The respiratory system’s job is to do the work of breathing for the body. Breathing involves inhaling, which causes oxygen to enter the lungs. Inside the lungs, the oxygen moves into very tiny air sacs called alveoli. In the alveoli, the oxygen centers the bloodstream and carbon dioxide is removed from the blood in a process called exchange of gases. The last step of breathng is exhaling, which pushes the air in the lungs containing carbon dioxide back outside the body.

Breathing is an important life function.

Emma likes to climb trees. But last night she fell from one and broke her arm. Emma’s mom took her to the hospital. They put a cast on Emma’s arm and told her to come back in six weeks to have it removed. It didn’t’ stop Emma, though. The next week, she climbed to the top of the oak tree in her front yard.

Nothing will stop Emma from climbing trees.

She had on a cast for six weeks.

Walt Disney was a famous film-maker. The first cartoons that he made were short silent films. Disney was a pioneer in the use of color and sound in cartoons. Even though the Walt Disney films are old, they are still popular.

Walt Disney was a famous cartoonist.

My mom is taking us to the skating rink today. First we’ll pick up my friend, Kim. Then we’ll go to the rink. It opens at 10 o’clock. Kim has her own ice skates, but I’ll have to rent mine. I know we’ll have lots of fun.

Kim and I will have fun ice skating today.

Write a Summary Practice

What is the page mainly about? (Answer: C — Homes meet a basic need) A) House colors B) Old buildings C) Homes meet a basic need ✓ D) Travel What basic need do homes give? (Answer: B — Shelter) A) Food B) Shelter ✓ C) Clothes D) Money What does “basic need” mean here? (Answer: C — Something people must have to live) A) Something nice to have B) A school rule C) Something people must have to live ✓ D) A weekend plan Homes keep people ______ and ______. What are the two words from the page? (Answer: B — dry / safe) A) rich / tall B) dry / safe ✓ C) loud / fast D) clean / funny What else are homes for, according to the text? (Answer: B — Eating, sleeping, and being with family and friends) A) Shopping B) Eating, sleeping, and being with family and friends ✓ C) Driving D) Fighting What does the heading “Meeting our needs” tell you? (Answer: B — explains how homes fit the local climate) A) A joke is coming B) This part explains how homes fit the local climate ✓ C) It is a story D) It lists prices What does “climate” mean on this page? (Answer: C — the usual weather of a place) A) Yesterday’s forecast B) A big storm C) The usual weather of a place ✓ D) Room temperature Which roof is best for cold, snowy places? (Answer: D — Slanted) A) Flat B) Dome C) Glass D) Slanted ✓ Why is a slanted roof helpful in snowy places? (Answer: B — snow slides off more easily) A) It is cheaper B) Snow slides off more easily ✓ C) Birds like it D) It is quieter Why do many houses in hot places have many windows? (Answer: B — to let air move through and keep people cool) A) To block all light B) To let air move through and keep people cool ✓ C) To make walls heavy D) To reduce street noise Which detail best supports “climate changes home design”? (Answer: C — Hot → many windows; Snowy → slanted roofs) A) People like blue walls B) Kitchens are big C) Hot → many windows; Snowy → slanted roofs ✓ D) Cities are crowded What does the caption about a traditional Japanese house show? (Answer: C — People sit on mats on the floor to eat) A) People eat outdoors B) Families don’t eat together C) People sit on mats on the floor to eat ✓ D) People stand to eat Which sentence is LEAST connected to the main idea of the page? (Answer: D — Blue walls are relaxing) A) Homes protect people from weather B) Roofs can change with climate C) Windows help rooms stay cool D) Blue walls are relaxing ✓ Which text structure organizes the right paragraph? (Answer: C — Cause–effect) A) Timeline B) Problem–solution C) Cause–effect ✓ D) Description only What is the author’s purpose? (Answer: B — to explain how homes meet a human need) A) To sell houses B) To explain how homes meet a human need ✓ C) To tell a funny story D) To give building laws What can you guess about a flat roof in a snowy place? (Answer: B — snow can pile up and be unsafe) A) Best choice B) Snow can pile up and be unsafe ✓ C) Always cheaper D) Warmer in summer Which page feature helps you find ideas quickly? (Answer: C — Headings and photo captions) A) Rhyme B) Dialogue C) Headings and photo captions ✓ D) Footnotes Which sentence is the best summary of the page? (Answer: C — Homes give shelter; designs change with climate) A) Houses are beautiful in winter B) People prefer bright colors C) Homes give shelter; designs change with climate (slanted roofs, many windows) ✓ D) Windows are the most important part True/False or Short Answer (5) True/False: All homes have the same purpose, even if they look different. (Answer: True) True/False: In hot places, houses usually have fewer windows to keep heat in. (Answer: False — hot places → many windows for airflow/cooling) Short Answer (1–3 words): Homes provide shelter to keep people _____ and _____. (Answer: dry; safe) Short Answer (one example): Write one climate → design pair from the page. (Answer: cold/snowy → slanted roof OR hot → many windows) True/False: The photo shows people in Japan eating on mats on the floor. (Answer: True)

Covalent Molecules and Compounds Just as an atom is the simplest unit that has the fundamental chemical properties of an element, a molecule is the simplest unit that has the fundamental chemical properties of a covalent compound. Some pure elements exist as covalent molecules. Hydrogen, nitrogen, oxygen, and the halogens occur naturally as the diatomic (“two atoms”) molecules H2, N2, O2, F2, Cl2, Br2, and I2 (part (a) in Figure 3.1.1). Similarly, a few pure elements exist as polyatomic (“many atoms”) molecules, such as elemental phosphorus and sulfur, which occur as P4 and S8 (part (b) in Figure 3.1.1). Each covalent compound is represented by a molecular formula, which gives the atomic symbol for each component element, in a prescribed order, accompanied by a subscript indicating the number of atoms of that element in the molecule. The subscript is written only if the number of atoms is greater than 1. For example, water, with two hydrogen atoms and one oxygen atom per molecule, is written as H2O. Similarly, carbon dioxide, which contains one carbon atom and two oxygen atoms in each molecule, is written as CO2. Covalent compounds that predominantly contain carbon and hydrogen are called organic compounds. The convention for representing the formulas of organic compounds is to write carbon first, followed by hydrogen and then any other elements in alphabetical order (e.g., CH4O is methyl alcohol, a fuel). Compounds that consist primarily of elements other than carbon and hydrogen are called inorganic compounds; they include both covalent and ionic compounds. In inorganic compounds, the component elements are listed beginning with the one farthest to the left in the periodic table, as in CO2 or SF6. Those in the same group are listed beginning with the lower element and working up, as in ClF. By convention, however, when an inorganic compound contains both hydrogen and an element from groups 13–15, hydrogen is usually listed last in the formula. Examples are ammonia (NH3) and silane (SiH4). Compounds such as water, whose compositions were established long before this convention was adopted, are always written with hydrogen first: Water is always written as H2O, not OH2. The conventions for inorganic acids, such as hydrochloric acid (HCl) and sulfuric acid (H2SO4), are described elswhere. Note! For organic compounds: write C first, then H, and then the other elements in alphabetical order. For molecular inorganic compounds: start with the element at far left in the periodic table; list elements in same group beginning with the lower element and working up. Write the molecular formula of each compound. a. The phosphorus-sulfur compound that is responsible for the ignition of so-called strike anywhere matches has 4 phosphorus atoms and 3 sulfur atoms per molecule. b. Ethyl alcohol, the alcohol of alcoholic beverages, has 1 oxygen atom, 2 carbon atoms, and 6 hydrogen atoms per molecule. c. Freon-11, once widely used in automobile air conditioners and implicated in damage to the ozone layer, has 1 carbon atom, 3 chlorine atoms, and 1 fluorine atom per molecule. Solution: a. • A The molecule has 4 phosphorus atoms and 3 sulfur atoms. Because the compound does not contain mostly carbon and hydrogen, it is inorganic. • B Phosphorus is in group 15, and sulfur is in group 16. Because phosphorus is to the left of sulfur, it is written first. • C Writing the number of each kind of atom as a right-hand subscript gives P4S3 as the molecular formula. b. • A Ethyl alcohol contains predominantly carbon and hydrogen, so it is an organic compound. • B The formula for an organic compound is written with the number of carbon atoms first, the number of hydrogen atoms next, and the other atoms in alphabetical order: CHO. • C Adding subscripts gives the molecular formula C2H6O. c. • A Freon-11 contains carbon, chlorine, and fluorine. It can be viewed as either an inorganic compound or an organic compound (in which fluorine has replaced hydrogen). The formula for Freon-11 can therefore be written using either of the two conventions. • B According to the convention for inorganic compounds, carbon is written first because it is farther left in the periodic table. Fluorine and chlorine are in the same group, so they are listed beginning with the lower element and working up: CClF. Adding subscripts gives the molecular formula CCl3F. • C We obtain the same formula for Freon-11 using the convention for organic compounds. The number of carbon atoms is written first, followed by the number of hydrogen atoms (zero) and then the other elements in alphabetical order, also giving CCl3F. Write the molecular formula for each compound. a. Nitrous oxide, also called “laughing gas,” has 2 nitrogen atoms and 1 oxygen atom per molecule. Nitrous oxide is used as a mild anesthetic for minor surgery and as the propellant in cans of whipped cream. b. Sucrose, also known as cane sugar, has 12 carbon atoms, 11 oxygen atoms, and 22 hydrogen atoms. c. Sulfur hexafluoride, a gas used to pressurize “unpressurized” tennis balls and as a coolant in nuclear reactors, has 6 fluorine atoms and 1 sulfur atom per molecule. Answer: a. N2O b. C12H22O11 c. SF6. Ionic Compounds The substances described in the preceding discussion are composed of molecules that are electrically neutral; that is, the number of positively-charged protons in the nucleus is equal to the number of negatively-charged electrons. In contrast, ions are atoms or assemblies of atoms that have a net electrical charge. Ions that contain fewer electrons than protons have a net positive charge and are called cations. Conversely, ions that contain more electrons than protons have a net negative charge and are called anions. Ionic compounds contain both cations and anions in a ratio that results in no net electrical charge. Note! Ionic compounds contain both cations and anions in a ratio that results in zero electrical charge.An ionic compound that contains only two elements, one present as a cation and one as an anion, is called a binary ionic compound. One example is MgCl2, a coagulant used in the preparation of tofu from soybeans. For binary ionic compounds, the subscripts in the empirical formula can also be obtained by crossing charges: use the absolute value of the charge on one ion as the subscript for the other ion. This method is shown schematically as follows: Crossing charges. One method for obtaining subscripts in the empirical formula is by crossing charges. When crossing charges, it is sometimes necessary to reduce the subscripts to their simplest ratio to write the empirical formula. Consider, for example, the compound formed by Mg2+ and O2−. Using the absolute values of the charges on the ions as subscripts gives the formula Mg2O2:Polyatomic Ions Polyatomic ions are groups of atoms that bear net electrical charges, although the atoms in a polyatomic ion are held together by the same covalent bonds that hold atoms together in molecules. Just as there are many more kinds of molecules than simple elements, there are many more kinds of polyatomic ions than monatomic ions. Two examples of polyatomic cations are the ammonium (NH4+) and the methylammonium (CH3NH3+) ions. P. The method used to predict the empirical formulas for ionic compounds that contain monatomic ions can also be used for compounds that contain polyatomic ions. The overall charge on the cations must balance the overall charge on the anions in the formula unit. Thus, K+ and NO3− ions combine in a 1:1 ratio to form KNO3 (potassium nitrate or saltpeter), a major ingredient in black gunpowder. Similarly, Ca2+ and SO42− form CaSO4 (calcium sulfate), which combines with varying amounts of water to form gypsum and plaster of Paris. The polyatomic ions NH4+ and NO3− form NH4NO3 (ammonium nitrate), a widely used fertilizer and, in the wrong hands, an explosive. One example of a compound in which the ions have charges of different magnitudes is calcium phosphate, which is composed of Ca2+ and PO43− ions; it is a major component of bones. The compound is electrically neutral because the ions combine in a ratio of three Ca2+ ions [3(+2) = +6] for every two ions [2(−3) = −6], giving an empirical formula of Ca3(PO4)2; the parentheses around PO4 in the empirical formula indicate that it is a polyatomic ion. Writing the formula for calcium phosphate as Ca3P2O8 gives the correct number of each atom in the formula unit, but it obscures the fact that the compound contains readily identifiable PO43− ions.Summary • There are two fundamentally different kinds of chemical bonds (covalent and ionic) that cause substances to have very different properties. • The composition of a compound is represented by an empirical or molecular formula, each consisting of at least one formula unit.Contributors The atoms in chemical compounds are held together by attractive electrostatic interactions known as chemical bonds. Ionic compounds contain positively and negatively charged ions in a ratio that results in an overall charge of zero. The ions are held together in a regular spatial arrangement by electrostatic forces. Most covalent compounds consist of molecules, groups of atoms in which one or more pairs of electrons are shared by at least two atoms to form a covalent bond. The atoms in molecules are held together by the electrostatic attraction between the positively charged nuclei of the bonded atoms and the negatively charged electrons shared by the nuclei. The molecular formula of a covalent compound gives the types and numbers of atoms present. Compounds that contain predominantly carbon and hydrogen are called organic compounds, whereas compounds that consist primarily of elements other than carbon and hydrogen are inorganic compounds. Diatomic molecules contain two atoms, and polyatomic molecules contain more than two. A structural formula indicates the composition and approximate structure and shape of a molecule. Single bonds, double bonds, and triple bonds are covalent bonds in which one, two, and three pairs of electrons, respectively, are shared between two bonded atoms. Atoms or groups of atoms that possess a net electrical charge are called ions; they can have either a positive charge (cations) or a negative charge (anions). Ions can consist of one atom (monatomic ions) or several (polyatomic ions). The charges on monatomic ions of most main group elements can be predicted from the location of the element in the periodic table. Ionic compounds usually form hard crystalline solids with high melting points. Covalent molecular compounds, in contrast, consist of discrete molecules held together by weak intermolecular forces and can be gases, liquids, or solids at room temperature and pressure. An empirical formula gives the relative numbers of atoms of the elements in a compound, reduced to the lowest whole numbers. The formula unit is the absolute grouping represented by the empirical formula of a compound, either ionic or covalent. Empirical formulas are particularly useful for describing the composition of ionic compounds, which do not contain readily identifiable molecules. Some ionic compounds occur as hydrates, which contain specific ratios of loosely bound water molecules called waters of hydration.

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