7 July 2020- What is Chemistry?- Part 1

INTRODUCTION 

The science of chemistry is the study of matter and the chemical changes that matter undergoes. Research in chemistry not only answers basic questions about nature but also affects people’s lives. Chemistry has been used, for example, to make stronger metals, to enrich soil for growing crops, to destroy harmful bacteria, and to measure levels of pollution in the environment. It has also made possible the development of such new substances as plastics, fibers, and new medicines.

Knowledge of chemistry is important in such fields as biology, medicine, agriculture, archaeology, and geology. Chemists themselves are employed by educational and research institutions, government, and industry.

The function of the chemical industry is to create new chemicals and to supply the products of chemical research to the world. Using chemical reactions, the industry converts raw materials such as water, salt, metals and minerals, petroleum, coal, natural gas, plant cellulose and starch, and atmospheric gases into other products. Some of these products are used by manufacturers. They include the chemicals needed to make metal and paper products; wallboard, pipe, insulation, and other construction materials; polymers for plastic toys, bottles, films, paints, and other items; and fibers for carpets and fabrics. Other chemical products that are used by consumers more directly include medicines, dyes, paints, fertilizers, shampoos, detergents and waxes, perfumes, cosmetics, and flavourings.

CHEMISTRY AIDS IN UNDERSTANDING THE WORLD

The work of chemistry is generally described as analysis and synthesis. Chemists analyze substances by taking them apart to find out what they are made of. Chemists synthesize substances by putting them together in different, possibly more useful combinations. Assisted by specialized instruments and computers, chemists study materials as small as single atoms and as large and complex as DNA (deoxyribonucleic acid), which contains millions of atoms.

The chemist wants to understand how the universe is put together and how the substances in it can be changed to the advantage of humans. Chemists and physicists have made great advances in these efforts. They have discovered that all things in the universe can be classified as matter or energy and that matter and energy can be converted from one to the other.

Matter itself can be classified according to its physical state: solid, liquid, gas, or plasma. A study of the physical states has led to the conclusion that their characteristics can be explained by assuming that matter consists of particles in motion. This moving-particle theory, called the kinetic-molecular theory of matter, explains many common phenomena such as the evaporation of liquids and the diffusion of gases. Matter can also be grouped according to the nature of its composition: element, compound, or mixture.

ATOMIC THEORY

The study of matter has helped chemists discover that the whole universe is made up of chemical building blocks called elements. More than 90 elements are known to exist in nature, and scientists have made about 20 additional elements artificially. The science of chemistry involves a study of these elements and of the compounds that are formed when different elements combine.

Chemists also now know that elements are made of atoms. Some early Greek philosophers had suggested that matter was made of atoms, but not until the beginning of the 19th century did physicists and chemists begin to collect the evidence needed to prove the theory and to understand the nature of atoms.

The atomic theory can be summarized as follows: (1) Ordinary matter is made of small particles called atoms. (2) Atoms of the same elements have the same average masses, and atoms of different elements have different average masses. (3) Chemical reactions take place between atoms or groups of atoms.

CHEMICAL CHANGE

When gasoline is burned, bread is eaten, or plastics are produced from petroleum, chemical changes occur. When a substance undergoes a chemical change, mass is conserved—that is, the amount of mass in the substance is the same before and after the chemical change takes place. The atoms keep their original identity and their number stays the same, but they are rearranged into new combinations. However, the chemical properties of the starting materials vanish, and the properties of the new materials appear. The process by which substances are changed into other substances is called a chemical reaction.

In addition to chemical change, matter can also undergo two other kinds of changes—physical and nuclear. The boiling of water, the melting of ice, and the dissolving of table sugar in tea are all examples of physical changes. In all these reactions the chemical composition of the substances involved remains the same. As with chemical changes, mass is conserved when matter undergoes a physical change. The only change is in physical form.The phenomenon of radioactivity, the fission of uranium-235, and the fusion of hydrogen atoms (to form helium atoms), with the resultant release of atomic energy, are examples of nuclear changes. In each case the atomic nucleus changes and one kind of atom is transformed into another. One element may be changed, or transmuted, into one or more other elements, and mass is conserved.

ELEMENTS, COMPOUNDS, AND MIXTURES

A sample of a pure element contains atoms that are chemically the same, but different from those of all other elements. Pure copper contains only copper atoms, pure oxygen only oxygen atoms. The tendency of atoms of different elements to combine makes possible a great variety and number of compounds. A compound is made up of two or more elements that have undergone a chemical change. Water, sugar, baking soda, and salt are examples of familiar compounds.

Compounds are formed by a process called a chemical reaction. Elements combine, or react, to form compounds that have physical and chemical properties different from those of the original elements. For example, when atoms of hydrogen gas and oxygen gas are combined at room temperature, they undergo a chemical reaction and form water. Water is a compound that is liquid at room temperature and is unlike either hydrogen or oxygen. Methane gas, made from carbon and hydrogen, and table sugar, made from carbon, hydrogen, and oxygen, are other examples of compounds.

A compound has a definite composition—its constituent elements always occur in the same proportion, or formula. Water, for example, always contains two hydrogen atoms for every oxygen atom. The number of known compounds, both natural and artificial, is in the millions. Countless others remain to be discovered or synthesized in the laboratory.

Substances such as milk, paint, ink, air, and muddy water are mixtures, as are rocks and metal alloys. The atoms and other particles in a mixture are intermingled, but they have not combined with each other and undergone chemical changes. Unlike compounds, mixtures do not have definite composition; this is why their composition cannot be described by a fixed formula.

Mixtures can usually be separated by physical methods, such as chromatography, evaporation, distillation, or filtration. The choice of method depends largely on the nature of the mixture and the type of substances it contains.

CHROMATOGRAPHY

Chromatography consists of a collection of methods that can be used to separate the substances dissolved in a liquid or gas mixture. The process has two main parts, or phases—a stationary phase and a moving, or mobile, phase. The stationary phase is usually a liquid or solid medium; the mobile phase is generally a liquid or gas solvent. The mixture being analyzed is applied to the medium (stationary phase), which is then placed in contact with the solvent (mobile phase). The solvent moves through the medium by capillary action, carrying the mixture substances with it. The individual substances in the mixture move at different rates based on certain properties—such as molecular size. Each substance in the mixture separates out from the moving solvent and adsorbs to the medium at a different point, enabling identification.

The different methods of chromatography include gas chromatography, thin-layer chromatography, and paper chromatography. Paper chromatography is especially useful for separating pigmented mixtures, such as inks and plant material, and identifying their components by color. A small amount of the study mixture is applied near the edge of a special filter paper. The paper is hung over a container of a solvent such as water or alcohol, so that the edge of the paper is in contact with the solvent. The solvent travels up the paper, carrying the mixture with it. Different pigments in the mixture separate out and adhere to the paper at different points, allowing their identification by colour.

EVAPORATION

Evaporation is used to separate a soluble solid from a liquid mixture. In the lab, an open container of the mixture is heated. As the temperature of the mixture rises, the liquid part evaporates, or vaporizes, into the air. The solid part of the mixture is left behind in the container, where it can be collected and analyzed. Evaporation can also occur without heat: if a container of a liquid mixture is left open, the liquid portion will gradually evaporate into the air, leaving the solid component behind. This is the basis of the method used commercially to harvest sea salt from seawater.

DISTILLATION

Distillation is used to separate the liquid from a solution. The liquid is heated to its boiling point, converting it to a vapour, or gas state, which is then cooled and condensed back to its liquid form in a separate apparatus. Distillation methods include simple distillation and fractional distillation.

Simple distillation is used to separate one liquid from a solution. The solution is heated to its boiling point so that the liquid evaporates. The rising vapoir is captured in a separate apparatus, where it is cooled and condensed into a liquid state again. The solid material that was in the solution is left behind. Simple distillation can be used to separate and collect the water in a saltwater solution; the water evaporates, and the vapour is cooled and condensed to form liquid water in a separate container, leaving the salt behind.

Fractional distillation is similar to simple distillation but entails more steps. It is used to separate two or more liquids that have different boiling points. The solution is raised first to the lowest of the boiling points, allowing the liquid to vapourize and then condense separately into liquid form. The remaining solution is then heated to the next highest boiling point, allowing vapourization and condensation to take place. Fractional distillation would be used, for example, to separate a solution containing ethanol and water. The solution would first be heated to 173 °F (78 °C), the boiling point of ethanol. The ethanol would vapourize, and its vapour would be cooled and condensed back to liquid ethanol in a separate container. The remaining solution would then be heated to 212 °F (100 °C), the boiling point of water. The water vapour would then be cooled and condensed back to its liquid form. Any other liquids, as well as any solid material, would be left behind.

FILTRATION

Filtration is a useful method for separating an insoluble solid from a liquid mixture, such as a mixture of sand, pebbles, and water. The mixture is poured through a filtering device such as a sieve or a funnel lined with filter paper. The liquid portion of the mixture passes through the filter paper; even a simple coffee filter can be used. The solid sand and pebbles will not pass through the paper, however, and will be “trapped” and retained in the filter.

That's it for today. Tomorrow, we will look at this more deeply. Until next time, stay curious, and stay sciencey!