Lead(II) chloride is insoluble in water. It can be prepared by mixing dilute hydrochloric acid with aqueous lead(II) nitrate. Which step is involved in the preparation of lead(II) chloride?

collect the precipitate of lead(II) chloride by filtration.

add dilute HCl until no more gas is evolved.

evaporate the filtrate until crystals of lead(II) chloride appear.

titrate HCl against aqueous lead(II) nitrate.

Which two reagents could be used to prepare the insoluble salt copper(II) carbonate?

copper(II) sulfate (aq) + sodium carbonate (aq)

copper(II) oxide (s) + magnesium carbonate (s)

copper(II) oxide (s) + sodium carbonate (aq)

copper(II) sulfate (aq) + magnesium carbonate (s)

Which pair of substances produces a precipitate when their aqueous solutions are mixed?

sodium sulfate and barium chloride

sodium nitrate and silver nitrate

sodium chloride and barium nitrate

sodium nitrate and barium chloride

Preparation of salts

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.

Growing up in Sioux Falls, South Dakota, a small city surrounded by endless plains, I've found unexpected echoes of home in China's smaller towns — from the warmth of locals in Huaihua, Central China's Hunan province, to the quiet charm of Yangshuo, South China's Guangxi Zhuang autonomous region. With an itch to see more of China's lesser-visited regions, I began planning a trip to the northwest with seven friends — five Americans, one Pakistani, one Zimbabwean, and one Colombian. We bought round-trip tickets from Shanghai to Yinchuan, Ningxia Hui autonomous region, for less than $120 each. From there, we planned to rent a car and drive to Xining in Qinghai, then on to Qinghai Lake, and finally to Lanzhou, Gansu. To make that possible, several of us applied for Chinese driver's licenses, a process that involved translating our US licenses into Mandarin and passing a short test on traffic laws. Within a day, we were licensed. As we piled into two rental cars in late March to begin our eight-day journey, it became clear that this wasn't just a road trip — it was the culmination of our four years in China, the Mandarin we had so diligently studied, and our ongoing effort to contribute to US-China people-to-people relations. Right away, we drew curious reactions. At the Yinchuan airport, taxi drivers offered us rides into the city, only to stare in astonishment when we told them we had rented cars. "You're driving? In China?" one driver asked, visibly surprised. It was a reaction we'd encountered multiple times during our trip, as foreign drivers are rare in China, especially in remote regions. In Yinchuan, we stocked up on snacks and adjusted to the chilly desert air. From there, we headed west, navigating wide highways framed by dramatic landscapes: arid plains, jagged mountains, and occasionally, a herd of sheep crossing the road. The vastness of the Northwest was humbling — and as someone who grew up on the wide-open prairies of South Dakota, it felt oddly familiar. One of the highlights of our trip was camping by Qinghai Lake, the largest saltwater lake in China. A few summers ago, Santiago Solano, one of my classmates from the US, cycled from Xi'an in Shaanxi to Urumqi in the Xinjiang Uygur autonomous region over the course of a month and met many kind strangers along the way. One of them was Geng San, a Tibetan lamb herder who managed a piece of land right next to Qinghai Lake and graciously invited us to camp there. "That's what China is — it's the people. The quiet generosity of an old Tibetan nomad who, years after we first met, still offered us a place to rest on his land," said Solano, who is also part of the group on this trip. But apparently, we underestimated just how cold it would be to camp next to Qinghai Lake in late March. It was deathly freezing. In preparation for the trip, we had ordered two tent kits and eight sleeping bags. However, when the temperature eventually dropped to — 10 C, all of us piled into the cars and turned the heaters on. So much for camping. From Qinghai Lake, we drove to Lanzhou, where we visited many food markets and tried every type of noodle on offer. Since we are college students, we rented a gaming hotel room — something I've only ever seen in China. At night, instead of attending local parties as we had before, we stayed in the hotel and gamed late into the morning. For me, the trip was as much about the journey as it was about the destinations. Driving through Northwest China gave us a unique perspective on the region's natural beauty and its people. At gas stations, shopkeepers greeted us with curiosity and kindness, often offering recommendations for nearby attractions. At roadside carts, we sampled local specialties, grabbing a quick skewer and a mango for the road. And at every stop, we were touched by the warmth and hospitality that make traveling in China so rewarding. As an American who has lived in China for several years, I'm often asked about my experiences here. Trips like this one remind me of the similarities between the two countries, despite their differences. Just as road trips are a quintessential part of American culture, they've become my favorite way to explore China. Whether it's driving through the rolling hills of South Dakota or the deserts of Ningxia, there's something universal about the freedom and camaraderie that come with having complete control over where you end up. Written by Charlie Howes, a 22-year-old American who has lived in China since 2019. He completed his final year of high school at Beijing No 80 High School and is currently studying at New York University Shanghai. He has founded a company in China focused on facilitating US-China trade and plans to continue living in Shanghai long term. He enjoys road trips, cycling around the world, learning languages, and meeting new people.

Make a vocabulary quiz with below.Crisis: a very difficult situation or emergency; “Investors are worried that continuing problems in the stock market might lead to another financial crisis.” Short-staffed: not having enough employees or workers; “We laid off dozens of employees to save money, but now we’re short-staffed and can’t fill orders quickly.” To drive someone nuts: to make someone upset or annoyed; “The autocorrect feature on my mobile phone really drives me nuts.” To arise: to happen or start to develop, especially for problems or situations; “Problems arose for the developers when local residents voted against construction of the new shopping mall.” To moan: to complain about something, especially in a way that bothers other people; “Every Monday morning my colleagues moan about having to come to the office so early.” Optimism: the feeling or belief that good things will happen; “Analysts are expressing optimism that the real estate market will bounce back quickly.” Pessimism: the tendancy or belief that the worst will happen. “Now is not the time for pessimism, we need to believe we can do this or we will fail.” Hurdle: a problem that must be overcome or dealt with in order to reach a goal or do something; “Lack of capital is a major hurdle for many young entrepreneurs with good business ideas.” © 2013 All rights reserved: www.businessenglishpod.com 2 To give up: to stop trying to do or achieve something; “After being rejected by over a dozen universities, Roger gave up on his plans to study for an MBA.” To keep your eye on the prize: to stay motivated and focused on your final goal or reward; “Okay folks, I know things have been really busy, but let’s keep our eye on the prize and we’ll get this done before Christmas.” Down the road: in the future; “Sales our good in our retail outlets, but 10 years down the road we’re going to have to be more competitive in the online market.” To overcome: to deal with a problem successfully; “You are going to have to overcome your lack of confidence if you want to be a good salesman.” Decisiveness: having the ability to make decisions and act firmly. “Decisiveness is a quality we look for in new managers.” Bumpy: we can use “bumpy” to describe a road or process that has many problems; “We had a pretty bumpy start-up phase, but we finally managed to turn a profit in our fifth year of operations.” To display: to show a feeling, emotion, or characteristic; “David displayed tremendous preparation and diplomacy during the negotiations.” On the fence: someone who is “on the fence” is having difficulty making a decision or taking a position; “While the executives support expansion wholeheartedly, the shareholders are still on the fence about it.” All-in: in poker, if you are “all-in” you put all your money on one bet, but we can use this expression more generally to say you are completely committed to something, especially a plan; “This sounds like a good idea, but before I go all-in I need to know more about the project partners.” To have what it takes: to have the ability or intelligence to do something; “I thought I’d enjoy law, but after a few years as a junior lawyer I realized I don’t really have what it takes.

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