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General Science · Class 9 · Chapter 2: Our Life and Chemistry

8 min

Key points

Chemistry is the branch of science which deals with the composition, structure, properties and reactions of matter
Matter exists in three states: solid, liquid and gas - each having different properties and molecular arrangements
Atoms are the basic building blocks of matter and cannot be divided further by chemical means
Elements are pure substances made up of only one type of atoms, while compounds are formed by chemical combination of two or more elements
Mixtures can be separated by physical means while compounds require chemical methods for separation
Chemical reactions involve breaking of old bonds and formation of new bonds resulting in new substances
Balancing chemical equations follows the law of conservation of mass - atoms are neither created nor destroyed
Acids turn blue litmus red and have sour taste, while bases turn red litmus blue and have bitter taste
pH scale ranges from 0-14, with 7 being neutral, below 7 acidic, and above 7 basic
Fertilizers like urea and DAP used in Pakistan's agriculture contain nitrogen, phosphorus and potassium compounds

Definitions

Chemistry: Chemistry is the branch of science which deals with the composition, structure, properties and reactions of matter.
Element: An element is a pure substance which is made up of only one type of atoms and cannot be broken down into simpler substances by chemical means.
Compound: A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass.
Mixture: A mixture is formed when two or more substances are mixed together in any proportion without any chemical combination.
Chemical Reaction: A chemical reaction is a process in which one or more substances are converted into new substances with different properties.

Worked example

Define chemistry and explain any three branches of chemistry with one example each. (5 marks)

Chemistry is the branch of science which deals with the study of composition, structure and properties of matter and the changes it undergoes during chemical reactions. Three branches are: (1) Organic Chemistry: The branch of chemistry that deals with the study of carbon compounds. Example: Study of methane (CH₄). (2) Inorganic Chemistry: The branch of chemistry that deals with the study of all elements and their compounds except carbon compounds. Example: Study of sodium chloride (NaCl). (3) Physical Chemistry: The branch of chemistry that deals with the study of physical properties of substances. Example: Study of boiling point of water.

Common mistakes

Students confuse elements with compounds - remember elements have only one type of atoms
Writing unbalanced chemical equations - always count atoms on both sides
Mixing up acid-base indicators - blue litmus turns red in acid, red litmus turns blue in base
Confusing mixtures with compounds - mixtures can be separated physically
Not learning exact textbook definitions - BSEK expects word-for-word definitions

Quick recap

Chemistry studies composition, structure and properties of matter. Matter exists as elements (one type of atom), compounds (chemically combined elements), or mixtures (physically combined substances). Three main branches: Organic (carbon compounds), Inorganic (non-carbon compounds), Physical (physical properties). Chemistry applications include medicine, food production, cosmetics, and industrial processes.

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