What are physical properties and chemical properties?

Question

As the question title says, I would like to know what the physical and chemical properties are. If possible, please include examples. Since it seems to be related, I would also like to know what physical and chemical changes are. I believe that things such as crushing a can is a physical change, but I am not sure what would constitute a chemical change (perhaps doing something that rearranges the molecules?)

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Edit: I originally wanted to ask if cooking an egg constitutes as a chemical or physical change, but I thought that that would be closed as too vague. If possible, please explain which one it is too. Thanks!

Answer 1

This is a tricky question.

I have to concur with Paul - it's difficult to give a truly hard and fast definition of the distinction between a chemical change and a physical change as there is a fairly smooth continuum between 'physical' and 'chemical' interactions. However, a good rule of thumb is that chemical changes involve changes in electronic structure, usually in the form of chemical bonding or redox. The problem is that traditionally 'physical' properties may have big implications for chemistry - I'll give some examples.

The Van der Waals and dipole-dipole interactions are mediated respectively by 'instantaneous' and permanent polarisations of electron density with respect to positive charges. Dipole-dipole interactions such as those found in water are quite significant and give rise, for instance, to water's frankly freaky liquid range as well as water's ability to mediate a wide range of chemical processes. These interactions don't involve significant changes in electronic structure however they have chemical implications in, for example, making a choice between liquid water or liquid nitrogen as a solvent.

The atomic mass (and hence the isotope) of an element is typically considered a physical property, however it has implications for reaction kinetics. This can be illustrated (expensively) by giving living organisms nothing but heavy water to drink. They will eventually experience toxicity due to the slowing of key biochemical reactions.

The magnetic properties of an atomic nucleus are typically considered to be physical, however they can be exploited in the observation that e.g. different isotopes of uranium have slightly differing hyperfine splittings and hence different ionisation energies. This allows isotopes to be enriched by selective ionisation.

Regarding the egg:

The dramatic changes that occur when you cook an egg are the result of denaturation of the proteins in the egg. Proteins are long chains of amino acids that take on their characteristic active conformations through a process called 'folding', so called because that's what it looks like (if you've heard of Folding@Home, that simulates the folding of proteins). If you raise the temperature much higher than physiological temperature, the protein degrades through unfolding into a different conformation. This involves the Van der Waals and dipole-dipole interactions holding together the secondary/tertiary/quaternary structure of the protein being broken en mass as well as the formation and/or scission of covalent bonds.

Answer 2

Paul's comment and Richard's answer are correct that that chemical and physical change span a continuum. However, at the introductory level, there is a difference between physical and chemical properties.

Physical changes do not involved the conversion of one substance into another. They can be reversible (ice melting and freezing at 0 °C) or essentially irreversible (free expansion of a gas).

Chemical changes are those that involve the conversion of one substance into another. They can be reversible (acid-base equilibria) or essentially irreversible under normal conditions (combustion).

Physical Properties are those properties of a substance or mixture than seem to match with physical descriptions of objects and with the physical changes, especially changes of state.

Intensive physical properties include such characteristic data as densities, specific heat capacities, boiling points, freezing points, Henry's Law constants, etc.

Extensive physical properties vary with the sample (are not characteristic to the substance): temperature, pressure, volume, mass, size, shape, etc.

Chemical Properties are those properties which, as Richard mentions, seem to have to do with electronic transitions (aka chemical changes). These include all of the known reaction, reactivity, thermochemistry, kinetics, and equillibrium data.

In addition, (and non-intuitively) most of the properties of substances and mixture that interact with our five senses are also chemical properties: hardness, color, taste, smell, etc.

Finally, the following properties (and others like them) are also chemical properties: toxicity, flammability, conductivity, nutritional value, etc.

Answer 3

Physical changes are reversible, whereas chemical changes are irreversible. For example, boiling water is a physical change because you can reverse it by cooling it down, but setting fire to wood or boiling an egg cannot be undone.