Liquid Definition in Chemistry: Properties & Examples

A liquid is one of the most familiar states of matter, yet its behavior is more interesting than it may appear in everyday life. Water flowing from a faucet, cooking oil poured into a pan, and gasoline moving through a fuel system are all examples of liquids. In chemistry, however, the definition of a liquid goes beyond simply describing something that can be poured. Liquids have specific molecular arrangements, physical properties, and behaviors that distinguish them from solids, gases, and plasma. Understanding these characteristics helps explain concepts such as density, viscosity, surface tension, evaporation, and phase changes. These ideas are important in chemistry, physics, engineering, biology, medicine, and many everyday activities.

The scientific study of liquids focuses heavily on how their particles interact and respond to temperature, pressure, and surrounding forces. Unlike particles in a solid, liquid particles are not locked into fixed positions, but they remain much closer together than particles in a gas. This combination allows a liquid to maintain a relatively constant volume while changing its shape to match its container. Different liquids also behave differently depending on their molecular structure and intermolecular forces. Some flow quickly, while others are thick and resistant to movement. Learning the liquid definition in chemistry provides a foundation for understanding everything from boiling water to industrial chemical processing.

What Is a Liquid in Chemistry?

A liquid is a state of matter that has a definite volume but does not have a fixed shape. Instead of maintaining its own shape, a liquid takes the shape of the container in which it is placed. The particles in a liquid are packed relatively close together, but they can move and slide past one another. This movement gives liquids their ability to flow while still remaining much denser than most gases. In chemistry, liquids are considered condensed phases because their particles remain close due to attractive intermolecular forces. Water at normal room temperature is one of the most common examples used to explain the properties of liquids.

The definition of liquid becomes clearer when its behavior is compared with other states of matter. A solid has both a fixed shape and a fixed volume because its particles remain in relatively stable positions. A gas has neither a fixed shape nor a fixed volume because its particles move freely and spread throughout available space. A liquid exists between these two states in terms of particle freedom and organization. Its molecules have enough energy to move around but generally not enough energy to separate completely from one another. This balance between molecular attraction and kinetic energy gives liquids many of their recognizable physical characteristics.

The particles that make up a liquid are constantly moving even when the liquid appears completely still. Molecules rotate, vibrate, collide, and change positions as thermal energy keeps them in motion. At the same time, intermolecular attractions prevent most of those molecules from escaping the liquid under ordinary conditions. These attractions may include hydrogen bonding, dipole-dipole forces, or London dispersion forces depending on the substance. Stronger intermolecular forces usually influence characteristics such as boiling point, viscosity, and surface tension. The microscopic behavior of molecules therefore determines many properties that can be observed at the macroscopic level.

Liquids can consist of pure substances, mixtures, solutions, or more complicated combinations of chemical materials. Pure liquid water contains primarily H₂O molecules, while liquid ethanol consists primarily of ethanol molecules. A saltwater solution, by comparison, contains dissolved ions distributed throughout liquid water. Milk is an even more complex liquid system containing water, fats, proteins, minerals, and other substances. Although these materials have very different chemical compositions, they can all display typical liquid behavior under appropriate conditions. The term liquid therefore describes a physical state rather than a particular type of chemical substance.

Temperature and pressure play important roles in determining whether a substance exists as a liquid. Many materials become liquid only within a specific temperature range and under suitable pressure conditions. Water, for example, commonly remains liquid between its freezing and boiling points at standard atmospheric pressure. Cooling it enough causes it to solidify into ice, while heating it enough eventually produces water vapor. Other substances have completely different melting and boiling temperatures because of differences in molecular structure and intermolecular forces. For this reason, whether a substance is solid, liquid, or gas depends partly on the environmental conditions surrounding it.

Key Properties of Liquids

One of the most important properties of liquids is that they have a definite or nearly constant volume. When water is transferred from a glass into a wider bowl, its shape changes but the amount of space occupied by the water remains approximately the same. This happens because liquid particles are already positioned close together and cannot easily be compressed. Gases behave differently because large spaces between gas particles allow compression to occur much more readily. The low compressibility of liquids makes them useful in hydraulic equipment and many mechanical systems. This characteristic is also one reason liquids are generally categorized as condensed forms of matter.

Liquids also have the ability to flow, which is why both liquids and gases are often described as fluids. The particles in a liquid are not arranged in a rigid structure, allowing neighboring molecules to move around each other. Gravity, pressure differences, and external forces can therefore cause liquid material to move from one location to another. The rate at which this movement occurs varies significantly between different liquids. Water flows relatively easily, while substances such as honey or thick motor oil move much more slowly. This resistance to flowing is measured through an important physical property known as viscosity.

Density is another major characteristic used to describe and compare liquids. Density represents the amount of mass contained within a certain volume of material and is commonly expressed in units such as grams per milliliter. Different liquids can have significantly different densities because of variations in molecular mass and particle arrangement. Oil usually floats on water because many common oils have lower densities than water and do not readily mix with it. Mercury, in contrast, is an unusually dense liquid under ordinary conditions. Understanding liquid density is useful in chemistry experiments, engineering calculations, manufacturing processes, and many everyday applications.

Surface tension is a distinctive liquid property created by attractive forces between molecules near the liquid’s surface. Molecules inside a liquid are pulled in many directions by surrounding particles, while surface molecules experience a less balanced set of attractions. This difference causes the surface to behave somewhat like a stretched elastic layer. Surface tension helps explain why small water droplets tend to form rounded shapes and why certain lightweight objects can sometimes remain on water without immediately sinking. The strength of surface tension varies depending on the liquid and environmental conditions. Temperature and dissolved substances can also change how strongly surface molecules attract one another.

Liquids also display a phenomenon known as capillary action, which allows them to move through narrow spaces without being pushed by an obvious external force. Capillary behavior results from a combination of cohesive forces between liquid molecules and adhesive forces between the liquid and another surface. Water moving upward through narrow plant tissues is a familiar example of this process. Similar behavior can occur when a paper towel absorbs spilled water or when liquid travels through a thin glass tube. The amount of capillary movement depends on properties such as surface tension, tube diameter, and molecular attraction. This behavior demonstrates how intermolecular forces can strongly influence visible liquid movement.

How Particles Behave in a Liquid

Particles in a liquid are positioned close together but are not organized into the rigid arrangement typically associated with crystalline solids. Their close spacing helps a liquid maintain a relatively stable volume even as its shape changes. At the molecular level, particles continually move into new positions while interacting with neighboring particles. They may slide around one another, rotate, vibrate, and collide as they respond to thermal energy. Because these motions occur constantly and randomly, a liquid can gradually redistribute itself throughout the bottom of a container. This molecular mobility is one of the major differences between liquids and most solid materials.

The amount of kinetic energy possessed by liquid particles depends strongly on temperature. When a liquid is heated, its molecules generally move faster because they gain kinetic energy. Faster molecular movement can weaken the relative influence of intermolecular attractions and make some physical processes occur more rapidly. For example, evaporation usually increases when the temperature of a liquid rises. Heating also tends to reduce the viscosity of many common liquids, allowing them to flow more easily. These effects show how changes at the particle level can create noticeable changes in the physical behavior of a liquid.

Intermolecular forces help hold liquid particles together even though the particles remain mobile. The type and strength of these forces depend on the chemical structure of the molecules involved. Water molecules experience strong hydrogen bonding, which contributes to water’s relatively high surface tension and boiling point for such a small molecule. Nonpolar liquid molecules may rely primarily on London dispersion forces, while polar substances can experience dipole-dipole attractions. Stronger intermolecular interactions often require more energy to separate molecules from one another. As a result, intermolecular forces influence boiling temperature, vapor pressure, viscosity, and several other measurable liquid properties.

Diffusion also occurs in liquids because particles are constantly moving and colliding. If a drop of food coloring is placed into still water, the color eventually spreads through the liquid even without mechanical stirring. Random molecular motion gradually moves particles from regions of higher concentration toward areas of lower concentration. Diffusion in liquids is generally slower than diffusion in gases because liquid particles are packed more closely and encounter greater resistance. Temperature can increase the rate of diffusion by increasing molecular movement. This process is important in chemistry, biological systems, food preparation, medicine, and environmental science.

Liquid particles can occasionally gain enough energy to overcome attractions and escape from the surface into the gas phase. This process is called evaporation and can occur at temperatures below the liquid’s boiling point. At any moment, different molecules within the same liquid possess slightly different kinetic energies. Molecules near the surface with sufficiently high energy may break away and enter the surrounding air as vapor. When high-energy molecules escape, the average kinetic energy of the remaining liquid can decrease, producing an evaporative cooling effect. Sweating helps cool the human body through a similar principle as liquid water evaporates from the skin.

Liquids vs Solids and Gases

The difference between solids, liquids, and gases begins with how their particles are arranged and how freely those particles can move. In a solid, atoms or molecules are generally held within relatively fixed positions by strong attractive forces or bonding structures. The particles may vibrate, but they do not normally move freely throughout the material. Liquid particles remain close together but can change positions and slide around their neighbors. Gas particles have much greater freedom and are usually separated by large distances compared with their particle size. These structural differences explain why the three common states of matter behave so differently.

Shape provides one of the easiest ways to distinguish a liquid from a solid. A solid object normally keeps its own shape unless enough force is applied to bend, break, melt, or otherwise deform it. A liquid does not maintain a permanent shape and instead conforms to the container holding it. Pouring water from a bottle into a bowl immediately changes the water’s shape without changing its chemical identity. The ability to change shape comes from the mobility of molecules within the liquid. Although liquids can flow and reshape themselves, their particles remain close enough for the liquid to retain a nearly constant volume.

Liquids differ from gases because they do not normally expand to fill every available part of a container. If a small amount of water is placed in a large sealed container, most of the water remains pooled at the bottom. A gas placed in the same container spreads throughout the available space because gas particles move widely and independently. Gases are also much more compressible because there are relatively large gaps between their particles. Compressing a liquid requires considerably greater pressure because its molecules are already close together. These differences are especially important in fluid mechanics, storage systems, industrial processes, and chemical equipment design.

Solids, liquids, and gases can often transform into one another through changes in temperature or pressure. When a solid absorbs enough energy, it may reach its melting point and become a liquid. Continued heating can eventually cause the liquid to boil and transform into a gas. Reversing the process can cause gas to condense into liquid and liquid to freeze into solid material. These phase transitions involve changes in particle energy, arrangement, and intermolecular interaction rather than necessarily changing chemical identity. Water demonstrates these changes clearly through ice, liquid water, and water vapor.

Some substances do not behave exactly like the simple solid-liquid-gas model might suggest. Glass, liquid crystals, supercritical fluids, gels, and other materials can display more complicated properties that fall between traditional classifications. Nevertheless, the three-state model remains extremely useful for introducing the behavior of ordinary matter. Liquids occupy a particularly interesting position because they combine the close particle spacing of solids with some of the mobility associated with gases. They can transport dissolved substances while maintaining significant density and cohesion. This combination makes liquids essential in biological organisms, chemical reactions, manufacturing, transportation, and many natural processes.

Common Examples of Liquids

Water is probably the most familiar example of a liquid and one of the most important substances studied in chemistry. At normal room conditions, water flows freely, takes the shape of its container, and maintains a relatively constant volume. Its polar molecular structure allows individual water molecules to form hydrogen bonds with neighboring molecules. These interactions contribute to water’s surface tension, heat capacity, boiling behavior, and ability to dissolve many substances. Because of its strong solvent properties, water plays an essential role in chemical reactions and biological processes. Oceans, rivers, beverages, cells, and laboratory solutions all demonstrate the widespread importance of liquid water.

Ethanol is another common liquid that appears in laboratories, cleaning products, fuels, manufacturing processes, and many commercial formulations. Its molecules contain both a polar hydroxyl group and a nonpolar carbon-based portion. This structure gives ethanol properties that differ from those of both water and purely nonpolar liquids. Ethanol mixes readily with water and can also dissolve certain organic substances that are not highly soluble in pure water. It has a lower boiling point than water under comparable pressure conditions. Studying ethanol helps chemistry students understand how molecular structure affects solubility, volatility, intermolecular forces, and other liquid properties.

Cooking oils provide useful examples of nonpolar or largely nonpolar liquid mixtures. Vegetable oils usually consist primarily of triglyceride molecules derived from fatty acids and glycerol. These molecules interact differently from water molecules, which explains why oil and water generally separate instead of forming a uniform mixture. Many common oils are also less dense than water, causing an oil layer to float when the two substances are combined. Their relatively high viscosity produces slower flow compared with water under similar conditions. Temperature can significantly change oil viscosity, which is noticeable when refrigerated oils become thicker or partially solidify.

Mercury provides an unusual example because it is a metallic element that remains liquid at typical room temperature. Most metals encountered in everyday life are solid under similar conditions, making mercury particularly distinctive. Liquid mercury has a silvery appearance, high density, strong surface tension, and electrical conductivity associated with metallic behavior. Historically, it was used in thermometers, barometers, electrical equipment, and scientific instruments. However, mercury and many mercury compounds can present serious toxicity concerns, so safer alternatives are now widely preferred. Its physical properties nevertheless make mercury an important example when explaining that liquids are not limited to water-like molecular substances.

Many household materials are also liquids even though they contain complex mixtures of ingredients. Shampoo, liquid soap, vinegar, milk, paint, perfume, gasoline, juice, syrup, and household cleaners can all behave as liquids under ordinary conditions. Their viscosities, densities, colors, odors, and chemical compositions vary considerably, yet they share basic characteristics associated with the liquid state. Some contain dissolved solids, suspended particles, emulsified droplets, or several different liquid components. Others may change noticeably with temperature or separate into layers over time. These examples demonstrate that a liquid can be chemically simple or extremely complex while still displaying the fundamental physical behavior of a liquid.

Important Physical Properties of Liquids

Viscosity describes a liquid’s resistance to flowing and is one of the most noticeable differences between various liquid substances. A low-viscosity liquid such as water pours quickly, while a high-viscosity material such as honey moves much more slowly. Viscosity results partly from friction and intermolecular interactions between molecules moving past one another. Large molecules, strong intermolecular attractions, and complicated molecular structures can all contribute to greater resistance to flow. Temperature also has an important effect because many liquids become less viscous when heated. Understanding viscosity is essential when designing lubricants, pipelines, paints, medicines, foods, and industrial processing equipment.

Vapor pressure measures the tendency of molecules to escape from a liquid and enter the gas phase at a particular temperature. In a closed container, some molecules evaporate from the liquid surface while vapor molecules simultaneously return through condensation. Eventually, the rates of evaporation and condensation may become equal, producing a dynamic equilibrium. The pressure produced by the vapor under these conditions is called the equilibrium vapor pressure. Liquids with weaker intermolecular attractions often have higher vapor pressures at the same temperature than liquids with stronger attractions. Vapor pressure is closely related to evaporation rates, volatility, boiling points, and temperature.

Boiling point is the temperature at which a liquid’s vapor pressure becomes equal to the pressure surrounding the liquid. At this stage, vapor bubbles can form throughout the liquid instead of evaporation occurring only at the surface. Boiling temperature is not always constant because external pressure can change the conditions required for boiling. At higher elevations, lower atmospheric pressure allows water to boil at a temperature below its familiar value at sea level. Increasing pressure has the opposite effect and can raise the boiling temperature. These principles are important in pressure cooking, industrial distillation, laboratory chemistry, and many engineering processes.

Surface tension reflects the cohesive attraction between molecules at the surface of a liquid. Water has relatively strong surface tension because of hydrogen bonding between its molecules. This property allows water to form droplets and contributes to the curved surface known as a meniscus inside certain containers. Detergents and surfactants can reduce surface tension by interfering with interactions between surface molecules. Lowering surface tension allows water to spread more effectively across surfaces and penetrate small spaces during cleaning. Surface tension is therefore important in washing, painting, printing, pharmaceuticals, coatings, biological systems, and numerous industrial applications.

Specific heat capacity and thermal behavior are also important when examining liquid properties. Different liquids require different amounts of energy to produce the same change in temperature. Water has a relatively high specific heat capacity, meaning it can absorb substantial thermal energy before its temperature rises dramatically. This property helps moderate temperatures in living organisms, oceans, buildings, and industrial cooling systems. Liquids also transfer heat through processes such as conduction and convection. Their ability to circulate makes liquid coolants particularly useful for moving thermal energy away from engines, electronics, machines, and chemical equipment.

Phase Changes Involving Liquids

Melting is the phase change in which a solid becomes a liquid after absorbing sufficient thermal energy. As a solid is heated, its particles gain kinetic energy and vibrate more strongly within their positions. Once conditions reach the melting point, the organized solid structure begins to break down as particles gain enough mobility to move past one another. The substance then enters the liquid state while generally maintaining its chemical identity. Ice melting into liquid water is a familiar example of this transition. Melting points differ significantly among substances because bonding, molecular structure, pressure, and intermolecular forces vary between materials.

Freezing is essentially the reverse of melting and occurs when a liquid loses enough energy to become a solid. As temperature decreases, liquid particles move more slowly because their average kinetic energy falls. Eventually, attractive forces can organize particles into a more stable and less mobile solid arrangement. The temperature at which this process occurs is generally called the freezing point under specified conditions. Pure substances often have characteristic freezing temperatures that can assist with identification and analysis. Dissolved substances may lower the freezing point, which helps explain why salt can influence the freezing behavior of water.

Evaporation occurs when molecules escape from the surface of a liquid and enter the gas phase. Unlike boiling, evaporation can happen at temperatures well below the boiling point. Molecules within a liquid have a range of kinetic energies rather than all moving at exactly the same speed. Higher-energy molecules near the surface may overcome intermolecular forces and move into the surrounding air. Warm temperatures, greater surface area, lower humidity, and air movement can often increase evaporation rates. Everyday examples include wet clothes drying, puddles disappearing, perfume spreading into the air, and sweat evaporating from skin.

Condensation is the opposite of vaporization and occurs when gas particles lose enough energy to enter the liquid state. Water droplets forming on the outside of a cold beverage container provide a familiar example. These droplets usually come from water vapor already present in the surrounding air rather than from liquid leaking through the container. When water vapor encounters a sufficiently cool surface, molecules lose energy and gather together as liquid droplets. Condensation also contributes to cloud formation, dew, fog, and many weather processes. Industrial condensers use similar principles to recover liquids from vapors during chemical processing and distillation.

Boiling represents rapid vaporization occurring throughout a liquid when its vapor pressure matches the surrounding pressure. Bubbles form inside the liquid and rise toward the surface as molecules transition into the vapor phase. During boiling, added energy is heavily involved in overcoming intermolecular attractions rather than simply raising temperature. This energy requirement is related to the enthalpy of vaporization of the substance. Different liquids therefore require different amounts of energy to convert from liquid to gas. Understanding boiling behavior is important in cooking, chemical separation, purification, energy production, refrigeration, and industrial manufacturing.

Factors That Affect Liquid Behavior

Temperature is one of the strongest influences on how a liquid behaves because it directly affects molecular kinetic energy. Heating usually causes molecules to move faster, increasing the frequency and energy of molecular collisions. Many liquids become less viscous as temperature rises because their particles can move past one another more easily. Higher temperature also generally increases vapor pressure and accelerates evaporation. Cooling produces the opposite trend and may eventually cause a liquid to freeze when enough thermal energy is removed. Temperature control is therefore essential in laboratories, chemical plants, food storage, transportation systems, and countless everyday processes.

Pressure can also change liquid behavior, especially when phase transitions are involved. Liquids themselves are relatively difficult to compress, but external pressure can strongly influence their boiling temperatures. Increasing surrounding pressure usually requires a liquid to reach a higher temperature before boiling occurs. Decreasing pressure allows boiling to happen at a lower temperature because less vapor pressure is required to overcome the surroundings. This relationship explains why cooking conditions differ at high elevations compared with locations near sea level. Industrial systems deliberately adjust pressure to control boiling, distillation, evaporation, and chemical reaction conditions.

Molecular structure is another major factor determining physical properties such as viscosity, solubility, boiling point, and surface tension. Small changes in molecular shape or functional groups can create significant differences in how molecules interact. Polar molecules may experience strong electrostatic attractions, while nonpolar molecules often depend largely on dispersion forces. Molecules capable of hydrogen bonding can display particularly noticeable effects on boiling point and cohesion. Large molecules may also experience more extensive intermolecular interactions than smaller molecules with similar chemical features. Understanding molecular structure allows chemists to predict and explain why different liquids behave differently under the same environmental conditions.

Dissolved substances can change the behavior of a liquid by altering its composition and particle interactions. Adding salt to water, for example, can change properties such as freezing point, boiling point, electrical conductivity, and density. Dissolving sugar produces different effects because sugar remains as neutral molecules rather than separating into charged ions. Surfactants can dramatically reduce surface tension, while polymers may greatly increase viscosity even at relatively low concentrations. These changes are important when chemists formulate cleaning products, medicines, foods, paints, cosmetics, and industrial fluids. Solutions therefore often behave differently from the pure liquids used to create them.

Impurities and suspended particles can also affect how a liquid flows, freezes, boils, or interacts with surfaces. Natural water commonly contains dissolved minerals, gases, organic compounds, and microscopic particles that distinguish it from highly purified laboratory water. Crude oil contains a complex range of hydrocarbons with different molecular sizes and boiling characteristics. Milk contains dispersed fat droplets and proteins that influence its texture and appearance. Even small amounts of contamination can alter sensitive chemical measurements in laboratory situations. For this reason, chemists carefully control liquid purity when accurate experiments, manufacturing standards, or product performance depend on predictable physical properties.

Why Liquids Matter in Chemistry and Everyday Life

Liquids are extremely important in chemistry because many chemical reactions are performed in liquid solutions. Dissolving reactants in a suitable solvent allows molecules and ions to move freely and come into contact with one another. This mobility can increase reaction efficiency and makes concentrations easier to measure and control. Water is frequently used as a solvent, but organic liquids such as ethanol, acetone, and other compounds may be chosen for specific reactions. Solvents can influence reaction rates, chemical equilibria, and the stability of intermediates. Choosing an appropriate liquid medium is therefore a fundamental part of experimental chemistry and industrial chemical design.

Biological systems also depend heavily on liquids, especially water-based fluids. Cells contain large amounts of water that provide an environment for biochemical reactions and transport. Blood carries oxygen, nutrients, hormones, waste products, and other substances throughout the body in a liquid medium. Digestive fluids help break down food, while intracellular and extracellular fluids maintain conditions required for normal cellular function. Plants depend on water to transport dissolved minerals from roots and support photosynthesis and cellular structure. Without the unique chemical and physical properties of liquids, life as we know it would function very differently.

Industrial applications rely on liquids for cooling, lubrication, transportation, cleaning, extraction, and chemical manufacturing. Lubricating oils reduce friction between moving mechanical components and help manage heat generated during operation. Coolants transfer thermal energy away from engines, industrial machinery, electronic systems, and manufacturing equipment. Liquid fuels store chemical energy in forms that can be pumped, transported, and controlled efficiently. Chemical plants move liquid raw materials through networks of tanks, valves, pumps, and pipelines. Understanding viscosity, density, vapor pressure, corrosiveness, and temperature behavior is essential for keeping these systems efficient and safe.

Liquids also play major roles in food preparation and household activities. Cooking commonly involves water, milk, oils, sauces, syrups, and other liquid ingredients whose properties change with heating and cooling. Cleaning products depend on liquids to dissolve dirt, carry active ingredients, and spread across contaminated surfaces. Paints and coatings must have carefully controlled viscosity so they can be applied smoothly before forming a solid layer. Medicines may be prepared as solutions, suspensions, syrups, or injectable liquids depending on their purpose. Familiar household activities therefore demonstrate many chemical principles involving liquid flow, solubility, evaporation, temperature, and intermolecular attraction.

Understanding the liquid definition in chemistry ultimately makes many everyday observations easier to explain scientifically. Water droplets form because of surface tension, oil separates from water because of molecular polarity, and wet clothing dries through evaporation. Honey flows more slowly than water because it has a much higher viscosity, while pressurized cooking changes boiling behavior by affecting surrounding pressure. Each example connects an ordinary experience to interactions taking place between atoms and molecules. Studying liquids therefore provides much more than a basic definition of a state of matter. It creates a foundation for understanding chemistry, biology, environmental science, engineering, technology, and the physical world around us.

Frequently Asked Questions

What is the simplest definition of a liquid in chemistry?

A liquid is a state of matter that has a definite volume but no fixed shape. It flows and takes the shape of its container because its particles can move past one another while remaining relatively close together.

What are five important properties of liquids?

Important liquid properties include definite volume, fluidity, relatively low compressibility, viscosity, and surface tension. Liquids can also display capillary action, diffusion, evaporation, and characteristic boiling and freezing behavior.

What are some common examples of liquids?

Common examples include water, cooking oil, milk, vinegar, ethanol, gasoline, liquid soap, juice, and mercury. Each has a different chemical composition, but all can demonstrate typical liquid behavior under suitable temperature and pressure conditions.

Why does a liquid take the shape of its container?

Liquid particles are close together but are not locked into fixed positions. Because they can move and slide around one another, the liquid flows until it conforms to the available shape of its container.

What is the main difference between a liquid and a gas?

A liquid has a relatively definite volume and particles that remain close together, while a gas expands to fill the available container. Gases are also much easier to compress because their particles are separated by considerably larger spaces.