Why Do Substances Change States? Key Concepts for Students
Change in State Definition
In the field of physics, the matter can exist in distinct forms or states in the universe. However, there are only the primary four fundamental states of matter solid, liquid, gas, and plasma. We observe these matters every day around us in our lives. For instance, water exists in various states around us, such as it exists as water vapour in the gaseous state, ice in the solid-state, and water in the liquid state. In addition to that, there are several other intermediate states of matter such as liquid crystal, glass, quark-gluon plasma etc. that can only exist due to harsh conditions such as increased temperature, high pressure, extreme density and high energy. The different states of process exhibit different or change in the properties. Going further let us study the change in state definition and different states of process in depth.
States of Matter
As we already know, the four fundamental states of matter are solid, liquid, gas, and plasma. Solids exhibit tightly packed constituent particles or atoms or molecules. The movement of the particle is restricted due to a strong force between them that only enables them to vibrate. This movement results in the solids having a specific volume and specific shape. It is impossible to change its structure. They only change structure when an external force is applied, for instance, breaking or cutting. Liquid has a fluid tendency that is mostly incompressible. They retain a specific volume; however, they take the shape of their container. If the external pressure and temperature remain constant, then the size won’t change. The existence of liquids depends upon the maximum temperature called critical temperature.
In addition to that, gases are compressible fluids. Unlike solids and liquids, gases can expand to fill their container and take the shape of the box. The gas molecules have adequate kinetic energy to minimize the effect of intermolecular forces. Also, the distance between the adjacent molecules of a gas is way higher than the size of the molecule. Gases do not have a fixed volume or shape. Similar to gas plasma do not have a fixed volume or shape. The main difference is that plasmas are strongly conducive to electricity and can produce currents, magnetic fields, electromagnetic forces etc. Let us look at the change in state definition and an example of Change in State in Chemical reaction.
Change in State
Matter can transition from one state to another under certain conditions. The main requirement is the application of temperature and pressure. Let us look at the change in states for the three most common matter solid, liquid, and gas.
Solid to Liquid or Liquid to Solid:- Solids can change their state to liquid state when the temperature is increased. On increasing the temperature, the kinetic energy of the particles also increases. And with that, as the energy rises, the particles start vibrating with much higher frequency than before. Hence, the attractive forces between particles start decreasing. That results in the particles detaching themselves from their fixed positions and starts moving in a free manner. Subsequently the solid goes through a phase transition and becomes a liquid. This phenomenon is referred to as melting, and the temperature at which this process occurs is called the melting point. Similarly, when the temperature is reduced in case of liquids (example below zero degree celsius for water). Hence, liquids go through a phase transition to become solids, and the process is referred to as freezing. The temperature at which this process occurs is called the freezing point.
Liquid to Gas or Gas to Liquid:- when the temperature is increased for liquids (heat) their particles start to move at increased speeds. The energy supplied due to the temperature enables the particles of liquid to overcome the force of attraction. It ultimately leads them to go through a phase change and transition into a gas (vapour). The process is called evaporation, and this temperature is referred to as the boiling point. For instance, when we boil the water at high temperatures, it starts evaporating from the container. Similarly, gases can change their phase into the liquid state by the process of condensation.
Gas to Solid or Solid to Gas:- The process that leads the transition of solid to a gas is known as sublimation. One typical example of this phenomenon is dry ice or solid carbon dioxide. At room temperatures, it turns into a gas. When a gas becomes a solid by skipping the liquid state, then that process is called deposition. Let us look at the Example of Change in State in Chemical Reaction.
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FAQs on Change of States in Chemistry: Definitions & Examples
1. What does a change of state mean in chemistry?
A change of state refers to the physical transformation of a substance from one state of matter (solid, liquid, or gas) to another, without any change in its chemical composition. This transformation is typically caused by changes in temperature or pressure, which alter the energy of the particles and the strength of intermolecular forces holding them together. For example, when ice (solid water) melts into liquid water, its chemical formula, H₂O, remains the same.
2. What are the six main processes for the change of state of matter?
The six primary processes through which matter changes its state are:
- Melting (or Fusion): The process where a solid turns into a liquid upon heating.
- Freezing (or Solidification): The process where a liquid turns into a solid upon cooling.
- Vaporisation (or Evaporation/Boiling): The process where a liquid turns into a gas.
- Condensation: The process where a gas turns into a liquid upon cooling.
- Sublimation: The process where a solid turns directly into a gas without passing through the liquid state.
- Deposition: The process where a gas turns directly into a solid without passing through the liquid state.
3. How do temperature and pressure cause a change of state?
Temperature and pressure are the two key factors that induce a change of state. Increasing temperature provides particles with more kinetic energy, causing them to vibrate more vigorously. This increased energy can overcome the intermolecular forces, leading to a change from solid to liquid (melting) or liquid to gas (vaporisation). Conversely, increasing pressure forces particles closer together, strengthening the intermolecular forces. This can cause a gas to liquefy (condensation) even without a significant drop in temperature. For example, LPG (Liquefied Petroleum Gas) is kept in a liquid state inside cylinders by applying high pressure.
4. What is the difference between evaporation and boiling?
While both evaporation and boiling involve a liquid changing into a gas, they are different processes. Evaporation is a surface phenomenon that can occur at any temperature below the boiling point. Only the particles at the surface with enough kinetic energy escape. In contrast, boiling is a bulk phenomenon that occurs at a specific temperature (the boiling point). During boiling, bubbles of vapour form throughout the entire volume of the liquid, not just at the surface.
5. Why does the temperature of water remain constant at 100°C while boiling, even if more heat is supplied?
The temperature of water remains constant at its boiling point (100°C at standard pressure) because the extra heat energy supplied is used to overcome the forces of attraction between water molecules, allowing the liquid to change into steam (gas). This absorbed energy, which does not increase the temperature, is called the latent heat of vaporisation. It is 'hidden' energy that is solely used for the phase transition, not for raising the kinetic energy (and thus temperature) of the substance.
6. Can you give some examples of changes of state in everyday life?
Changes of state are all around us. Some common examples include:
- Melting: Ice cubes melting in a drink.
- Freezing: Water turning into ice in a freezer.
- Evaporation: Puddles drying up after rain.
- Condensation: Water droplets forming on the outside of a cold glass.
- Sublimation: Dry ice (solid carbon dioxide) turning directly into gas, or the gradual shrinking of mothballs in a closet.
- Deposition: Frost forming on windows on a cold morning, where water vapour in the air turns directly into ice crystals.
7. Besides solid, liquid, and gas, what are other less common states of matter?
While solid, liquid, and gas are the most common states of matter on Earth, scientists recognise others, including:
- Plasma: This is an ionised gas consisting of super-energetic and super-excited particles. It is the most abundant state of matter in the universe, found in stars and lightning.
- Bose-Einstein Condensate (BEC): This state occurs when a gas of bosons at very low densities is cooled to temperatures extremely close to absolute zero. At this point, the atoms clump together and behave as a single quantum entity or 'super atom'.
