CHAPTER 1: Chemistry, An Introduction science: the study of nature to explain what one observes 1.4 THE SCIENTIFIC METHOD: How Chemists Think Applying the Scientific Method 1. Make an observation, and propose a hypothesis to explain what is observed. 2. Test the validity of the hypothesis by carrying out experiments: controlled observations designed specifically to verify or disprove a hypothesis. Record observations and analyze the data on the system being studied. It s important to keep good records, so others can reproduce the work. 3. Conduct additional experiments to test the hypothesis under various conditions. If all or part of the hypothesis does not hold up to testing, then it is adjusted or a new hypothesis is proposed to explain the observations. If hypothesis holds up to extensive testing, it can lead to the development of a scientific (or natural) law and/or a scientific theory. scientific (or natural) law: a simple statement or equation that summarizes past observations and predicts future ones scientific theory: a tested broader and deeper explanation of observed natural phenomena Thus, a scientific law summarizes what happens; a scientific theory explains why it happens. Example: Many news reports and articles claim that global warming is just a theory. How does this illustrate the media and general public s lack of understanding regarding scientific theories. 1.2 Chemicals Compose Ordinary Things Ex. 1: Are there chemicals in a cup of coffee? Give some examples. CHEM 139: Chapter 1 & 3 page 1 of 8
Thus, chemicals are not necessarily hazardous. In fact, almost everything consists of chemicals since any substance consisting of more than one type of atom is a chemical. For example, the glass of soda above contains water (H 2 O) molecules and carbon dioxide (CO 2 ) molecules; the lead in a pencil is actually graphite which consists of carbon atoms; and we are made of DNA and various other biological molecules or chemicals. Consider the water and carbon dioxide molecules: The atoms making up these molecules and their structure at the atomic, molecular, or microscopic level determine their properties and behavior at the macroscopic level (i.e. what we can observe with the naked eye). 3.1 WHAT IS MATTER? CHAPTER 3: Matter matter: anything that has mass and occupies volume or space CHEM 139: Chapter 1 & 3 page 2 of 8
STATES OF MATTER Matter exists in one of three physical states: solid, liquid, gas gas: Volume is variable, particles are widely spaced, Takes the shape of the container because particles are in constant motion If container volume expands, particles move apart to fill container If container volume decreases, particles move closer together liquid: Fixed (or constant) volume, but shape can change Takes the shape of the container because particles are moving Particles are packed closely together but move around each other solid: Has definite shape, rigid volume Particles can only vibrate in place 3.2 PHYSICAL AND CHEMICAL PROPERTIES AND CHANGES Physical Properties: physical state (solid, liquid, gas) color density melting and boiling points conducts electricity and/or heat dissolves in water or other liquids hardness odor Chemical Properties: how a substance reacts with other substances e.g. hydrogen reacts explosively with oxygen CHEMICAL VERSUS PHYSICAL CHANGES Physical Change: a process that does not alter the chemical makeup of the starting materials e.g. changing shape, changing physical state, dissolving Note: Water remains H 2 O in the images above showing water as a solid, liquid, gas. Other examples of physical changes include hammering gold into foil, dry ice subliming Dissolving table salt or sugar in water is also a physical change. A substance dissolved in water is the fourth physical state, aqueous. Know the terms for transitions from one physical state to another! freezing: liquid solid condensing: gas liquid melting: solid liquid evaporating (or vaporizing): liquid gas CHEM 139: Chapter 1 & 3 page 3 of 8
Two less common transitions: sublimation: solid gas (e.g. dry ice sublimes) deposition: gas solid (e.g. water vapor deposits on an icebox) Chemical Change: a process that changes the chemical makeup of the substances. Starting materials are used up and new substances are formed. e.g. oxidation of matter (burning or rusting) release of gas bubbles (fizzing) release of heat or light We can show H 2 and O 2 reacting to form water (H 2 O) below. Notice that the H 2 O has a different chemical makeup than H 2 and O 2. Other examples of chemical changes: e.g. oxidation of matter (burning or rusting), release of gas bubbles (fizzing), mixing two solutions to form an insoluble solid (precipitation), and other evidence indicating the starting materials (reactants) were changed to a different substance. The following examples are all chemical changes that convert the reactants to completely different compounds and/or elements. release of gas bubbles (fizzing) formation of insoluble solid (precipitation) oxidation (burning or rusting) Chemical changes usually show evidence such as color changes, formation of a new solid or gas substance, or by releasing or absorbing heat. CHEM 139: Chapter 1 & 3 page 4 of 8
3.3 3.5 Classifying Matter According to Its Composition: Elements, Compounds, and Mixtures We can classify matter into pure substances and mixtures: pure substance: a single chemical, consisting of only one kind of matter There are two types of pure substances: elements and compounds. In the figure below, copper rods are an example of an element, and sugar is an example of a compound. mixture: consists of two or more elements and/or compounds Mixtures can be homogeneous or heterogeneous: Homogeneous mixtures have a uniform appearance and composition because the particles in them mix uniformly (e.g. solutions like sweetened tea below) Heterogeneous mixtures do not have a uniform composition. e.g. chocolate chip cookie, water and C 8 H 18 mixture below shown as separate layers CHEM 139: Chapter 1 & 3 page 5 of 8
elements: consist of only one type of atom atoms cannot be broken down into smaller components by chemical reaction e.g. copper wire (Cu), sulfur powder (S 8 ) Examples also include sodium (Na), barium (Ba), hydrogen gas (H 2 ), oxygen gas (O 2 ), and chlorine gas (H 2 ). compounds: consist of more than one type of atom and have a specific chemical formula Examples include hydrogen chloride (HCl), water (H 2 O), sodium chloride (NaCl) which is table salt, barium chloride (BaCl 2 ) Two or more pure substances combine to form mixtures. mixtures: consist of many compounds and/or elements, with no specific formula Matter having variable composition with definite or varying properties can be separated into component elements and/or compounds e.g., any alloy like brass, steel, 10K to 18K gold; sea water, carbonated soda; air consists of nitrogen, oxygen, and other trace gases. Mixtures can be separated by physical changes, such as filtration, evaporation, or distillation. The image at the right shows that air is a mixture of mostly nitrogen (N 2 in blue) and some oxygen (O 2 in red) while salt water consists of salt (Na + and Cl - ions or charged particles) dissolved in water. Example: Is salt water a homogeneous or heterogeneous mixture? Explain. CHEM 139: Chapter 1 & 3 page 6 of 8
Example 1: Consider the following molecular-level representations of different substances: A D B E C F For each figure above, indicate if it represents an element, a compound, or a mixture AND if it represents a solid, liquid, or gas. A: element compound mixture solid liquid gas B: element compound mixture solid liquid gas C: element compound mixture solid liquid gas D: element compound mixture solid liquid gas E: element compound mixture solid liquid gas F: element compound mixture solid liquid gas Ex. 2: Circle all of the following that are chemical changes: burning condensing dissolving rusting vaporizing precipitating CHEM 139: Chapter 1 & 3 page 7 of 8
Ex. 3: Classify the following as pure substances or mixtures: A homogeneous liquid whose temperature stays constant while boiling. Granite a rock with several visible minerals in it. A red solid that turns blue when heated and releases water that is always 30% of the solid s mass. A gas that when cooled and compressed, a liquid condenses out but some gas remains. Pure substance Pure substance Pure substance Pure substance Mixture Mixture Mixture Mixture Ex. 3: Classify the following as pure elements or compounds: Chlorine, Cl 2 Element Compound Table sugar, C 12 H 22 O 11 Element Compound A red solid that turns blue when heated and releases water that is always 30% of the solid s mass. A brown-red liquid that, when energy is applied to it in any form, causes only physical changes in the material, not chemical. Element Element Compound Compound SCIENTIFIC JUDGEMENT There are times when a scientist confronts evidence that is not clear-cut, and a judgement call must be made. Specifically, in Chapter 2 we will see that a scientist must make a best guess to interpolate (make the best guess based on the marked scale) between two markings on a measuring device. Consider the questions below and discuss what evidence you would look for to decide when making the following determinations. 1. Consider the image on Page 4 that shows fizzing. Compare that fizzing to the bubbles one would see in a beaker of boiling water (or boiling a kettle for tea). Both of these exhibit bubbles rising in the liquid. Are these physical or chemical changes? How do you know? (Consider the context, and other chemical evidence you could consider.) CHEM 139: Chapter 1 & 3 page 8 of 8