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What is the difference between a physical change and a chemical change?

Class 7 ScienceNCERT chapter: Changes Around Us: Physical and Chemicalइसे हिंदी में पढ़ें

Exam-ready answer

A physical change is a change in which only properties such as shape, size, colour or state of a substance change and no new substance is formed, for example melting of ice. A chemical change is a change in which one or more new substances are formed, for example rusting of iron or burning of paper.

Explanation

In a physical change, the particles of the substance stay the same; they are only rearranged. When ice melts, it becomes liquid water, and freezing turns it back into ice. Dissolving salt in water, folding paper or stretching a rubber band are also physical changes. Many physical changes can be reversed, but not all: a torn sheet of paper cannot be joined back, yet it is still paper.

In a chemical change, substances react and new substances with different properties are formed. Common signs are a change of colour, release of a gas, production of heat or light, or a new smell. When a little baking soda is added to vinegar, bubbles of carbon dioxide form; passing this gas through lime water turns it milky. Curdling of milk into curd, cooking food and burning paper are chemical changes.

Rusting is a common chemical change around us. When iron is left in moist air, it reacts with oxygen and water to form a brown, flaky substance called rust. Rust is not iron; it is a new substance. Painting, greasing or galvanising, which means coating iron with zinc, keeps air and moisture away and prevents rusting. Most chemical changes cannot be reversed easily.

Points to remember

  1. Physical change: only shape, size, colour or state changes and no new substance is formed.
  2. Chemical change: one or more new substances with new properties are formed.
  3. Signs of a chemical change: change in colour, gas given off, heat or light produced, new smell.
  4. Physical change examples: melting of ice, dissolving salt, tearing paper, stretching a rubber band.
  5. Chemical change examples: rusting of iron, burning of paper, curdling of milk, baking soda reacting with vinegar.

Exam tip

Write a one-line definition of each change, then make a two-column table with points like 'new substance formed?' and 'easily reversed?' plus two examples each. Add a word equation for one chemical change, such as vinegar + baking soda → carbon dioxide + other substances.

Questions students also ask

Is burning of a candle a physical or chemical change?
Both happen together. The wax melting near the flame is a physical change, because melted wax becomes solid again on cooling. The wax vapour burning in the flame is a chemical change, because it produces new substances, mainly carbon dioxide and water vapour, along with heat and light.
Is dissolving salt in water a physical change?
Yes. Salt spreads evenly through the water but no new substance is formed. The solution still tastes salty because the salt is present, and when the water is evaporated, the salt is left behind and can be recovered.
How can rusting of iron be prevented?
Rusting needs both oxygen and water (moisture). Keeping these away from iron stops rust from forming. Painting, applying grease or oil, and galvanising, which is coating iron with a layer of zinc, are common methods used for gates, bridges and tools.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.