What Happens When You Apply Heat To A Substance?

Heat is a form of energy that flows between objects or systems due to their temperature difference. It results in the transfer of kinetic energy at the molecular level. Temperature is a measure of the average kinetic energy of molecules and atoms in a substance.

Applying heat to a substance can have various effects depending on the amount of heat and the properties of the substance. When heat is added, the molecules gain more kinetic energy and start moving faster and vibrating more. This can lead to changes in volume, state, color, chemical bonds, and other properties of the substance.

In this article, we will explore the key effects that occur when heat is applied to matter. We will cover thermal expansion, phase changes, increased molecular motion, changes in chemical bonds, decomposition reactions, combustion, and color changes. Understanding heat and its effects has important applications in science, engineering, cooking, and daily life.

Thermal Expansion

Thermal expansion is the tendency for matter to change volume in response to a change in temperature. When most substances are heated, their particles begin moving more vigorously and maintain a greater average separation. This results in an increase in volume and is referred to as thermal expansion. The degree of expansion varies with the material and the change in temperature.

Thermal expansion arises due to the anharmonic nature of interatomic potentials. When heat is applied to a substance, it causes the atoms and molecules to vibrate more rapidly and with larger amplitudes. Since atomic bonds are not perfectly harmonic, the interatomic spacing increases as atoms move farther apart during vibrations, resulting in an increase in volume.

Thermal expansion applies to solids, liquids, and gases. In solids, the atoms are relatively fixed in place but vibrate about their mean positions. As the temperature increases, the vibration amplitudes of the atoms increase, causing the average separation between atoms to increase and the solid to expand. In liquids, the molecules are closely packed but can move past one another. When thermal energy is added, they begin colliding and sliding past each other more vigorously, forcing an increase in average intermolecular spacing. For gases, heating causes an increase in the velocity and kinetic energy of gas molecules, allowing them to spread out more in a volume and reducing intermolecular collisions.

A common example of thermal expansion in solids is the expansion of railroad tracks as temperatures increase from night to day. Gaps must be left between rail sections to allow for expansion to prevent buckling. Liquids such as alcohol or mercury expand noticeably when heated in thermometers, enabling temperature measurement. Hot air rises because it is less dense than surrounding cooler air, demonstrating the expansion of heated gases.

Phase Changes

When heat is applied to a substance, it can undergo phase changes between solid, liquid, and gas states. Some common phase changes that occur with heating include melting, boiling, evaporation, and condensation.

Melting refers to the phase change of a substance from solid to liquid state. As a solid is heated, the kinetic energy of its molecules increases until the molecular forces binding the molecules together are overcome and the solid transitions to a liquid. Melting is an endothermic process, meaning it requires energy input in the form of heat.

Boiling is the phase change of a liquid to a gas state that occurs when the liquid reaches its boiling point temperature. At this point, the kinetic energy of the liquid molecules is high enough that a significant portion transition to the gaseous state. Boiling is also an endothermic process that requires heat energy input.

Evaporation refers to the phase change from liquid to gas that occurs on the surface of a liquid below its boiling point. As liquid molecules near the surface gain enough kinetic energy, they can escape from the liquid and enter the gas phase. Evaporation causes a cooling effect as the highest energy molecules leave the liquid.

Condensation is the reverse process of evaporation, where gas phase molecules transition to the liquid state by losing kinetic energy. This commonly occurs when water vapor in the air condenses into liquid water on cool surfaces, resulting in dew.

Phase change diagrams plot temperature vs. heat energy input and illustrate the amount of energy required for transitions between solid, liquid, and gas states. The flat parts of the graph represent phase changes taking place at constant temperature, demonstrating their endothermic nature.

Increased Kinetic Energy

When heat is applied to a substance, it causes the molecules in that substance to move faster. This increase in molecular motion is directly related to an increase in the kinetic energy of the molecules. Kinetic energy is the energy associated with motion. So as more heat is added, the molecules gain more kinetic energy and move faster.

Temperature is a measure of the average kinetic energy of the molecules in a substance. So when the temperature rises, it indicates the molecules have more kinetic energy on average. However, at any given temperature, there is a distribution of kinetic energies among the molecules. Some molecules will have more kinetic energy than others as they move at different velocities.

This increase in molecular motion and kinetic energy is responsible for many of the changes we observe when heat is applied to a substance. The faster motion of molecules can lead to thermal expansion as molecules take up more space. It can also break intermolecular bonds, allowing phase changes to occur. The higher energy state of molecules often enables chemical reactions as well. Overall, the addition of heat increases the energy available to molecules to move and undergo physical and chemical changes.

Changes in Chemical Bonds

Applying heat to substances can cause chemical bonds between atoms to break. Bonds require energy to break, known as activation energy. The higher the temperature, the more energy atoms and molecules have, allowing them to more easily overcome the activation energy barrier and break bonds. This increases chemical reaction rates.

For example, heating hydrocarbon fuels like wood, oil and gas provides enough energy for the hydrocarbon molecules to break apart into hydrogen and carbon atoms. The carbon and hydrogen can then react with oxygen to release energy as heat and light, producing combustion and fire.

Food cooking also relies on increased temperature causing chemical changes. Heating proteins like meat and eggs denatures them, changing their chemical structure. Starch molecules in foods like bread, rice and potatoes break down upon heating as well. Chemical changes like these alter the taste, texture and digestibility of food through cooking.

Many chemical manufacturing processes also depend on heat to drive necessary chemical reactions. Reactions like cracking heavy hydrocarbons into more useful fuels and synthesizing ammonia for fertilizers occur at high temperatures where bonds can break and re-form more favorably.

Decomposition

Decomposition is another common reaction that occurs upon heating. This is when a substance breaks down into simpler substances upon heating. For example, many carbon-based organic compounds will decompose into carbon dioxide and water when heated. This is because the heat provides enough energy to break apart the bonds holding the molecules together.

Other substances like metal carbonates and nitrates will also decompose upon heating. For instance, calcium carbonate (found in limestone and shells) will break down into calcium oxide and carbon dioxide gas when heated. This process is called calcination and is used to make quicklime, an important material in making cement and mortar.

Metal nitrates like copper nitrate or silver nitrate will decompose into metal oxides and nitrogen dioxide gas. Alkali metal nitrates like potassium nitrate will decompose into alkali metal oxides, oxygen, and nitrogen gas.

In all cases of thermal decomposition, heat provides the activation energy needed to break bonds and decompose the original compounds into simpler substances. The decomposition temperature depends on how stable the compounds are and the strength of their chemical bonds.

Combustion

One dramatic effect that can occur when heat is applied to a substance is combustion, which is an exothermic oxidation reaction between a fuel and an oxidant, usually oxygen. For combustion to occur, three elements are required: fuel, oxygen, and an ignition source. The fuel can be a solid, liquid or gas, and provides the matter that will chemically react with oxygen. Common fuels include wood, gasoline, natural gas, and hydrogen. Oxygen serves as the oxidizing agent and must be present for the chemical reaction to proceed. Finally, an ignition source provides the activation energy needed to initiate the combustion reaction. This is often a spark or flame, but can also be generated from concentrated light, electromagnetic energy, or chemical reactions. Once a combustion reaction is underway, it is self-sustaining due to the heat it produces.

The interdependence of fuel, oxygen, and heat in sustaining a combustion reaction is commonly illustrated using a “fire triangle” diagram. Each side of the triangle represents one of the three required elements. If any one element is removed, the fire triangle is broken and combustion stops. This principle is used in fire suppression techniques like smothering flames with blankets or baking soda, which cut off the oxygen supply. It also explains why fuels with low ignition temperatures are more flammable and hazardous. The fire triangle provides a simple model for understanding how combustion occurs and how it can be controlled.

Color Changes

When heat is applied to many materials, the increased energy can cause electrons to move to higher energy states. As electrons transition between different energy levels, the material absorbs and emits different wavelengths of light, resulting in color changes.

One example of heat-induced color change can be seen when cooking meat. Raw beef is reddish in color due to the presence of a protein called myoglobin that contains iron atoms. As meat is heated, the myoglobin protein denatures, causing the iron atoms to oxidize and take on a grayish color.

The leaves of deciduous trees also undergo color changes in autumn as chlorophyll breaks down in response to decreasing temperatures and sunlight. The green chlorophyll fades away to reveal yellow and orange pigments known as carotenoids that were present in the leaves all along.

Transition metal compounds often exhibit striking color changes when heated. For example, white chromium(III) oxide turns green when heated as the crystal field splitting energy increases. Meanwhile, cobalt chloride shifts from blue to pink due to a spin state transition induced by the temperature change.

Applications

Heat is applied in many important real-world processes and techniques:

Cooking – Applying heat is fundamental to cooking food. Gentle heating causes proteins to denature and firm up or coagulate, it caramelizes sugars to develop deeper flavors, and it softens the structures of fruits and vegetables to make them more palatable. More intense heat applied in cooking techniques like sautéing, pan-frying, grilling, and broiling leads to the Maillard reaction between amino acids and sugars, which creates desirable browning and complex, robust flavors.

Metallurgy – Heating metals is critical for shaping, joining, and treating them. Blacksmiths and other metalsmiths heat iron, steel, and other metals to soften them for bending, forging, welding, and other manipulations. Heat treating like quenching and tempering can strengthen and harden metals by altering their internal crystalline structures. And smelting requires intense heat to extract pure metals from ore.

Chemistry lab techniques – Heating is used in many basic and advanced chemistry techniques. Gentle heating can be used to encourage dissolution of a solute in a solvent. More intense heating is used in distillation to vaporize and then condense liquids to purify or isolate chemical compounds. Digestion, used to break down organic matter, relies on heat. And heating drives many common chemical reactions performed in labs.

Summary

When heat is applied to matter, it can induce a wide variety of changes. We explored several key effects that heating causes:

– Thermal expansion – Heating makes molecules vibrate more intensely, causing them to take up more space and expand in volume.

– Phase changes – With enough added heat energy, substances can change phase from solid to liquid to gas as the molecular bonds break down.

– Increased kinetic energy – Heat boosts the random molecular motion, increasing the average kinetic energy of particles in a substance.

– Chemical changes – Heat can break bonds completely and enable molecules to rearrange into new compounds through decomposition and combustion reactions.

– Color shifts – Some materials exhibit color changes when heated as their electronic structures are altered.

Understanding these thermal effects allows us to purposely utilize heat in many practical applications. From cooking food to powering engines, heat drives countless essential processes by energizing molecules.

In summary, heat flow causes transformations in matter ranging from subtle to drastic. Through purposeful heating, we can harness these changes for human benefit. The thermal behavior of materials underpins many technologies and activities that society depends on.