied by physical changes. For example, the reaction of hydrogen gas and oxygen gas produces liquid water.

40 40 A physical change can be achieved by changing temperature. Continual heating of a solid produces the following temperature behavior.

41 41 Some terms pertaining to phase changes: (1) Melting temperature: The temperature at which solid changes to liquid (2) Boiling temperature: The temperature at which liquid changes to gas (3) Freezing temperature: The temperature at which liquid changes to solid (4) Condensation temperature: The temperature at which gas changes to liquid (5) Sublimation: The process in which solid turns to gas directly without passing through the liquid phase (6) Deposition: The process in which gas turns to solid directly without passing through the liquid phase

42 42 Energy flow during phase changes: When solid is heated, heat is converted mainly to the rotational and vibrational energy, causing the molecules to oscillate and collide to overcome the attraction, and the solid melts gradually. When liquid is heated, heat is converted mainly to the translational energy, making the molecules to move faster, eventually pulling away from one another to form a gas. During melting or boiling, heat is used to change phase so that temperature does not change!