study-notes — Chemistry (Elements, Compounds and Mixtures)

LEARNING OBJECTIVES By the end of this unit, you should be able to: • Define what a physical change is. • Give examples of physical changes. • Define what a chemical change is. • Give examples of chemical changes. • Explain the differences between physical and chemical changes. CORE CONCEPTS PHYSICAL CHANGES A physical change is a change in the form or appearance of a substance, but not its chemical identity. This means that no new substance is formed. The substance remains the same, even though it might look different. Here are the main features of physical changes: • No new substance is formed: For example, when ice melts, it becomes water. Both ice and water are still H₂O. • Usually reversible: You can often change the substance back to its original form easily. For instance, you can freeze water back into ice. • Little or no energy change: Physical changes usually do not involve a large amount of heat or light being absorbed or given out. • Easy to separate: If a physical change involves mixing, the original substances can often be separated easily using physical methods like filtration or distillation. Examples of physical changes: • Melting ice: Ice changes to liquid water. • Boiling water: Liquid water changes to steam (water vapour). • Dissolving sugar in water: Sugar mixes with water, but it's still sugar and water. You can evaporate the water to get the sugar back. • Tearing paper: The paper is in smaller pieces, but it's still paper. • Bending a metal wire: The wire changes shape but is still the same metal.

PHYSICAL CHANGES OF WATER

CHEMICAL CHANGES A chemical change (also called a chemical reaction) is a process that involves the rearrangement of the structure of molecules or ions. This results in the formation of one or more new substances with different properties from the original substances. Here are the main features of chemical changes: • New substances are formed: The original substances are used up and completely new substances are made. For example, when wood burns, it turns into ash, smoke, and gases. • Usually irreversible: It is often very difficult or impossible to change the new substances back to the original ones using simple physical methods. You cannot turn ash back into wood. • Significant energy change: Chemical changes usually involve a noticeable amount of energy being absorbed (like in cooking) or released (like in burning) as heat, light, or sound. • Hard to separate: The new substances are chemically bonded and cannot be separated by simple physical means. Examples of chemical changes: • Burning wood: Wood reacts with oxygen to form ash, carbon dioxide, and water vapour. Heat and light are released. • Rusting of iron: Iron reacts with oxygen and water to form iron oxide (rust). • Cooking an egg: The proteins in the egg change permanently due to heat. • Souring of milk: Bacteria cause chemical reactions that change the milk's composition and taste. • Digestion of food: Food is broken down into simpler nutrients in your body.

CHEMICAL CHANGE: BURNING WOOD

DIFFERENCES BETWEEN PHYSICAL AND CHEMICAL CHANGES The table below summarises the key differences between physical and chemical changes.

Comparison of Physical and Chemical Changes

Feature	Physical Change	Chemical Change
New Substance	No new substance is formed.	New substance(s) with different properties are formed.
Reversibility	Easily reversible.	Often irreversible.
Energy Change	Small or no energy change.	Significant energy change (heat, light, sound).
Composition	Chemical composition remains the same.	Chemical composition changes.
Separation	Original substances can be separated by physical means.	Original substances cannot be separated by physical means.

Figure: Key differences between physical and chemical changes

SUMMARY • Physical changes alter a substance's appearance but not its chemical identity. No new substances are formed, and they are often reversible. Examples include melting, boiling, and dissolving. • Chemical changes result in new substances with different properties. They are usually irreversible and involve significant energy changes. Examples include burning, rusting, and cooking. PRACTICE QUESTIONS EASY 1. What is a physical change? 2. Give two examples of a physical change. 3. What is a chemical change? 4. Give two examples of a chemical change. MEDIUM 5. Classify each of the following as either a physical or a chemical change: a) Cutting vegetables b) Baking bread c) Evaporation of water from a pot d) Digesting your food e) Lighting a match HARD 6. Explain why dissolving salt in water is a physical change, while burning paper is a chemical change. 7. Imagine you mix sand and iron filings. Is this a physical or chemical change? How would you separate the sand from the iron filings? ANSWERS 1. A physical change is a change in the form or appearance of a substance, but no new substance is formed. 2. Melting ice, boiling water, dissolving sugar, tearing paper. (Any two correct examples) 3. A chemical change is a process where new substances with different properties are formed. 4. Burning wood, rusting iron, cooking an egg, souring milk. (Any two correct examples) 5. a) Physical change b) Chemical change c) Physical change d) Chemical change e) Chemical change 6. Dissolving salt in water is a physical change because the salt and water keep their chemical identities. You can evaporate the water to get the salt back. Burning paper is a chemical change because the paper turns into ash and smoke, which are completely new substances. You cannot turn ash back into paper. 7. Mixing sand and iron filings is a physical change. You can separate the iron filings from the sand using a magnet because iron is magnetic and sand is not. COMMON MISTAKES TO AVOID • Confusing dissolving with chemical reaction: Not all dissolving is a chemical change. For example, sugar dissolving in water is physical. A chemical change happens if you see bubbles (gas formed), a new solid (precipitate), or a big temperature change. • Thinking all changes are reversible: While many physical changes are reversible, some might be difficult to reverse (like tearing paper into very tiny pieces). However, the key is still that no new substance is formed. • Ignoring the formation of new substances: Always ask yourself: "Is a completely new material created?" If yes, it's chemical. If it's just a different form of the same material, it's physical. EXAM TIPS • Look for clues: In an exam, look for words like "burns," "rots," "rusts," "cooks," "explodes" – these usually mean a chemical change. Words like "melts," "freezes," "dissolves," "boils," "cuts," "bends" usually mean a physical change. • Consider reversibility: Can you easily get the original substance back? If yes, it's likely physical. If no, it's likely chemical. • Think about new properties: Do the substances after the change have completely different properties (like colour, smell, state) from the original ones? If yes, it's chemical. QUICK REVISION SUMMARY • Physical Change: Changes form, not identity. No new substance. Often reversible. Small energy change. E.g., melting ice. • Chemical Change: Forms new substances. Changes identity. Usually irreversible. Big energy change. E.g., burning wood. • The main difference is whether a new substance is formed.

study-notes — Chemistry (Elements, Compounds and Mixtures)

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