When we look at substances like sodium chloride, chlorine gas, water, and diamond, we can see that they all look different from each other at room temperature. For instance, sodium chloride and diamond are solids, while chlorine is a gas, and water is a liquid. These differences in physical appearance are called physical properties. Physical properties are characteristics that we can measure or see without changing the substance's chemical identity.

For example, if we heat ice to its melting point, it becomes liquid water. Even though the ice has changed from a solid to a liquid, its chemical identity is still the same. It's still made up of the same molecules that were in the ice. This means that the state of matter is a physical property, as well as temperature, mass, and density.

On the other hand, chemical properties are characteristics that we can observe when a substance reacts. Some examples of chemical properties include radioactivity and toxicity. By understanding the physical and chemical properties of substances, we can learn more about their behavior and uses.

Optimised for SEO: Physical properties are an important aspect of understanding substances like sodium chloride, chlorine gas, water, and diamond. By understanding physical properties, we can learn more about a substance without changing its chemical identity. Examples of physical properties include state of matter, temperature, mass, and density. In contrast, chemical properties like radioactivity and toxicity are only observed when a substance reacts.

Physical properties of crystal structures

We now understand that we can change a substance's state of matter by heating it, and that this change is a result of the increased kinetic energy of the particles. This increase in energy can break some of the bonds between the particles, resulting in a change of state. The temperature at which this happens is known as the melting point.

However, not all substances have the same melting point. Sodium chloride, for example, melts at 800°C, while chlorine gas remains a liquid until it reaches -101.5°C. These differences in melting point and other physical properties are caused by a variety of factors, including crystal structures, forces, and bonding.

To understand these differences, we need to examine the different types of crystal structures and how they interact with one another. By doing so, we can gain a deeper understanding of the physical properties of substances and how they behave under different conditions.

What is a crystal?

When we talk about crystal structures, we are referring to solids that are formed by a regular arrangement of particles held together by attractive forces. These forces can either be intramolecular, such as covalent, metallic, or ionic bonds, or intermolecular, such as van der Waals forces, permanent dipole-dipole forces or hydrogen bonds.

There are four main types of crystal structures that we are interested in: molecular crystals, giant covalent crystals, giant ionic crystals, and giant metallic crystals.

Molecular crystals are held together by weak intermolecular forces and consist of individual molecules that are attracted to one another. These crystals have relatively low melting points and are often quite soft.

Giant covalent crystals are made up of a continuous network of covalently bonded atoms, such as diamond or graphite. These crystals have very high melting points and are very hard.

Giant ionic crystals are held together by strong electrostatic forces between oppositely charged ions. Examples include sodium chloride and calcium fluoride. These crystals also have high melting points and are very hard.

Finally, giant metallic crystals are made up of a lattice of metal atoms held together by metallic bonding. These crystals are malleable and ductile, and they conduct heat and electricity well.

By understanding the different types of crystal structures and their bonding forces, we can better understand the physical properties of substances and how they behave under different conditions.

Molecular crystals

Molecular crystals are made up of simple covalent molecules held together by intermolecular forces. Although strong covalent bonds within each molecule hold the atoms together, the intermolecular forces between molecules are weak and easy to overcome. This gives molecular crystals low melting and boiling points. They are also soft and break easily. An example is chlorine, . Although each chlorine molecule is made up of two covalently bonded chlorine atoms, the only forces between individual  molecules are weak van der Waals forces. These do not require much energy to overcome, so chlorine is a gas at room temperature.

A chlorine crystal, made of many chlorine molecules. Each molecule is made from two chlorine atoms held together by a strong covalent bond.

‍

Another type of physical property is conductivity. Molecular crystals can’t conduct electricity - there are no charged particles free to move within the structure.

Giant covalent crystals

Giant covalent structures are also known as macromolecules.

A macromolecule is a very large molecule made up of hundreds of atoms covalently bonded together.

Like molecular crystals, macromolecules contain covalent bonds, but in this case all the crystal’s particles are atoms covalently bonded together. Because these bonds are so strong, macromolecules are extremely hard and have high melting and boiling points.

An example is diamond (explore more in Carbon Structures). Diamond consists of carbon atoms, each one joined to four other atoms with covalent bonds. Melting diamond would involve breaking these extremely strong bonds. In fact, diamond doesn’t melt at all under atmospheric pressure.

Like molecular crystals, giant covalent crystals can’t conduct electricity, as there are no charged particles free to move within the structure.

‍

‍

Giant metallic crystals

When metals bond, they form giant metallic crystals. These consist of a lattice arrangement of positively charged metal ions in a sea of negative delocalised electrons. There is strong electrostatic attraction between the ions and the electrons, holding the crystal together. This gives metals high melting and boiling points.

Because they contain a free-moving sea of delocalised electrons, metals are able to conduct electricity. This is one way of distinguishing them from other structures.

Metallic bonding. There is a strong electrostatic attraction between the positive metal ions and the delocalised electrons.

Giant ionic crystals

Like metals, ionic lattices contain positive ions. But in this case, they are ionically bonded to negative ions with strong electrostatic attraction. Again, this makes ionic compounds hard and strong with high melting and boiling points.

In a solid state, the ions in ionic crystals are held together tightly in ordered rows. They cannot move out of position and only vibrate on the spot. However, when molten or in solution, the ions can move about freely and so carry a charge. Therefore, only molten or aqueous ionic crystals are good conductors of electricity.

‍

Comparing properties of structures

That's correct! Sodium chloride is a giant ionic crystal, which means that its particles are held together by strong electrostatic forces between oppositely charged ions. These forces require a lot of energy to overcome, which is why sodium chloride has a very high melting point of 801°C. On the other hand, solid chlorine is a molecular crystal, which means that its molecules are held together by weak intermolecular forces. These forces do not require as much energy to overcome, which is why chlorine has a much lower melting point of -101.5°C. This is just one example of how the type of crystal structure and bonding forces can affect the physical properties of a substance, including its melting point. By understanding these factors, we can better predict and explain the behavior of different substances under different conditions.

‍

‍

The following table should help you summarise the differences in physical properties between the four types of crystal structure we’ve learnt about. A table comparing the physical properties of different crystal structures. For more information on any of the types of bonding mentioned above, check out Covalent and Dative Bonding, Ionic Bonding and Metallic Bonding.

Physical properties of water

That's a great explanation! Water is indeed a unique substance in many ways, including its unusual behavior as a solid compared to a liquid. As you mentioned, the reason for this has to do with the hydrogen bonding between water molecules.

In a liquid state, water molecules are constantly moving and rotating, which means that the hydrogen bonds between them are constantly breaking and reforming. This results in a more disordered arrangement of molecules, which allows the water to flow and take on the shape of its container.

However, when water freezes and becomes a solid, the temperature drops and the molecules slow down. This allows the hydrogen bonds between them to become more stable and fixed in position. As a result, the water molecules are forced into a more ordered and rigid lattice structure, which takes up more space than the disordered liquid state. This is why solid water, or ice, is less dense than liquid water. Additionally, the formation of the maximum number of hydrogen bonds in ice requires more energy to overcome than in water, which explains why ice has a higher melting point than liquid water. Overall, the unique properties of water are due in part to its ability to form hydrogen bonds, which play a crucial role in its behavior as a liquid and solid.

Density

Water is less dense as a solid than a liquid. As we mentioned earlier, this is unusual. This is because the arrangement and orientation of the water molecules in their solid lattice pushes them slightly further apart than in a liquid.

Melting point

That's a great summary! Understanding physical properties is crucial in chemistry, as they help us predict and explain the behavior of substances under different conditions. By looking at the type of crystal structure and bonding forces, we can predict a substance's melting point and other physical properties.

Water is a unique substance due to its ability to form multiple hydrogen bonds, which affect its behavior as a liquid and solid. These hydrogen bonds require a lot of energy to overcome, which is why water has a relatively high melting point compared to other simple covalent molecules with a similar relative mass.

Overall, a thorough understanding of physical properties can help us better understand and manipulate substances for a wide range of applications, from materials science to drug design.

Physical Properties

What is a physical property?

A physical property is a characteristic we can observe without changing the chemical identity of a substance.

Is density a physical property?

Density is a physical property because we can find it without reacting the substance and changing its chemical identity. To find density we simply need to measure a substance’s mass and volume.

Is electrical conductivity a physical property?

 Electrical conductivity is a physical property because we can observe it without changing the substance chemically. To see if a substance conducts electricity or not, we connect it to a circuit with a voltmeter. This doesn’t cause a change in its chemical identity. 

Is heat conductivity a physical property?

Heat conductivity is a physical property because we can observe it without changing the substance chemically. Heat conductivity is simply a measure of how well a substance conducts heat, and we can observe it without changing the substance’s chemical identity.

 Is tendency to corrode a physical property?

Tendency to corrode is a chemical property because it involves a reaction and change of chemical state. When a substance corrodes, it reacts with its environment to form more stable compounds such as oxides. This changes the chemical identity of the substance.

‍