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CBSE Class 9 Chemistry-Basic Concepts of Chemistry. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.
IMPORTANT POINTS OF SOME BASIC CONCEPTS OF CHEMISTRY
1. Metric System was based on the decimal system.The International System of Units (SI)The International System of Units (in French Le Systeme International d’Unites–abbreviated as SI) was established by the 11th General Conference on Weights andMeasures (CGPM from ConferenceGenerale des Poids at Measures). The SI systemhas seven base units
2. Prefixes in SI system Multiple Prefix Symbol
10‐12 pico m
10‐9 nano m
10‐6 micro m
10‐3 milli m
10‐2 centi m
3. Mass and Weight‐‐ Mass of a substance is the amount of matter present in it whileweight is the force exerted by gravity on an object. The mass of a substance isconstant whereas its weight may vary from one place to another due to change ingravity. The mass of a substance can be determined very accurately by using ananalytical balance
4. A common unit, litre (L) is not an SI unit, is used for measurement of volume ofliquids. 1 L = 1000 mL, 1000 cm3= 1 dm3
5. Density: Density of a substance is its amount of mass per unit volume.SI unit of density = SI unit of mass/SI unit of volume = kg/m3or kg m–3
6. Temperature‐‐There are three common scales to measure temperature — °C (degreecelsius), °F (degree Fahrenheit) and K (kelvin). Here, K is the SI unit.K = °C + 273.15Note—Temperature below 0 °C (i.e. negative values) are possible in Celsius scale but in Kelvin scale, negative temperature is not possible.
7. Scientific Notation In which any number can be represented in the form N × 10n(Where n is an exponent having positive or negative values and N can vary between 1 to 10).e.g. We can write 232.508 as 2.32508 x102in scientific notation. Similarly, 0.00016can be written as 1.6 x 10–4
8. Precision refers to the closeness of various measurements for the same quantity. Accuracy is the agreement of a particular value to the true value of the result
9. Significant FiguresThe reliability of a measurement is indicated by the number of digits used torepresent it. To express it more accuratelywe express it with digits that are knownwith certainty. These are called as Significant figures. They contain all thecertaindigits plus one doubtful digit in a number.
Rules for Determining the Number of Significant Figures All non‐zero digits are significant. For example,
6.9 has two significant figures, while 2.16 has three significant figures. The decimal place does notdetermine the number of significant figures.
A zero becomes significant in case it comes in between non zero numbers. Forexample, 2.003 has four significantfigures, 4.02 has three significant figures.
Zeros at the beginning of a number are not significant. For example, 0.002 hasone significant figure while 0.0045has two significant figures.
All zeros placed to the right of a number are significant. For example, 16.0 hasthree significant figures, while 16.00has four significant figures. Zeros at theend of a number without decimal point are ambiguous.
In exponential notations, the numerical portion represents the number ofsignificant figures. For example, 0.00045 isexpressed as 4.5 x 10‐4in terms of scientific notations. The number of significant figures in this number is 2,while inAvogadro's number (6.023 x 1023) it is four.The decimal point does not count towards the number of significant figures.For example, the number 345601 has sixsignificant figures but can be writtenin different ways, as 345.601 or 0.345601 or 3.45601 all having same numberofsignificant figures.
10 Retention of Significant Figures ‐ Rounding off Figures The rounding off procedure is applied to retain the required number of significant figures.
1. If the digit coming after the desired number of significatance is its am figures happens tobe more than 5, the precedingsignificant figure is increased by one, 4.317 isrounded off to 4.32.
2. If the digit involved is less than 5, it is neglected and the preceding significantfigure remains unchanged, 4.312 isrounded off to 4.31.
3. If the digit happens to be 5, the last mentioned or preceding significant figureis increased by one only in case ithappens to be odd. In case of even figure, the 4preceding digit remains unchanged. 8.375 is rounded off to 8.38 while8.365 isrounded off to 8.36.
11. Dimensional Analysis During calculations generally there is a need to convert unitsfrom one system to other. This is called factor label method or unit factor methodor dimensional analysis.For example‐ 5 feet and 2 inches (height of an Indian female) is to converted in SIunit1 inch = 2.54 x 10‐2mthen, 5 feet and 2 inch = 62 inch.
12. Physical Classification of Matter
Properties Solid Liquid Gas
1. volume Definite Definite Indefinite
2. Shape Definite Indefinite Indefinite
3. Inter molecularforce of attractionVery high Moderate Negligible / Verylow
4. arrangement ofmoleculesOrderly arranged Free to movewithin the volumeFree to move everywhere
5. Inter molecularspaceVery small Slightly greater Very great
6. Compressibility Not compressible Not compressible Highly compressible
7. Expansion on heating Very little Very little Highly expand
8. Rigidity Very rigid Not rigid knownasfluidNot rigid andknown as fluid
9. Fluidity Can’t flow Can flow Can flow
10 .Diffusion They can diffusedue to kineticenergy ofliquid/gasesCan diffuse Andvery fastCan diffuseAndrate of diffusion isvery fast
Chemical Classification of matter‐‐‐
ElementsAn element is the simplest form of matter that cannot be split into simpler substancesor built from simpler substances by any ordinary chemical or physical method. Thereare 114 elements known to us, out of which 92 are naturally occurring while the resthave been prepared artificially.
Elements are further classified into metals, non‐metals and metalloids.CompoundsA compound is a pure substance made up of two or more elements combined in adefinite proportion by mass, which could be split by suitable chemical methods.
Characteristics of compoundCompounds always contain a definite proportion of the same elements bymass.The properties of compounds are totally different from the elements fromwhich they are formed.Compounds are homogeneous.
Compounds are broadly classified into inorganic and organic compounds.
Inorganic compounds are those, which areobtained from non‐living sourcessuch as minerals. For example, common salt, marble and limestone.Organiccompounds are those, which occur in living sources such as plants andanimals. They all contain carbon. Commonorganic compounds are oils, wax,fats etc.
13. Mixtures - A mixture is a combination of two or m compounds in any proportionso that the components do not lose their identity. Air is an example of a mixtureMixtures are of two types, homogeneous and heterogeneous.Homogeneous mixtures have the same composition throughout the sample.
Thecomponents of such mixtures cannot be seen under a powerful microscope. They arealso called solutions. Examples of homogeneous mixtures are air, seawater, gasoline,brass etc.
Heterogeneous mixtures consist of two or more parts (phases), which have different compositions. These mixtures have visible boundaries of separation between the different constituents and can be seen with the naked eye e.g., sand and salt, chalk powder in water etc.
14. LAWS OF CHEMICAL COMBINATIONS -Law of Conservation of Mass (Given by Antoine Lavoisier in 1789).It states that matter (mass) can neither be created nor destroyed.
Law of Definite Proportions or Law of Constant Composition:
This law was proposed by Louis Proust in 1799, which states that:'A chemical compound always consists of the same elements combined together inthe same ratio, irrespective of the method of preparation or the source from where itis taken'.
15. Mole is defined as the amount of a substance, which contains the same number ofchemical units (atoms, molecules, ions or electrons) as there are atoms in exactly 12grams of pure carbon‐12.A mole represents a collection of 6.022 x1023( Avogadro's number) chemical units..The mass of one mole of a substance in grams is called its molar mass.
16. Molar Volume -The volume occupied by one mole of any substance is called its molar volume. It isdenoted by Vm. One mole of all gaseous substances at 273 K and 1 atm pressureoccupies a volume equal to 22.4 litre or 22,400 mL. The unit of molar volume is litreper mol or millilitre per mol
17.PERCENTAGE COMPOSITION - The mass percentage of each constituent element present in any compound is calledits percentage composition
Mass % of the element=Mass of element in 1 molecule of the compound x 100
Molecular mass of the compound
Empirical Formula and Molecular Formula—
An empirical formula represents the simplest whole number ratio of various atoms present in a compound. E.g. CH is the empirical formula of benzene.
The molecular formula shows the exact number of different types of atoms present in a molecule of a compound. E.g. C6H6 is the molecular formula of benzene.
18. Relationship between empirical and molecular formulae The two formulas are relate Chemical Equation Shorth and representation of a chemical change in terms ofsymbols and formulae ofthe substances involved in the reaction is called chemical equation..The substances that react among themselves to bring about the chemical changes areknown as reactants, whereas the substances that are produced as a result of thechemical change, are known as products.
19. Limiting Reagent‐ The reactant which gets consumed first or limits the amount ofproduct formed is known as limiting reagent
20. Reactions in Solutions‐‐ The concentration of a solution can be expressed in any ofthe following ways.
- Mass Percent is the mass of the solute in grams per 100 grams of the solution.A 5 % solution of sodium chloride means that 5 g of NaCl is present in 100gof the solution.
- Volume percent is the number of units of volume of the solute per 100 unitsof the volume of solution. 5 % (v/v) solution of ethyl alcohol contains 5 cm3of alcohol in 100 cm3ofthe solution
- Molarity of the solution is defined as the number of moles of solute dissolvedper litre (dm3) of the solution. It is denoted by the symbol M. Measurements in Molarity can change with the change in temperature because solutions expand or contract accordingly.
Molarity of the solution = No. of moles of the solute = n
Volume of the solution in litre V
The Molarity of the solution can also be expressed in terms of mass and molar mass
21. Molarity of the solution = Mass of the solute
Molar mass of the solute X volume of the solution in liter
In terms of weight, molarity of the substance can be expressed as: Molarity equation
To calculate the volume of a definite solution required to prepare solution of other molarity, the following equation is used:
M1V1 = M2V2, where M1= initial molarity, M2= molarity of ed as Molecular formula = n x empirical formult on in 1803 ore elements
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