Ch 1Some Basic Concepts of Chemistry — Class 11 Chemistry, concept-first.

Chemistry is the branch of science that studies the composition, structure, properties, and reactions of matter.

Chemistry is not an isolated subject. It sits at the heart of science, constantly interacting with physics, biology, geology, and even computer science.

Anything which has mass and occupies space is called matter. Everything around us — a book, a pen, a pencil, water, air, and all living beings — is composed of matter: each has mass, and each occupies…

Matter around us exists in three familiar physical forms: solid, liquid, and gas. What distinguishes these states is not the kind of particles (atoms or molecules) they are made of, but how those part…

At the macroscopic level — the level we can see and handle — matter is first divided into two broad categories: pure substances and mixtures.

Every substance around us has unique physical and chemical characteristics that define its identity. To study these systematically, we measure properties like mass, volume, temperature, and density us…

Every substance possesses a set of properties that are unique to it — these are what allow us to identify one substance from another.

Quantitative measurement of properties is required for scientific investigation. Many properties of matter — length, area, volume and so on — are quantitative in nature, and any quantitative observati…

Science depends on measurement, and measurement depends on a consistent system of units. Before 1960, different countries used different systems — the British Imperial system, the CGS (centimetre-gram…

Mass and weight are often used interchangeably in daily life, but in science they describe fundamentally different properties.

Volume is the amount of three-dimensional space that a substance occupies. Since space is measured in three dimensions, the fundamental unit of volume is the cube of a length unit.

The two properties discussed just before this — mass (1.3.4) and volume (1.3.5) — are related through density:

Temperature is a measure of how hot or cold an object is. It is a fundamental physical quantity, and in chemistry, controlling and measuring temperature is essential because reaction rates, equilibriu…

Many a time in the study of chemistry, one has to deal with experimental data as well as theoretical calculations.

Chemistry deals with numbers that span an extraordinary range. A single gram of hydrogen contains roughly 602,200,000,000,000,000,000,000 molecules, while the mass of a single hydrogen atom is about 0…

Every measurement you make in a laboratory carries some uncertainty. The source of this uncertainty is twofold: the limitations of the instrument you are using, and the skill of the person making the…

When you work through numerical problems in chemistry, you often need to convert a quantity expressed in one set of units into the equivalent quantity in another set of units.

Chemistry is the science of change, and at its heart lies the question: how much? The Laws of Chemical Combinations provide the foundational rules that govern how elements and compounds react in fixed…

The law of conservation of mass was put forward by the French chemist Antoine Lavoisier in 1789. Before Lavoisier, chemistry was largely qualitative — alchemists and early chemists had not yet develop…

This law — which the book notes is sometimes also referred to as the Law of Definite Composition (equivalently, of constant composition) — states that a given chemical compound always contains its con…

This law was proposed by Dalton in 1803. According to it, if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are…

In 1808, Joseph Louis Gay Lussac published a law that described a striking pattern in the way gases react.

In 1811, Amedeo Avogadro put forward a bold idea that would eventually become one of the cornerstones of chemical science.

The idea that matter is made of tiny, indivisible particles is ancient — the Greek philosopher Democritus called them atomos, meaning “uncuttable” or “indivisible”.

The journey from the microscopic world of atoms to the measurable quantities in a laboratory begins with understanding their masses.

Atoms are unimaginably small. A single hydrogen atom has a mass of roughly grams — a decimal so small it is nearly impossible to work with directly.

Most elements in nature are not composed of atoms that are all identical. Instead, they exist as a mixture of isotopes — atoms of the same element that have the same number of protons but different nu…

Molecular mass is the sum of the atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together.

Some substances, such as sodium chloride, do not contain discrete molecules as their constituent units.

Atoms and molecules are extremely small in size, and their numbers in even a small amount of any substance are really very large.

So far we were dealing with the number of entities present in a given sample; many a time, information regarding the percentage of a particular element present in a compound is required.

An empirical formula represents the simplest whole-number ratio of various atoms present in a compound.

The word stoichiometry is derived from two Greek words — stoicheion (meaning element) and metron (meaning measure).

In any chemical reaction, the reactants are rarely present in the exact ratio demanded by the balanced chemical equation.

A majority of reactions in the laboratories are carried out in solutions. Therefore, it is important to understand how the amount of substance is expressed when it is present in a solution.

Mass per cent is the simplest way to express concentration. It is obtained by using the following relation:

Mole fraction is the ratio of the number of moles of one component to the total number of moles of all components in the solution. If substance A dissolves in substance B, with and moles respectively:

Molarity is the most widely used concentration unit in the laboratory, denoted by M. It is defined as the number of moles of solute per litre of solution:

Molality is defined as the number of moles of solute present in 1 kg of solvent. It is denoted by m:

A handful of the chapter's end-of-chapter exercise questions come with their own data table printed as part of the question.

- Matter is anything with mass and volume; classified as mixtures (homogeneous/heterogeneous) or pure substances (elements/compounds).

Higher-order thinking problems from the NCERT Exemplar.