Why do some compounds and elements exist as a liquid at room temperature, while others are solids and gases? What forces are involved?
Intermolecular forces and surface area, I believe.
On the microscopic level, it is all due to the force of cohesive forces, such as https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/States_of_Matter/Properties_of_Liquids/Cohesive_and_Adhesive_Forces
The short version is...
From a physical perspective,, there are 3 principle forces (that we know of) in the universe that give rise to three types of particle-particle interactions (bonds) that hold matter together as we know it in the scientific community. These are nuclear bonds (proton-neutron interactions), chemical bonds (Ionic and Molecular/Covalent) and physical bonds (weak electrostatic +/- attractions). The physical bonds (with some chemical bond considerations) are responsible for the state of existence (solid, liquid, gas) of matter. I'm thinking 'Plasma' is not a part of this question.
Physical bonds are in themselves divided into three classes...
=> Dipole - Dipole Interactions
=> Hydrogen Bonding
=> London (or, Vanderwaal) Forces
All are electrostatic +/- interactions between the molecules (or, elements) making up the larger structure of the material. The attractive strength between the + and - fragments of molecules defines the state (s, l, g) of the material of interest.
Some examples ...
Water (H2O) is liquid at room temperature (25^oC) b/c the +/- fragments of the molecule have a high degree of affinity and exist very close together. At high temperatures (>100^oC where high degree of movement occurs) the +/- interactions are weakened and the molecules separate into gas (steam). At low temps (<0^oC where low degree of movement occurs) the +/- attractions are very strong and water becomes solid (ice). Other factors are also taken into account such as elements in the structure and geometry of the molecule, but all work symbiotically with respect to the atmospheric conditions around the substance.
Another example could be of interest => Why is Methane a gas at room temp whereas water is a liquid at room temp? Both have approximately the same molecular weight but one is a gas (Methane) and one is a liquid (water) at room temp.
Ans => Methane has little to no +/- attractive forces inherent in the molecular structure b/c of its symmetrical molecular geometry and all +/- forces cancel themselves out. So, methane, essentially is considered to be a 'non-polar' molecule and does not attract other methane molecules. At room temperature conditions the particles move about much like ping-pong balls move in a lotto globe. Water, because of its structural geometry forms what is called a 'dipole' with part of the molecule + and part -. This gives water the unique properties that we observe.
One last example... Carbon Dioxide (CO2) is a molecule having a linear geometry O=C=O but Carbon - Oxygen bonds are dipoles with +/- character. However, because of the linear geometry of the molecule the +/- attractive forces on one side of the molecule cancel out the +/- attractive forces on the other side of the molecule and the molecule exists in the gas phase at room temperature.
Hope this helps. Doc
Doc 48 gave a very comprehensive answer concerning intermolecular forces. The reason that I said surface area is because the exposure of surface area, depending on the types of intermolecular forces experienced between two molecules, can determine if a substance is a liquid or gas at a specific temperature. One substance can be a liquid at room temperature because of intermolecular forces, and another substance can become a gas at the same temperature. Two proteins that are made of the same number of amino acids but very differently in the types of amino acids can have boiling and melting points that are considerably different because of the intermolecular forces that they experience exposes varying amounts of surface area between the proteins.
Ohh okay, all these answers make sense. Thank you so much!
The different states of matter (solid, liquid, and gas) are determined by the arrangement and movement of their particles. The forces between particles play a crucial role in determining the state of a compound or element at a given temperature. Let's break it down further:
1. Solid: In a solid, particles are tightly packed and held together by strong forces of attraction called intermolecular forces. These forces prevent the particles from moving freely, resulting in a fixed shape and volume. The intermolecular forces in solids are usually strong, such as ionic bonds in sodium chloride (NaCl), covalent bonds in diamond (C), or metallic bonds in iron (Fe).
2. Liquid: In a liquid, particles are still attracted to one another, but the forces of attraction are weaker compared to solids. The particles have more freedom of movement, allowing them to flow and take the shape of their container. The intermolecular forces in liquids are generally weaker than those in solids. Examples include water (H2O), ethanol (C2H5OH), and mercury (Hg).
3. Gas: In a gas, the forces between particles are very weak compared to solids and liquids. The particles move freely, have high energy, and are not held together by strong forces. Gases fill their container completely and do not have a fixed shape or volume. Examples of gases include oxygen (O2), nitrogen (N2), and carbon dioxide (CO2).
The strength of intermolecular forces depends on factors such as the type of chemical bonding (ionic, covalent, metallic), polarity, molecular shape, and size of particles. Remember, intermolecular forces are not as strong as intramolecular forces (the forces that hold atoms together within a molecule).
To determine why specific compounds or elements exist in different states, it is important to consider their chemical composition and the intermolecular forces present. Experimental data, including melting and boiling points, can provide valuable information about a substance's physical state at different temperatures. Additionally, theoretical calculations and computer simulations can help predict the behavior of different materials in terms of their states of matter.