Chemistry: Matter, acids and batteries

Matter, its changes, and why water is special

All matter is made of atoms. Atoms join to form molecules. Elements contain one kind of atom, and compounds contain two or more kinds in a fixed ratio. Mass is conserved in both physical and chemical changes (NCERT Science, Class 9, Chapter 9).

There are two kinds of change (NCERT Science, Class 9, Chapter 9, Class 10, Chapter 1):

• A physical change alters only the form or state, and no new substance forms. Examples: dissolving sugar, melting ice, crystallising salt from its solution.
• A chemical change makes new substances. Signs include a change in state or colour, a gas given off and a change in temperature. Cooking an egg is one: you cannot uncook it.

UPSC asked which are chemical changes. Crystallisation of sodium chloride and melting of ice are physical, while souring of milk is a chemical change, since new substances form (Prelims 2014: answer (b), 3 only).

In a water molecule the two O–H bonds sit at an angle, so the molecule is bent, not straight. Oxygen pulls the shared electrons harder, so one end is slightly negative and the other slightly positive. This gives a net dipole moment of 1.85 D (NCERT Chemistry, Class 11, Chapter 4, dipole moment table). Because it is dipolar, water surrounds and pulls apart ions and polar molecules, like tiny magnets tugging at salt crystals. That is why it dissolves more substances than any other liquid and is called the universal solvent (Prelims 2021: answer (a)).

The modern periodic table arranges elements by atomic number, the number of protons in an atom. It now runs to element 118. IUPAC announced the approved names and symbols of the last four, elements 113, 115, 117 and 118, on 28 November 2016 (IUPAC).

Acids, bases, salts and acid rain

Acids, like lemon juice and vinegar, turn blue litmus red and give H⁺ ions in water. Bases, like soap solution, turn red litmus blue and give OH⁻ ions. An acid and a base neutralise each other to form a salt and water (NCERT Science, Class 10, Chapter 2).

The pH scale runs from 0 to 14. A neutral solution has pH 7. Values below 7 are acidic and above 7 are basic. Our body works within a narrow pH range. Tooth decay starts when the pH in the mouth falls below 5.5. Plants and aquatic life also survive only in a narrow pH range (NCERT Science, Class 10, Chapter 2).

Rain is naturally slightly acidic, about pH 5.6, because carbon dioxide dissolves in it to form weak carbonic acid. Acid rain forms when sulphur dioxide and nitrogen oxides from burning fuels are carried by wind and turn into sulphuric and nitric acids (US EPA). In UPSC’s list, only nitrogen oxide and sulphur dioxide cause acid rain. Carbon monoxide and ozone do not (Prelims 2022: answer (b), 2 and 4 only). NCERT calls rain with pH below 5.6 acid rain. It lowers the pH of rivers and harms aquatic life.

Common salts and their uses:

Source: NCERT Science, Class 10, Chapter 2 (Reprint 2026-27). Definitions, not data.

Batteries and electrochemistry

A battery turns chemical energy into electrical energy. Electrons flow out of the anode (the negative terminal while discharging) through the circuit to the cathode. In a lithium-ion cell, lithium ions shuttle between the electrodes through the electrolyte (the liquid or gel between them) as the cell charges and discharges.

The 2019 Chemistry Nobel went to John Goodenough, M. Stanley Whittingham and Akira Yoshino “for the development of lithium-ion batteries” (nobelprize.org). These batteries now power phones, laptops and electric vehicles.

EV battery cathodes are usually made of lithium compounds with nickel, manganese and cobalt (NMC) or lithium iron phosphate (LFP). Graphite is the usual anode, as in the IEA’s model of an NMC811 cathode with a graphite anode (IEA, Global EV Outlook 2024). So of cobalt, graphite, lithium and nickel, three usually make up battery cathodes: cobalt, lithium and nickel (Prelims 2025: answer (c)).

Source: IEA, Global EV Outlook 2024, Trends in electric vehicle batteries. Shares refer to 2023-24. China’s near-total hold on battery materials is why India is pushing its own critical mineral and battery plans.

What UPSC asks: Prelims: chemical change, pH 7, 118 elements. Mains: acid rain, cathode metals.