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Physical vs Chemical Properties & Changes
Physical and Chemical Properties
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Matter can be described by its properties and by the changes it goes through. Physical properties can be observed or measured without making a new substance, such as color, mass, density, and melting point. Chemical properties describe how a substance can react to form new substances, such as flammability or reaction with acid. Knowing the difference helps students classify changes they see in labs, kitchens, weather, and everyday materials.
A physical change changes the form, size, shape, or state of matter, but the particles remain the same substance. A chemical change rearranges atoms into new substances with different properties. Evidence such as gas bubbles, color change, temperature change, light, odor, or a solid forming can suggest a chemical reaction, but the best test is whether a new substance forms. In both physical and chemical changes, mass is conserved when the system is closed.
Key Facts
- Density is a physical property: density = mass ÷ volume or ρ = m/V.
- Physical changes do not create a new substance, such as melting ice, dissolving sugar, or cutting paper.
- Chemical changes create new substances, such as burning wood, rusting iron, or baking a cake.
- Mass is conserved in a closed system: mass of reactants = mass of products.
- A change of state is physical: solid ⇄ liquid ⇄ gas.
- Signs of chemical change can include gas formation, precipitate formation, color change, energy change, odor, or light.
Vocabulary
- Physical property
- A characteristic of matter that can be observed or measured without changing the substance into something new.
- Chemical property
- A characteristic that describes a substance's ability to react and form new substances.
- Physical change
- A change in the form, size, shape, or state of matter that does not create a new substance.
- Chemical change
- A change in which atoms are rearranged and one or more new substances form.
- Precipitate
- A solid that forms when two solutions react chemically.
Common Mistakes to Avoid
- Calling every color change a chemical change is wrong because some color changes are physical, such as mixing dyes or shining colored light through a liquid.
- Thinking dissolving always creates a new substance is wrong because dissolving sugar in water is usually a physical change and the sugar molecules remain sugar.
- Using bubbles alone as proof of a chemical reaction is wrong because boiling water makes bubbles of water vapor without forming a new substance.
- Forgetting the closed system in mass conservation is wrong because mass may appear to decrease if gas escapes into the air during a reaction.
Practice Questions
- 1 A metal cube has a mass of 54 g and a volume of 20 cm3. Calculate its density using ρ = m/V, and identify whether density is a physical or chemical property.
- 2 In a closed flask, 10 g of vinegar reacts with 4 g of baking soda. If 6 g of liquid remains, what mass of gas must be present in the flask?
- 3 A student tears paper into pieces, then burns another sheet of paper. Explain which change is physical, which is chemical, and how you know.