WBCS Preliminary (Chemistry):Matter and Its Nature

1. MATTER AND ITS NATURE:

   • Anything that possesses mass, occupies space, offers resistance and can be perceived through one or more of our sense is called matter.
   • Matter is made up of particles. Particles of matter have space between them and are continuously moving and attract each other.
   • Matter can exist in three states-
     1. Solid
     2. Liquid
     3. Gas.
   • Solid has a definite shape, distinct boundaries and fixed volumes, Solids have a tendency to maintain their shape when subjected to outside force. Solids may break under force but it is difficult to change their shape, so they are rigid.
   • Liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids flow and change shape, so they are not rigid but can be called fluid.
   • A gas has no definite volume or shape. gases are highly compressible as compared to solids and liquids. The liquefied petroleum gas (LPG) cylinder that we get in our home for cooking or the oxygen supplied to hospitals in cylinders is compressed gas. Compressed natural gas (CNG) is used as fuel these days in vehicles.
   • The forces of attraction between the particles(inter-molecular force) are maximum in solids, intermediate in liquids and minimum in gases. The spaces in between the constituent particles and kinetic energy of the particles are minimum in the case of solids, intermediate in liquids and maximum in gases.
   • The arrangement of particles is most ordered in the case of solids, in the case of liquids layers of particles can slip and slide over each other while for gases, there is no order, particles just move about randomly.
   • In spite of above differences all kinds of matter have a common property, the property of having a mass.
   • The states of matter are inter-convertible. The state of matter can be changed by changing temperature or pressure.
   • On increasing the temperature of solids, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions and start moving more freely. A stage is reached when the solid melts and is converted to a liquid. The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point.
   • The process of melting, that is, change of solid state into liquid state is also known as fusion.
   • During the melting, the temperature of the system does not change after the melting point is reached, till all the ice melts. This happens even though we continue to heat the beaker, that is, we continue to supply heat. This heat gets used up in changing the state by overcoming the forces of attraction between the particles. As this heat energy is absorbed by ice without showing any rise in temperature, it is considered that it gets hidden into the contents of the beaker and is known as the latent heat.
   • The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.
   • The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point.
   • Latent heat of vaporisation is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point.
   • Sublimation is the change of gaseous state directly to solid state without going through liquid state, and vice versa.
   • Evaporation is a surface phenomenon. Particles from the surface gain enough energy to overcome the forces of attraction present in the liquid and change into the vapour state. The rate of evaporation depends upon the surface area exposed to the atmosphere, the temperature, the humidity and the wind speed. Evaporation causes cooling.
   • During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool.
   • Let us take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.
   • Properties such as shape, size, colour and state of a substance are called its physical properties. A change , which does not involve any alteration in composition of the substance is called a physical change. A physical change is generally reversible. In such a change no new substance is formed.
   • Some substances can be obtained in pure state from their solutions by crystallisation.
   • A change that alters the composition of a substance or substances taking part in the change is termed a chemical change. A chemical change is also called a chemical reaction.All new substances are formed as a result of chemical changes.
   • Burning of coal, wood or leaves is a chemical change. Explosion of a firework is a chemical change. If you leave a piece of iron in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting. The process of rusting can be represented by the following equation: Iron (Fe) + Oxygen (O2, from the air)water (H₂O) → rust (iron oxide- Fe₂O₃) For rusting, the presence of both oxygen and water (or water vapour) is essential. It is a chemical change.
   • Prevent iron articles from coming in contact with oxygen, or water, or both. One simple way is to apply a coat of paint or grease. Another way is to deposit a layer of a metal like chromium or zinc on iron. This process of depositing a layer of zinc on iron is called galvanisation.
   • Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.
   • Changes attended with absorption of heat are called endothermic changes, while those which occur with evolution of heat are called exothermic changes. The reactions in which heat is absorbed are known as endothermic reactions, while chemical reactions which evolve heat are called exothermic. The compounds formed from their elements with absorption of heat are called endothermic compounds, whilst those formed from their elements with evolution of heat are called exothermic compounds.
2. Classifications:
   • A pure substance is one that contains one kind of materials throughout its body. A substance cannot be separated into other kinds of matter by any physical process. Mixtures are constituted by more than one kind of pure form of matter, known as a substance. Mixtures can be separated into pure substances using appropriate separation techniques like filteration, sublimation, decantation, chromatography, crystallization, etc.
   • A substance is said to be homogeneous if it has one and the same composition and properties in all its parts. On the other hand, if the composition and properties are not identical throughout the body the substance is heterogeneous. A pure substance must be homogeneous.
   • Pure substance are classified into elements and compounds.
     Elements: An element is a form of matter that cannot be broken down by chemical reactions into simpler substances.Robert Boyle was the first scientist to use the term element in 1661. Elements can be normally divided into metals, non-metals and metalloids.
   • Compound: A compound is a substance composed of two or more different types of elements, chemically combined in a fixed proportion. Properties of a compound are different from its constituent elements.
   • Symbols: The symbol is an abbrevaiation for the full name of an element. In many cases the initial capital letter of the common name of element is used as abbrevaiation for it. H stands for Hydrogen, N for Nitrogen, etc. Two letters are used in cases of two or more elements having the same initial letter. A second prominent letter ( small) from its name is added to the initial letter. Al stands for Aluminium, Cl stands for chlorine, etc. In some cases the symbols are derived by taking letter or letters from the Latin name of the element. Cu stands for Copper ( Latin name Cuprum), Au stands for Gold ( Latin name Aurum), etc.
   • Symbol represents one atom and naturally stands for a perfectly definite amount of the element concerned. Every substance is an aggregate of its molecules, and the symbolic representation of a molecule of the substance is called its formula. The number of atoms per molecule of the element is known as the atomicity of the molecule. If the molecule of an element contains one atom, then the molecule is represented by the symbol only, i.e., in such a case symbol represents also the formula.
   • Valency: The number of chemical substances, except the element themselves, are composed of two or more of these elementary materials combined together. The valency of an element is the combining capacity of an atom of the element and is measured by the number of hydrogen atoms with which it can be combined. Hydrogen is chosen as the standard of referrence because the combining capacity of hydrogen is least. Though the combining capacity of an atom of the element is by and large fixed, valancy may vary; some elements exhibit different valancies. The highest valancy known being 8, the valancies range between 0 and eight. Helium, argon, etc., the so-called inert gases have no combining capacity and hence they are regarded as zero valent element. Valancy is always a whole number.
   • Compounds too like elements are represented by molecular formula. To build up the formula of a compound the symbols of the constituent elements are written side by side and the number of atoms of each is indicated by putting numerals to the lower right of the symbols. But the subscript one is not written in formula.
3. Solution:
   • A solution is a homogeneous mixture of two or more substances. The major component of a solution is called the solvent, and the minor, the solute. Lemonade, soda water etc. are all examples of solutions. We can also have solid solutions (alloys) and gaseous solutions (air).
   • The particles of a solution are smaller than 1 nm (10^-9 metre) in diameter. So, they cannot be seen by naked eyes. The solute particles cannot be separated from the mixture by the process of filtration. The solute particles do not settle down when left undisturbed, that is, a solution is stable.
   • The concentration of a solution is the amount of solute present per unit volume or per unit mass of the solution/solvent.
   • Materials that are insoluble in a solvent and have particles that are visible to naked eyes, form a suspension. A suspension is a heterogeneous mixture.
4. Alloys:
   • Alloys are homogeneous mixtures of metals and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, brass is a mixture of approximately 30% zinc and 70% copper.
   • Non-homogeneous systems, in which solids are dispersed in liquids, are called suspensions. A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye.
   • Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. Colloids are useful in industry and daily life. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium.
5. METALS AND NON-METALS: Elements can be normally divided into metals, non-metals and metalloids. Metals usually show some or all of the following properties:
   • They have a lustre (shine).Exception: Mercury, though a metal is liquid.
   • They have silvery-grey or golden-yellow colour.
   • They conduct heat and electricity. Silver is the best while copper stands second.
   • They are ductile (can be drawn into wires).Gold is the most ductile metal.
   • They are malleable (can be hammered into thin sheets). Exception: Metals like antimony and bismuth are brittle.
   • They are sonorous (make a ringing sound when hit).
   • Metals have high melting points. Exception:Gallium and Caesium have very low melting points.
   • Metals can form positive ions by losing electrons to non-metals. In electrolysis metals get deposited at the negative electrode(cathode).
   • Metals combine with oxygen to form basic oxides. Aluminium oxide and zinc oxide show the properties of both basic as well as acidic oxides. These oxides are known as amphoteric oxides.Different metals show different reactivities towards oxygen. Metals such as potassium and sodium react so vigorously that they catch fire if kept in the open. Hence, to protect them and to prevent accidental fires, they are kept immersed in kerosene oil.
   • Different metals have different reactivities with water and dilute acids.Metals above hydrogen in the Activity series can displace hydrogen from dilute acids and form salts.
   • Metals occur in nature as free elements or in the form of their compounds.The extraction of metals from their ores and then refining them for use is known as metallurgy.
   • The surface of some metals, such as iron, is corroded when they are exposed to moist air for a long period of time. This phenomenon is known as corrosion.