A liquid represents one of the fundamental states of matter, characterized by particles that possess the ability to flow. While maintaining a definite volume, a liquid lacks a fixed shape. These liquids are composed of atoms or molecules held together by intermolecular bonds. Water, the most prevalent liquid on Earth, covers a significant portion of the surface in its liquid state. However, this state occurs only within the temperature range of 0 degrees Celsius (32 degrees Fahrenheit) to 100 degrees Celsius (212 degrees Fahrenheit). Below this range, water transforms into a solid state, forming ice, while at higher temperatures, it transitions into a gaseous state as water vapor.
It is crucial to note that, despite existing in different states, the molecular structure of water remains consistent. Whether as a liquid, solid (ice), or gas (water vapor), the substance remains water. Nevertheless, it’s essential to clarify that frozen water and water vapor do not fall under the category of liquids, as they are distinct states of matter.
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What is Liquid State?
A liquid state refers to one of the primary phases of matter, characterized by a substance composed of small particles, commonly known as molecules. In this state, the intermolecular forces of attraction between these molecules exhibit a notable strength. Unlike gases, the molecules in a liquid are in relatively close proximity to each other. Despite their closeness, these molecules maintain constant, random motion within the liquid.
Heating a liquid results in an increase in the kinetic energy of its molecular entities. If the temperature rises sufficiently, the liquid undergoes a phase transition to become a gas or may react chemically with the surrounding environment. For instance, gradual heating can cause water to transition to a gaseous state, while sudden and intense heating may lead to the combustion of alcohol when combined with oxygen. Conversely, cooling a liquid reduces the kinetic energy of its molecular entities, potentially causing it to solidify when the temperature drops significantly.
Characteristics of Liquid
Adaptable Shape: Liquids don’t have a fixed shape and take on the shape of the container they’re in. Weak interactions between particles allow them to slide past each other, giving liquids their flexibility.
Constant Volume: Despite their shape-shifting ability, liquids have a fixed volume similar to solids. They resist compression because strong forces between particles prevent them from being squeezed, ensuring their volume remains unchanged.
Flowing Flexibility: Liquids flow instead of staying rigid, a property known as fluidity. The speed at which liquids flow varies based on the strength of their intermolecular forces. For example, water flows more easily than denser substances like honey.
Lighter Density: Liquids are often less dense than solids. This is due to the liquid form having more gaps between particles compared to the solid state. For instance, ice floats on water despite being chemically identical; ice’s porous structure gives it lower density.
Higher Kinetic Energy: Particles in liquids are less tightly packed than in solids, resulting in weaker forces between them. As a result, liquid particles have higher kinetic energy than their solid counterparts. This energy increases with rising temperatures.
Easy Particle Movement: Liquid particles can disperse easily due to weaker intermolecular forces. This property allows different liquids, such as water and alcohol, to mix seamlessly, forming liquid mixtures or solutions. Changes in a liquid’s physical conditions can alter its fundamental nature.
Properties of Liquid
Evaporation
Process: Evaporation is the transition of a liquid into vapors, occurring as molecules with higher kinetic energy overcome intermolecular forces and turn into vapor.
Influencing Factors:
- The liquid’s nature determines the evaporation rate; stronger intermolecular forces slow it down.
- Higher temperatures accelerate evaporation.
- A larger surface area speeds up the evaporation process.
Vapour Pressure
Vapour pressure, a result of kinetic processes, is the pressure exerted by vapors in equilibrium with a liquid at a specific temperature, dependent solely on the liquid’s temperature.
Influencing Factors:
- Weaker intermolecular forces in the liquid result in higher equilibrium vapour pressure.
- Vapour pressure increases with rising liquid temperature.
Boiling Point
The boiling point is the temperature at which a liquid’s vapour pressure matches the surrounding air pressure. External pressure and intermolecular forces significantly impact this property.
Factors Influencing Boiling Point:
- External pressure variations affect the boiling point; lower pressure leads to a reduced boiling point.
- The intensity of intermolecular forces influences the boiling point.
Surface Tension
Surface tension arises from the inward pull on a liquid’s surface molecules, creating a stretched membrane effect.
Factors Affecting Surface Tension:
- Surface tension decreases with increasing temperature due to heightened kinetic energy.
- The liquid’s intermolecular forces govern surface tension; higher forces result in increased tension.
Viscosity
Viscosity is a liquid’s internal resistance to flow, influenced by various intermolecular forces present in different liquids.
Factors Controlling Viscosity:
- Viscosity decreases with rising temperatures.
- Liquids with stronger intermolecular forces exhibit higher viscosity.
- Molecular mass directly influences viscosity; greater mass correlates to higher viscosity.
Physical Properties of Liquid
Shape Conformation and Volume Retention:
- Liquids exhibit the distinct physical property of adapting to the shape of their container, making them take on the vessel’s form.
- Despite this adaptability, liquids retain their volume, ensuring that the quantity of the substance remains constant unless affected by factors like vaporization or temperature changes.
Pouring Characteristics:
- When poured into a new container, a liquid will seamlessly adjust to the shape of the vessel, emphasizing its fluid nature.
- The volume of the liquid remains unchanged during pouring, provided there is no vaporization or alteration in temperature.
Distinguishing Criteria:
- The ability of liquids to retain both volume and adapt to container shape serves as practical criteria to distinguish them from solids and gases.
- Solids maintain both shape and volume when transferred between containers, while gases expand to fill their container, maintaining an equivalent volume.
Categories of Liquids:
- Liquids can be categorized into pure liquids and liquid mixtures, with water being a prevalent example.
- Liquid mixtures, such as blood and seawater, involve dissolved substances, showcasing the diversity of compositions in liquid states.
Complexity in Classification:
- Distinction between solid, liquid, and gas becomes challenging for substances with a high number of atoms, exceeding about 20.
- Large molecules may form glasses at temperatures below the typical melting point, blurring the traditional boundaries between solid and liquid.
Liquid Crystal States:
- Large, rigid, and planar or linear molecules may give rise to liquid crystal states, characterized by anisotropic properties.
- Liquid crystals, like cholesteryl acetate, exhibit unique optical effects due to changes in refractive index, finding applications in various technologies.
Spatial Order and Density:
- Liquids lack the strong spatial order of solids but share the high density of solids, setting them apart from the more disorderly and low-density nature of gases.
Challenges in Theoretical Understanding:
- Quantitative theories of liquids pose challenges due to the combination of high density and partial order in their molecular arrangement.
- The kinetic molecular theory, which describes matter as particles in constant motion driven by thermal energy, contributes to the understanding of the liquid state.
Examples of Liquid
- Water
- Oil
- Blood
- Mercury
- Milk
- Coffee
- Vinegar
- Gasoline
- Soft drinks
- Juice
- Alcohol
- Liquid dishwashing detergent
- Magma
- Household bleach
- Bromine
- Acetone
- Honey
- Lubricating oil
- Perfume
- Maple syrup
- Shampoo
- Ink
- Liquid chocolate
- Lemonade
Transitions Between States of Matter
Particle Behavior in Gases: In gases, particles like molecules and atoms are widely spaced and move rapidly, exhibiting various influences such as electrical charges and attractive or repulsive forces. Particle motion is linear, and collisions are elastic, preserving energy.
Cooling of Gases: Cooling a gas slows down particle movement. Particles with lowered kinetic energy coalesce due to attractive forces, forming a liquid. This transition releases latent heat of liquefaction.
Liquid State Characteristics: In the liquid state, particles move at the same speed, and distances between them are molecular in scale. Further cooling leads to decreased particle speed until the liquid solidifies into a denser state at the freezing temperature, releasing latent heat of fusion.
Solid-to-Liquid-to-Gas Transitions: Heating a solid provides heat of fusion, allowing particles to move into the liquid state. Additional heating supplies heat of evaporation, enabling particles to enter the gaseous state by overcoming mutual influences.
Average Kinetic Energy Consideration: The simplified view accounts for average kinetic energy, recognizing that particles in gases, liquids, and solids may have different speeds. It emphasizes that the motion is random.
Complexities and Equilibrium: At interfaces between liquid and gas or liquid and solid, particle exchange occurs. An equilibrium state is achieved in a closed system, ensuring an equal exchange of particles in both directions.
Statistical Approach to Kinetic Energy: Due to the varying speeds of particles, discussions about the liquid state involve statistical formulations, acknowledging the probabilistic nature of kinetic energy values.
Dynamic Exchange at Interfaces: At the interface between liquid and gas or liquid and solid, a continuous exchange of particles takes place. Slow gas molecules condense at the liquid surface, while fast liquid molecules escape into the gas, establishing equilibrium.
Misconception about Liquids
- Misconception: All liquids contain water.
Clarification: Liquids like oil and mercury don’t contain water but are still liquids. Wetting a surface doesn’t necessitate water; for instance, rubbing alcohol wets the skin before evaporating.
- Misconception: Soft substances are always liquids.
Clarification: Hardness isn’t indicative of a substance’s state. A bar of chocolate is solid but malleable, transforming into a liquid when melted. Items like pillows and fabrics, though compressible, contain solid fibers with fixed arrangements.
- Misconception: Sand and powders are liquids since they take the shape of their container.
Clarification: Despite conforming to containers, sand and powders are composed of tiny solid particles. Analogously, hard candies, although solid, can fill a container and be poured out.
- Misconception: Gels, oobleck, slime, clay, and mayonnaise neatly fit into either the liquid or solid category.
Clarification: These substances exhibit characteristics of both states, making them poor examples for classification. Gels and mayonnaise, for instance, may display liquid or solid properties based on conditions, defying straightforward categorization.
References
- https://www.britannica.com/science/liquid-state-of-matter
- https://www.techtarget.com/whatis/definition/liquid
- https://www.livescience.com/46972-liquids.html
- Helmenstine, Anne Marie, Ph.D. “Liquid Definition in Chemistry.” ThoughtCo, Apr. 5, 2023, thoughtco.com/definition-of-liquid-604558.
- https://www.embibe.com/exams/liquid-state/
- https://www.geeksforgeeks.org/liquid-state-of-matter/
- https://sciencenotes.org/liquid-definition-examples-of-liquids/
How do we define temperature in a liquid? What is the internal energy of a liquid?
Or how do we approach it?
What is the average velocity of molecules in water compared to a gas?
And do you know why most physicists believe that saturated air is never reached in manner of the absolute zero K temperature?
While in fact saturated air + water is a stable equilibrium as it maximizes entropy for a given amount of enthalpy.
Is it because we never got beyond the Aristotle description of clouds? That says when warm humid air rises, and cools down, the max water content of the air is reached and it starts to rain.
While actually the air expands and cools down, and if the air is humid enough it gets over saturated. If there are nucleation sites droplets will form but it takes a while because of diffusion of water molecules to water droplet and heat flow (heat of condensation) away from the droplet. As the cloud, on a larger scale, does not cool down and keeps at the same temperature because of the condensation (jus tlike we usually say condensation occurs at constant temp, but in reality it gets a bit over saturated to drive the condensation), then the cloud can rise higher and higher, all the time condensing out water vapour to compensate for the work done by expansion of the cloud. This assures that the cloud remains lighter than the surrounding air and it will further rise (air cools down because the pressure is Lower higher up, not because it is colder higher up.).
Seems like everyone is stuck on the weird belief that a relative humidity of 90% is ridiculously high.
And yeah, then you will find that droplets with corona always evaporate, and hence that we don’t understand airborne contagion of corona as dried out corona particles can’t be infectious on microbiologist considerations.
And the issue is: physicists themselves don’t understand that air is not always under saturated.
Now lung physiologists have learnt that exhaled air is 100% saturated. That the relative humidity is 100% in exhaled air.
So definitely, in exhaled air, one will not have ‘droplet nuclei’ the dried remnants of droplets with corona. Dried droplet nuclei form the heart of the explanation of airborne contagion.
Why don’t engineers and physicists explain to medical doctors that in saturated air, droplets won’t evaporate and hence that the current theory that ‘ dried droplet nuclei’ are the agents of dessimination of airborne deseases is not necessarily true.
the issue is: physicists themselves don’t understand that air is not always under – saturated. Somehow, all of us think that Relative humidity is 90% at most.
What needs to be stressed is that there are two situations: one in which tiny dried droplets in unsaturated air dry almost immediately to form dried droplet nuclei and one situation in which the air is satiated containing very tiny non-dried droplets with corona that remain in air for longer periods and distances.
That the latter situation exists, is not what many people know.
But exhaled air is saturated, air above the sea is saturated and air on the brink of condensing out dew droplets is saitrated (or better slightly over saturated). And the latter happens almost every night.
As our breath is saturated at 37 C it actually contains 4 times as much water vapour as air at 17 C.
So air close to saturation due to cooling (night, cold meat plant, etc) mixed with (a lot of) exhaled air for ( prolonged) times, will also lead to saturated air.
Ventilation is therefore understandingly effective, as long as we are not ventilating with saturated air. Which may happen in the rainy season or autumn. Thinks churches made of stone in which people keep their coat on.