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.
Understanding Physical vs Chemical Properties & Changes
At the particle level, a material has its own pattern of atoms, ions, or molecules. Its observable features come from that pattern. The spacing between particles affects how much space a sample occupies.
The strength of attraction between particles affects melting and boiling behavior. Electrical conductivity depends on whether charged particles can move. Some properties depend on sample amount.
A large block of aluminum has more mass than a small piece. Other properties stay the same for any pure sample under the same conditions.
This makes them useful for identifying an unknown material. Scientists compare several measurements because one feature alone, such as color, is rarely enough.
Changes can be examined by following the particles before and after an event. Crushing a crystal creates smaller pieces with more exposed surface, yet each piece contains the same kind of particles. Heating a liquid makes its particles move faster and spread farther apart.
In a mixture, substances can often be separated by physical methods. Filtration can remove sand from water. Evaporation can recover a dissolved solid from a solution.
These methods work because the substances keep their identities. A reaction is different because old particle groupings break apart and new groupings form. The products may have properties that neither starting material had.
Observations in an experiment need careful interpretation. Bubbles do not automatically prove a reaction. Water can bubble simply because it is boiling, and trapped air can escape from a solid.
A color change may come from mixing colored materials or from a change in lighting. Stronger evidence comes from a repeatable investigation that compares the starting materials with what remains afterward. Students can record temperature, mass, appearance, and whether a material can be separated using physical techniques.
Heating must be controlled because heat can cause either a state change or a reaction. The amount of energy transferred does not by itself decide the type of change.
Conservation ideas help explain confusing results. In an open container, a reaction may seem to lose mass when a gas leaves into the room. A process may seem to gain mass when a substance takes in a gas from the air.
Rust provides a useful example because iron combines with oxygen, so the rusted object can weigh more than the original iron. Measuring the whole closed container avoids this mistake.
In class, pay attention to the exact material being studied, the conditions of the test, and the boundary of the system. Good classification comes from evidence about particle identity, not from one dramatic observation.
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.