Everything around you keeps changing. Ice melts, milk turns sour, iron rusts and food cooks. In chemistry we sort all these into two big families: physical changes and chemical changes. Knowing the difference — and what happens during changes of state like melting and boiling — is a guaranteed scorer in the NDA General Science paper.
Why This Topic Matters For NDA
The NDA General Studies paper asks 3–5 chemistry questions almost every year, and "types of change" is one of the most repeated themes. Examiners love it because it is easy to frame: they simply give you an example — say, burning of a candle or dissolving sugar in water — and ask whether it is a physical or chemical change.
If your basics are sharp, these are free marks. There is no calculation and no formula to memorise — only clear understanding. That is exactly what this page builds.
Where this fits in your syllabus
This chapter is the foundation for almost everything else in chemistry. Before you can understand chemical reactions, acids and bases, metals and non-metals, or even biology topics like respiration and photosynthesis, you must first be able to recognise what kind of change is taking place. So the few minutes you spend here pay off across the whole paper.
The good news is that this is one of the most scoring and least time-consuming topics in the entire General Science section. The questions are short, direct and rarely tricky if you have understood the core idea: did a new substance form, or not?
Matter exists in three common states: solid, liquid and gas. A "change of state" simply means matter moving from one of these to another, like ice (solid) becoming water (liquid). The particles stay the same; only their arrangement and energy change.
What Is A Physical Change?
A physical change is a change in which no new substance is formed. Only the physical properties — shape, size, state or appearance — change. The chemical composition of the substance stays exactly the same.
Most physical changes are reversible, meaning you can get the original substance back.
Common features of a physical change
- No new substance is produced.
- It is usually easily reversible.
- The mass stays the same.
- Little or no energy change (and it is small if any).
Everyday examples
- Melting of ice into water
- Boiling of water into steam
- Dissolving sugar or salt in water
- Breaking a glass or tearing paper
- Stretching a rubber band
- Magnetising an iron rod
- Mixing iron filings and sand (you can separate them again)
In each case you could, in principle, recover the starting material — you can refreeze water, evaporate the salt solution to get the salt back, or pull apart the iron and sand. That recoverability is a strong hint that the change is physical.
In a physical change the molecules stay the same — H2O is still H2O whether it is ice, water or steam. Only the arrangement and spacing of molecules changes.
What Is A Chemical Change?
A chemical change (also called a chemical reaction) is a change in which one or more new substances are formed with completely different properties from the original.
Chemical changes are usually irreversible — you cannot easily get the original substances back. During a chemical change, the chemical bonds between atoms break and new bonds form, rearranging the atoms into different molecules. This is why the product behaves so differently from the starting material.
Signs that a chemical change has happened
- A new substance with new properties forms
- Change in colour (e.g. iron turning reddish-brown when rusting)
- Evolution of a gas (bubbles, fizzing)
- Formation of a precipitate (an insoluble solid)
- Change in temperature (heat given out or absorbed)
- Sometimes light or smell is produced
Everyday examples
- Rusting of iron
- Burning of paper, wood or a candle
- Cooking of food
- Souring of milk into curd
- Digestion of food in our body
- Photosynthesis in green plants
- Setting of cement and concrete
- Ripening of fruits
Notice that most of these are processes we see daily but rarely think about as chemistry. The clue is always the same: the end product is a genuinely different material that you cannot turn back into the original by simple physical means like heating, cooling or filtering.
If the question describes burning, cooking, rusting, rotting or souring, it is almost always a chemical change. These are the NDA's favourite trick examples.
Physical vs Chemical Change At A Glance
Here is the quick comparison the NDA expects you to know cold. The single most reliable test is to ask: has a new substance with new properties been formed? If yes, it is chemical; if no, it is physical.
Physical change
- No new substance formed
- Usually reversible
- Composition unchanged
- Energy change is small
- Example: melting of wax
Chemical change
- New substance(s) formed
- Usually irreversible
- Composition changes completely
- Energy change is often large
- Example: burning of wax
A candle shows both at once! Melting of the wax is a physical change, but burning of the wax (producing CO2 and water vapour) is a chemical change. Read which part the question asks about.
Changes Of State Explained
A change of state is a physical change where matter shifts between solid, liquid and gas. These changes happen when we add or remove heat. Each has a special name you must memorise.
How particles behave
In a solid, particles are packed tightly and only vibrate, so a solid has a fixed shape and volume. In a liquid, particles are looser and can slide past one another, so a liquid takes the shape of its container. In a gas, particles are far apart and move freely in all directions, filling any space available. Adding heat gives particles more energy to break free; removing heat lets them settle closer together.
The six key changes
- Melting (fusion): solid → liquid (ice → water)
- Freezing (solidification): liquid → solid (water → ice)
- Vaporisation / boiling: liquid → gas (water → steam)
- Condensation: gas → liquid (steam → water on a cold glass)
- Sublimation: solid → gas directly (camphor, naphthalene, dry ice)
- Deposition: gas → solid directly (frost forming)
Notice that the first, third and fifth changes need heat to be supplied, while the second, fourth and sixth happen when heat is removed. Spotting this direction quickly helps in exam questions.
Sublimation is a hot NDA favourite. Substances that sublime: camphor, naphthalene balls, iodine, ammonium chloride and solid CO2 (dry ice). Dry ice is used to keep ice cream cold because it turns straight to gas, leaving no wet liquid behind.
Melting Point And Boiling Point
The temperatures at which states change are fixed for a pure substance and are useful exam facts.
Melting point
The temperature at which a solid turns into a liquid. For pure ice it is 0°C (273 K).
Boiling point
The temperature at which a liquid turns into a gas throughout its bulk. For pure water it is 100°C (373 K) at normal atmospheric pressure.
Evaporation vs boiling
- Evaporation happens slowly, only at the surface, and at any temperature.
- Boiling happens fast, throughout the liquid, and only at the boiling point.
To convert Celsius to Kelvin, add 273. So 0°C = 273 K and 100°C = 373 K. The NDA sometimes gives temperatures in Kelvin to confuse you.
Latent Heat: The Hidden Energy
Here is something that surprises students. While ice is melting, its temperature stays at 0°C even though you keep supplying heat. Where does that heat go?
It is used to break the bonds holding the solid together. This hidden heat is called latent heat — the energy absorbed or released during a change of state without any change in temperature.
- Latent heat of fusion: heat needed to melt 1 kg of a solid at its melting point.
- Latent heat of vaporisation: heat needed to convert 1 kg of a liquid into gas at its boiling point.
This is why steam causes worse burns than boiling water at the same temperature — steam carries extra latent heat of vaporisation that it releases on your skin.
The same idea explains why we feel cool when we sweat: as sweat evaporates, it absorbs latent heat from our body, lowering our temperature. It is also why a desert cooler or an earthen pot (matka) keeps water cold — evaporation pulls heat out of the water inside.
Exothermic And Endothermic Changes
Changes also differ in whether they give out or take in heat.
Exothermic change
Heat is released to the surroundings, which feel warmer. Examples: burning of fuels, respiration, mixing acid with water, and freezing of water.
Endothermic change
Heat is absorbed from the surroundings, which feel cooler. Examples: melting of ice, evaporation, and photosynthesis.
Easy memory rule — EXO = EXit of heat (heat goes out, surroundings warm up). ENDO = ENter of heat (heat comes in, surroundings cool down).
Many chemical changes are exothermic (burning), and many changes of state are endothermic (melting, boiling). But the link is not fixed — for instance, freezing is a physical change that is exothermic, while the decomposition of limestone on heating is a chemical change that is endothermic.
Why this matters in real life
Cold packs used by athletes work on an endothermic process — a salt dissolves and absorbs heat, cooling an injury. Hand-warmers work on an exothermic process that releases heat. Recognising these in a question tells you instantly whether the surroundings get hotter or colder.
Worked Example
Let us classify a real situation step by step, the way you should think in the exam.
Sara lights a candle. Identify the type of change in each step: (1) the wax near the flame melts, (2) the molten wax burns to give light, heat, carbon dioxide and water vapour.
This both-at-once idea is exactly what NDA examiners test, so train your eye to separate the two events.
Common Mistakes To Avoid
Students lose easy marks on this topic for a handful of repeated reasons.
- Thinking all reversible changes are physical — mostly true, but focus on whether a new substance formed.
- Calling dissolving sugar a chemical change — it is physical; sugar can be recovered by evaporating the water.
- Confusing evaporation (surface, any temperature) with boiling (bulk, at boiling point).
- Forgetting that temperature stays constant during a change of state due to latent heat.
Many students say curd-from-milk and rust-from-iron are reversible. They are not — both are chemical changes that form new, permanent substances.
Previous-Year Style Question
Try this NDA-pattern multiple choice question before reading the answer.
Q. Which one of the following is a chemical change?
(a) Melting of ice (b) Dissolving salt in water (c) Rusting of iron (d) Boiling of water
Answer: (c) Rusting of iron. Rusting forms a brand-new substance, hydrated iron oxide (Fe2O3·xH2O), with properties different from iron — so it is a chemical change. Options (a), (b) and (d) are all physical changes because no new substance is formed and they are reversible.
Quick Revision
- Physical change: no new substance, usually reversible (melting, boiling, dissolving).
- Chemical change: new substance formed, usually irreversible (burning, rusting, cooking, souring).
- Changes of state: melting, freezing, vaporisation, condensation, sublimation, deposition.
- Sublimation: solid → gas directly (camphor, naphthalene, iodine, dry ice).
- Ice melts at 0°C (273 K); water boils at 100°C (373 K).
- Latent heat: hidden heat absorbed/released during a state change at constant temperature.
- Exothermic: heat out (burning). Endothermic: heat in (melting, evaporation).
Master these seven points and the NDA's change-of-state questions become guaranteed marks.
Frequently asked questions
Is melting of ice a physical or chemical change?
It is a physical change. Ice and water are both H2O, so no new substance is formed, and the change can be reversed by cooling the water back into ice.
Why is rusting of iron a chemical change?
Because iron reacts with oxygen and moisture to form a new substance, hydrated iron oxide (rust), which has completely different properties from iron and cannot be easily reversed.
What is sublimation and which substances show it?
Sublimation is the direct change of a solid into a gas without becoming a liquid. Common examples are camphor, naphthalene balls, iodine, ammonium chloride and dry ice (solid carbon dioxide).
What is the difference between evaporation and boiling?
Evaporation is slow, happens only at the liquid's surface and at any temperature. Boiling is fast, happens throughout the liquid, and occurs only at the boiling point (100 degrees Celsius for water).
Why does the temperature stay constant during melting or boiling?
Because the heat supplied is used to break the bonds between particles rather than to raise the temperature. This hidden energy is called latent heat.
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