01 · Explore
Development and Importance of Chemistry
Chemistry is the science of molecules and their transformations, playing a central role in understanding nature and improving human life.
Historically, chemistry in India was known as Rasayan Shastra or Rasvidya, encompassing metallurgy, medicine, and cosmetics. Ancient texts like the Charaka Samhita and Sushruta Samhita describe the preparation of acids and the use of metal oxides. Acharya Kanda, around 600 BCE, proposed the 'atomic theory' of Paramānu long before modern science.
Today, chemistry is essential for national economies and human needs. It provides life-saving drugs like cisplatin and taxol for cancer therapy and AZT for AIDS. It also addresses environmental challenges, such as developing safer alternatives to ozone-depleting chlorofluorocarbons (CFCs) and managing greenhouse gases like methane and carbon dioxide.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
What were the two primary goals of early alchemy?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 1, 2, 3, 4.
02 · Explore
Nature and Classification of Matter
Matter is anything that has mass and occupies space, existing in various physical states and chemical compositions.
Matter exists in three physical states: solid, liquid, and gas. In solids, particles are held close together in an orderly fashion with little freedom of movement. In liquids, particles are close but can move around. In gases, particles are far apart and move rapidly. These states are interconvertible by changing temperature and pressure.
At the macroscopic level, matter is classified into mixtures or pure substances. Mixtures contain two or more substances in any ratio and can be homogeneous (uniform composition, like air) or heterogeneous (non-uniform, like salt and sugar). Pure substances have a fixed composition and are further divided into elements (one type of atom) and compounds (different atoms combined in a fixed ratio).
| State | Volume | Shape | Particle Arrangement |
|---|---|---|---|
| Solid | Definite | Definite | Very close, orderly |
| Liquid | Definite | Indefinite | Close, move around |
| Gas | Indefinite | Indefinite | Far apart, fast movement |
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
How does a compound differ from a mixture?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 4, 5, 6.
03 · Explore
Measurement and SI Units
Scientific investigation requires quantitative measurement using a standardized system of units.
The International System of Units (SI) was established to provide a common standard. It consists of seven base units: metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity).
Mass is the amount of matter in an object and is constant, whereas weight is the force exerted by gravity and can vary. Volume is the space occupied (SI unit m³), and density is mass per unit volume (SI unit kg m⁻³). Temperature is measured on Celsius, Fahrenheit, and Kelvin scales, where K = °C + 273.15.
| Quantity | SI Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
Convert 25 °C to the Kelvin scale.
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 7, 8, 9, 10.
04 · Explore
Uncertainty and Significant Figures
Every experimental measurement has some uncertainty, which is managed using scientific notation and significant figures.
Scientific notation represents numbers as N × 10ⁿ, where N is between 1 and 10. For example, 232.508 is written as 2.32508 × 10². This makes handling very large or very small numbers easier during calculations.
Significant figures are the meaningful digits known with certainty plus one estimated digit. Rules include: all non-zero digits are significant; zeros between non-zeros are significant; and leading zeros are not significant. In addition/subtraction, the result is limited by the least number of decimal places; in multiplication/division, it is limited by the least number of significant figures.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
How many significant figures are in 0.0025 and 2.005?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 11, 12, 13.
05 · Explore
Laws of Chemical Combination
The formation of compounds from elements is governed by five fundamental laws.
The Law of Conservation of Mass (Lavoisier) states that matter can neither be created nor destroyed in a chemical reaction. The Law of Definite Proportions (Proust) states that a compound always contains the same elements in the same proportion by mass.
The Law of Multiple Proportions (Dalton) states that if two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers. Gay Lussac’s Law of Gaseous Volumes states that gases combine in a simple ratio by volume at constant temperature and pressure. Avogadro’s Law proposes that equal volumes of all gases contain an equal number of molecules.
Law of Multiple Proportions
H₂ + 1/2 O₂ → H₂O (16g O); H₂ + O₂ → H₂O₂ (32g O)
In water, 2g of hydrogen combines with 16g of oxygen. In hydrogen peroxide, 2g of hydrogen combines with 32g of oxygen. The ratio of oxygen masses (16:32) is 1:2, a simple whole number ratio.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
State Avogadro's Law.
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 14, 15, 16.
06 · Explore
Atomic and Molecular Masses
Masses of atoms and molecules are measured relative to the carbon-12 isotope.
One atomic mass unit (amu or u) is defined as 1/12th of the mass of one carbon-12 atom. Since many elements exist as isotopes, we use average atomic mass, calculated by multiplying the mass of each isotope by its fractional abundance.
Molecular mass is the sum of the atomic masses of all atoms in a molecule. For ionic compounds like NaCl, which do not have discrete molecules, the term 'formula mass' is used instead, summing the masses of the ions in the empirical formula.
Calculating Molecular Mass of Water
Molecular mass of H₂O = 2(1.008 u) + 16.00 u = 18.016 u
Multiply the atomic mass of hydrogen by 2 and add the atomic mass of oxygen.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
Define average atomic mass.
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 16, 17, 18.
07 · Explore
Mole Concept and Molar Mass
The mole is the SI unit for the amount of substance, providing a bridge between the microscopic and macroscopic worlds.
One mole (mol) contains exactly 6.02214076 × 10²³ elementary entities (atoms, molecules, or ions). This is known as the Avogadro constant (Nₐ). The mass of one mole of a substance in grams is called its molar mass, which is numerically equal to its atomic or molecular mass in 'u'.
Percentage composition describes the mass percentage of each element in a compound. This data is used to determine the empirical formula (the simplest whole-number ratio of atoms) and the molecular formula (the actual number of atoms in a molecule).
Determining Molecular Formula
- 1
Mass % to Grams
Assume a 100g sample to convert percentages directly to grams.
- 2
Grams to Moles
Divide the mass of each element by its atomic mass.
- 3
Mole Ratio
Divide all mole values by the smallest value to get the simplest ratio.
- 4
Empirical Formula
Write the formula using the whole number ratio.
- 5
Molecular Formula
Multiply the empirical formula by n, where n = Molar Mass / Empirical Formula Mass.
Sequential steps to derive the molecular formula from experimental mass percentage data.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
What is the molar mass of water?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 18, 19, 20.
08 · Explore
Stoichiometry and Reactions in Solutions
Stoichiometry deals with calculating the masses or volumes of reactants and products in chemical reactions.
A balanced chemical equation provides the molar ratios of reactants and products. The limiting reagent is the reactant that is completely consumed first, limiting the amount of product formed. Calculations must be based on the moles of the limiting reagent.
In laboratories, reactions often occur in solutions. Concentration is expressed in several ways: Mass percent (mass of solute/mass of solution × 100), Mole fraction (moles of component/total moles), Molarity (moles of solute/litres of solution), and Molality (moles of solute/kg of solvent). Molarity changes with temperature due to volume changes, while molality remains constant.
Calculating Molarity
M = (Mass of NaOH / Molar Mass) / Volume in L
To find the molarity of 4g NaOH in 250mL solution: Moles = 4g / 40g/mol = 0.1 mol. Volume = 0.250 L. Molarity = 0.1 / 0.250 = 0.4 M.
Pause & Try
Think it through first. Writing and checking your answer is free with an account.
Question
Why is molality preferred over molarity for temperature-dependent experiments?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 20, 21, 22, 23, 24.