Some Basic Concepts of Chemistry

1. Some Basic Concepts of Chemistry
   

The study of chemistry is very important as its domain encompasses every sphere of life. Chemists study the properties and structure of substances and the changes undergone by them. All substances contain matter, which can exist in three states – solid, liquid or gas. The constituent particles are held in different ways in these states of matter and they exhibit their characteristic properties. 

Classification of Matter

Matter can also be classified into elements, compounds or mixtures. An element contains particles of only one type, which may be atoms or molecules. The compounds are formed where atoms of two or more elements combine in a fixed ratio to each other. Mixtures occur widely and many of the substances present around us are mixtures.

When the properties of a substance are studied, measurement is inherent. 

Measurements of Substances in SI System

The quantification of properties requires a system of measurement and units in which the quantities are to be expressed. Many systems of measurement exist, of which the English and the Metric Systems are widely used. The scientific community, however, has agreed to have a uniform and common system throughout the world, which is abbreviated as SI units (International System of Units).

Since measurements involve recording of data, which are always associated with a certain amount of uncertainty, the proper handling of data obtained by measuring the quantities is very important. The measurements of quantities in chemistry are spread over a wide range of 10-31 to 1023. Hence, a convenient system of expressing the numbers in scientific notation is used. The uncertainty is taken care of by specifying the number of significant figures, in which the observations are reported. The dimensional analysis helps to express the measured quantities in different systems of units. Hence, it is possible to interconvert the results from one system of units to another.

Basic Laws of Chemical Combination

The combination of different atoms is governed by basic laws of chemical combination — these being the 

1. Law of Conservation of Mass, 2. Law of Definite Proportions, 3. Law of Multiple Proportions, 

4. Gay Lussac’s Law of Gaseous Volumes and 5. Avogadro Law. 

All these laws led to the Dalton’s atomic theory, which states that atoms are building blocks of matter. 

Atomic and Molecular Mass

The atomic mass of an element is expressed relative to 12C isotope of carbon, which has an exact value of 12u. Usually, the atomic mass used for an element is the average atomic mass obtained by taking into account the natural abundance of different isotopes of that element. The molecular mass of a molecule is obtained by taking sum of the atomic masses of different atoms present in a molecule. The molecular formula can be calculated by determining the mass per cent of different elements present in a compound and its molecular mass.

Avogadro Number

 

The number of atoms, molecules or any other particles present in a given system are expressed in the terms of Avogadro constant (6.022 × 1023). This is known as 1 mol of the respective particles or entities.

Stoichiometric Calculations

Chemical reactions represent the chemical changes undergone by different elements and compounds. A balanced chemical equation provides a lot of information. The coefficients indicate the molar ratios and the respective number of particles taking part in a particular reaction. The quantitative study of the reactants required or the products formed is called stoichiometry. Using stoichiometric calculations, the amount of one or more reactant(s) required to produce a particular amount of product can be determined . 

The Amount of Substance in a Solution

The amount of substance present in a given volume of a solution is expressed in number of ways, e.g., 1. Mass Per cent, 2. Mole fraction, 3. Molarity and 4. Molality.

Electric Generator

Electric Generator

Uses and Working Principle

Based on the phenomenon of electromagnetic induction, the experiments studied above generate induced current, which is usually very small. This principle is also employed to produce large currents for use in homes and industry. In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.

General Construction of an Electric Generator

An electric generator, as shown in Fig, consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet. The two ends of this coil are connected to the two rings R1 and R2. The inner side of these rings are made insulated. The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively. The two rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.

Working of Electric Generator

When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say the coil ABCD is rotated clockwise in the arrangement shown in Fig.

Figure Illustration of the principle of electric generator

By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus an induced current flows in the direction ABCD. If there are larger numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil. This means that the current in the external circuit flows from B2 to B1.

After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA. The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity of the current in the respective arms changes. Such a current, which changes direction after equal intervals of time, is called an alternating current (abbreviated as AC). This device is called an AC generator.

To get a direct current (DC, which does not change its direction with time), a split-ring type commutator must be used. With this arrangement, one brush is at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down. We have seen the working of a split ring commutator in the case of an electric motor (see Fig.). Thus a unidirectional current is produced. The generator is thus called a DC generator.

The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically. Most power stations constructed these days produce AC. 

Characteristics of Produced A.C. in India

In India, the AC changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz. An important advantage of AC over DC is that electric power can be transmitted over long distances without much loss of energy.

Electric Motor

Electric Motor

An electric motor is a rotating device that converts electrical energy into mechanical energy.

 

Uses of an Electric Motor

Electric motor is used as an important component in electric fans, refrigerators, mixers, washing machines, computers, MP3 players etc. 

Construction of an Electric Motor

An electric motor consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle. The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively, as shown in the Fig.

Working of an Electric Motor

Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y. Notice that the current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of current through arm AB. On applying Fleming’s left hand rule for the direction of force on a current-carrying conductor in a magnetic field. We find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus the coil and the axle O, mounted free to turn about an axis, rotate anti-clockwise. At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator. The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down. Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

The commercial electric motors use 

(i) An electromagnet in place of permanent magnet; 

(ii) Large number of turns of the conducting wire in the current- carrying coil; and 

(iii) A soft iron core on which the coil is wound. The soft iron core, on which the coil is wound, plus the coils, is called an armature. This enhances the power of the motor.

Class 10 Physics Chapter wise Synopsis

Light: Reflection and Refraction

• Light seems to travel in straight lines.

 

• Mirrors and lenses form images of objects. Images can be either real or virtual, depending on the position of the object.

 

• The reflecting surfaces, of all types, obey the laws of reflection. The refracting surfaces obey the laws of refraction.

 

• New Cartesian Sign Conventions are followed for spherical mirrors and lenses.

 

• Mirror formula, 

1/v + 1/u = 1/f

, gives the relationship between the object-distance (u), image-distance (v), and focal length (f) of a spherical mirror.

 

• The focal length of a spherical mirror is equal to half its radius of curvature.

 

• The magnification produced by a spherical mirror is the ratio of the height of the image to the height of the object.

 

• A light ray travelling obliquely from a denser medium to a rarer medium bends away from the normal. A light ray bends towards the normal when it travels obliquely from a rarer to a denser medium.

 

• Light travels in vacuum with an enormous speed of 3×10⁸ m s⁻¹. The speed of light is different in different media.

 

• The refractive index of a transparent medium is the ratio of the speed of light in vacuum to that in the medium.

 

• In case of a rectangular glass slab, the refraction takes place at both air-glass interface and glass-air interface. The emergent ray is parallel to the direction of incident ray.

 

• Lens formula, 

1/v - 1/u = 1/f

, gives the relationship between the object-distance (u), image-distance (v), and the focal length (f) of a spherical lens.

 

• Power of a lens is the reciprocal of its focal length. The SI unit of power of a lens is dioptre.

The Human Eye and the Colourful World

• The ability of the eye to focus on both near and distant objects, by adjusting its focal length, is called the accommodation of the eye.

 

• The smallest distance, at which the eye can see objects clearly without strain, is called the near point of the eye or the least distance of distinct vision. For a young adult with normal vision, it is about 25 cm.

 

• The common refractive defects of vision include myopia, hypermetropia and presbyopia. Myopia (short-sightedness – the image of distant objects is focussed before the retina) is corrected by using a concave lens of suitable power. Hypermetropia (far-sightedness – the image of nearby objects is focussed beyond the retina) is corrected by using a convex lens of suitable power. The eye loses its power of accommodation at old age.

 

• The splitting of white light into its component colours is called dispersion.

 

• Scattering of light causes the blue colour of sky and the reddening of the Sun at sunrise and sunset.

Electricity

• A stream of electrons moving through a conductor constitutes an electric current. Conventionally, the direction of current is taken opposite to the direction of flow of electrons.

 

• The SI unit of electric current is ampere.

 

• To set the electrons in motion in an electric circuit, we use a cell or a battery. A cell generates a potential difference across its terminals. It is measured in volts (V).

 

• Resistance is a property that resists the flow of electrons in a conductor. It controls the magnitude of the current. The SI unit of resistance is ohm (Ω ).

 

• Ohm’s law: The potential difference across the ends of a resistor is directly proportional to the current through it, provided its temperature remains the same.

 

• The resistance of a conductor depends directly on its length, inversely on its area of cross-section, and also on the material of the conductor.

 

• The equivalent resistance of several resistors in series is equal to the sum of their individual resistances.

 R = R₁ + R₂ + R₃

• A set of resistors connected in parallel has an equivalent resistance R given by

1/R = 1/R₁ + 1/R₂ + 1/R₃

• The electrical energy dissipated in a resistor is given by

 

• W = V × I × t

 

• The unit of power is watt (W). One watt of power is consumed when 1 A of current flows at a potential difference of 1 V. 

• The commercial unit of electrical energy is kilowatt hour (kWh).

 1 kW h = 3,600,000 J = 3.6 × 10⁶ J.

Magnetic Effects of Electric Current

• A compass needle is a small magnet. Its one end, which points towards north, is called a north pole, and the other end, which points towards south, is called a south pole.

 

• A magnetic field exists in the region surrounding a magnet, in which the force of the magnet can be detected.

 

• Field lines are used to represent a magnetic field. A field line is the path along which a hypothetical free north pole would tend to move. The direction of the magnetic field at a point is given by the direction that a north pole placed at that point would take. Field lines are shown closer together where the magnetic field is greater.

 

• A metallic wire carrying an electric current has associated with it a magnetic field. The field lines about the wire consist of a series of concentric circles whose direction is given by the right-hand rule.

 

• The pattern of the magnetic field around a conductor due to an electric current flowing through it depends on the shape of the conductor. The magnetic field of a solenoid carrying a current is similar to that of a bar magnet.

 

• An electromagnet consists of a core of soft iron wrapped around with a coil of insulated copper wire.

 

• A current-carrying conductor when placed in a magnetic field experiences a force. If the direction of the field and that of the current are mutually perpendicular to each other, then the force acting on the conductor will be perpendicular to both and will be given by Fleming’s left-hand rule. This is the basis of an electric motor. An electric motor is a device that converts electric energy into mechanical energy.

 

• The phenomenon of electromagnetic induction is the production of induced current in a coil placed in a region where the magnetic field changes with time. The magnetic field may change due to a relative motion between the coil and a magnet placed near to the coil. If the coil is placed near to a current-carrying conductor, the magnetic field may change either due to a change in the current through the conductor or due to the relative motion between the coil and conductor. The direction of the induced current is given by the Fleming’s right-hand rule.

 

• A generator converts mechanical energy into electrical energy. It works on the basis of electromagnetic induction.

 

• In our houses we receive AC electric power of 220 V with a frequency of 50 Hz. One of the wires in this supply is with red insulation, called live wire. The other one is of black insulation, which is a neutral wire. The potential difference between the two is 220 V. The third is the earth wire that has green insulation and this is connected to a metallic body deep inside earth. It is used as a safety measure to ensure that any leakage of current to a metallic body does not give any severe shock to a user.

 

• Fuse is the most important safety device, used for protecting the circuits due to short-circuiting or overloading of the circuits.

Thanks to Watch and stay with me till last words of synopsis.

Chapter wise Synopsis Science Class 10

Chapter wise Synopsis

Class 10

Subject Science

1. Chemical Reactions And Equations.

 • A complete chemical equation represents the reactants, products and their physical states symbolically.

 

• A chemical equation is balanced so that the numbers of atoms of each type involved in a chemical reaction are the same on the reactant and product sides of the equation. Equations must always be balanced.

 

• In a combination reaction two or more substances combine to form a new single substance.

 

• Decomposition reactions are opposite to combination reactions. In a decomposition reaction, a single substance decomposes to give two or more substances.

 

• Reactions in which heat is given out along with the products are called exothermic reactions.

 

• Reactions in which energy is absorbed are known as endothermic reactions.

 

• When an element displaces another element from its compound, a displacement reaction occurs.

 

• Two different atoms or groups of atoms (ions) are exchanged in double displacement reactions.

 

• Precipitation reactions produce insoluble salts.

 

• Reactions also involve the gain or loss of oxygen or hydrogen by substances. Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen.

2. Acids, Bases And Salts

• Acid-base indicators are dyes or mixtures of dyes which are used to indicate the presence of acids and bases.

 

• Acidic nature of a substance is due to the formation of H+(aq) ions in solution. Formation of OH–(aq) ions in solution is responsible for the basic nature of a substance.

 

• When an acid reacts with a metal, hydrogen gas is evolved and a corresponding salt is formed.

 

• When a base reacts with a metal, along with the evolution of hydrogen gas a salt is formed which has a negative ion composed of the metal and oxygen.

 

• When an acid reacts with a metal carbonate or metal hydrogencarbonate, it gives the corresponding salt, carbon dioxide gas and water.

 

• Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively.

 

• The strength of an acid or an alkali can be tested by using a scale called the pH scale (0-14) which gives the measure of hydrogen ion concentration in a solution.

 

• A neutral solution has a pH of exactly 7, while an acidic solution has a pH less than 7 and a basic solution a pH more than 7.

 

• Living beings carry out their metabolic activities within an optimal pH range.

 

• Mixing concentrated acids or bases with water is a highly exothermic process.

 

• Acids and bases neutralise each other to form corresponding salts and water.

 

• Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt.

 

• Salts have various uses in everyday life and in industries.

3. Metals And Non-Metals

• Elements can be classified as metals and non-metals.

 

• Metals are lustrous, malleable, ductile and are good conductors of heat and electricity. They are solids at room temperature, except mercury which is a liquid.

 

• Metals can form positive ions by losing electrons to non-metals.

 

• 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 have different reactivities with water and dilute acids.

 

• A list of common metals arranged in order of their decreasing reactivity is known as an activity series.

 

• Metals above hydrogen in the Activity series can displace hydrogen from dilute acids.

 

• A more reactive metal displaces a less reactive metal from its salt solution.

 

• 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.

 

• An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal.

 

• 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.

 

• Non-metals have properties opposite to that of metals. They are neither malleable nor ductile. They are bad conductors of heat and electricity, except for graphite, which conducts electricity.

 

• Non-metals form negatively charged ions by gaining electrons when reacting with metals.

 

• Non-metals form oxides which are either acidic or neutral.

 

• Non-metals do not displace hydrogen from dilute acids. They react with hydrogen to form hydrides.

4. Carbon and Its Compounds

Carbon is a versatile element that forms the basis for all living organisms and many of the things we use.

 

• This large variety of compounds is formed by carbon because of its tetravalency and the property of catenation that it exhibits.

 

• Covalent bonds are formed by the sharing of electrons between two atoms so that both can achieve a completely filled outermost shell.

 

• Carbon forms covalent bonds with itself and other elements such as hydrogen, oxygen, sulphur, nitrogen and chlorine.

 

• Carbon also forms compounds containing double and triple bonds between carbon atoms. These carbon chains may be in the form of straight chains, branched chains or rings.

 

• The ability of carbon to form chains gives rise to a homologous series of compounds in which the same functional group is attached to carbon chains of different lengths.

 

• The functional groups such as alcohols, aldehydes, ketones and carboxylic acids bestow characteristic properties to the carbon compounds that contain them.

 

• Carbon and its compounds are some of our major sources of fuels.

 

• Ethanol and ethanoic acid are carbon compounds of importance in our daily lives.

 

• The action of soaps and detergents is based on the presence of both hydrophobic and hydrophilic groups in the molecule and this helps to emulsify the oily dirt and hence its removal.

5. Periodic Classification of Elements
   

• Elements are classified on the basis of similarities in their properties.

 

• Döbereiner grouped the elements into triads and Newlands gave the Law of Octaves.

 

• Mendeléev arranged the elements in increasing order of their atomic masses and according to their chemical properties.

 

• Mendeléev even predicted the existence of some yet to be discovered elements on the basis of gaps in his Periodic Table.

 

• Anomalies in arrangement of elements based on increasing atomic mass could be removed when the elements were arranged in order of increasing atomic number, a fundamental property of the element discovered by Moseley.

 

• Elements in the Modern Periodic Table are arranged in 18 vertical columns called groups and 7 horizontal rows called periods.

 

• Elements thus arranged show periodicity of properties including atomic size, valency or combining capacity and metallic and non-metallic character.