What Happens To The Mass Of Most Matter When It Is Heated?

Matter is anything that has mass and takes up space. Mass is a measure of the amount of matter in an object. Mass is different from weight, which is the force of gravity on an object. While weight can be different depending on gravity’s pull, mass stays constant and does not change.

When matter is heated, different changes can occur – matter can melt, evaporate, expand, or undergo chemical changes. An important scientific concept is the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. This means the mass of the reactants (ingredients) equals the mass of the products. However, there are some exceptions to this law.

This article will explore what happens to the mass of most matter when heated, explaining the science behind changes in mass, exceptions to the law, and real-world examples.

The Law of Conservation of Mass

The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. This means the mass of the reactants (starting materials) in a chemical reaction will be equal to the mass of the products (resulting materials).

For example, if 10 grams of hydrogen gas reacts with 80 grams of oxygen gas, the resulting mass of the product (water) will be 90 grams. The total mass does not change before or after the reaction. This law was formulated in 1789 by French chemist Antoine Lavoisier and remains a fundamental concept in chemistry today.

Mass Remains Constant During Phase Changes

One of the key principles of the law of conservation of mass is that mass remains constant during phase changes. A phase change is when matter transitions between a solid, liquid, or gas state due to the addition or removal of energy. Common examples of phase changes include ice melting into liquid water, water boiling into steam, and water vapor condensing into liquid water.

During these phase changes, the chemical composition of the matter remains unchanged. An ice cube is still made of H2O molecules when it melts into liquid water. The main difference is the movement and spacing of the molecules. In a solid, the molecules are closely packed in a rigid arrangement. In a liquid, the molecules can slide past each other and move more freely. In a gas, the molecules are very spread out and have lots of space to move around.

So even though the physical state changes during a phase change, the number of atoms and molecules stays the same. No mass is gained or lost. This is why when you melt an ice cube, the resulting puddle of water has the same mass as the original ice cube. And if you boil water into steam and then condense it back into water, the same mass of water remains. The law of conservation of mass confirms that phase changes do not affect the total mass present.

Exceptions to the Law of Conservation of Mass

While the law of conservation of mass holds true for most physical and chemical changes, there are some important exceptions. The most notable exception is nuclear reactions. In nuclear processes like fusion, fission, and radioactive decay, some of the mass of the reactants is converted into energy. This is described by Einstein’s famous equation E=mc2, which shows that mass and energy are interchangeable. During nuclear reactions, matter is destroyed and energy is created, leading to an overall decrease in mass. For instance, when a uranium atom undergoes fission, it splits into lighter atoms and releases energy in the form of heat and radiation. The mass of the resulting atoms is less than the original uranium atom. This mass difference has been converted to energy. Such nuclear reactions demonstrate that mass is not always conserved, but rather can be transformed into tremendous amounts of energy. This exception only applies to nuclear changes and is negligible in chemical and physical changes.

Mass Increases When Heated

When most matter is heated, its mass remains constant. However, there are some exceptions to this rule. One exception is that matter undergoes thermal expansion when heated, causing it to take up more volume and space. The increase in volume occurs because the kinetic energy of the atoms or molecules increases as they gain heat energy. With more kinetic energy, the atoms or molecules vibrate and move faster, colliding with each other and pushing apart to occupy more space.

This increase in volume from the expansion of matter when heated can result in a slight increase in mass. As the atoms push apart, it causes the matter to become less dense overall. Since density is calculated as mass divided by volume, when the volume increases, the density decreases if the mass stays the same. In reality, a tiny bit of mass is gained as the atoms move farther apart because of the energy put into the system.

So in summary, when most matter gains heat energy, its mass remains constant while its volume increases. But a minute amount of mass is added to the system from the energy input required to expand the volume. The resulting decrease in density and slight increase in mass are consequences of the thermal expansion that occurs when matter is heated.

heating matter usually results in thermal expansion and phase changes, but mass is conserved.

Measuring Changes in Mass

Measuring small changes in mass requires precise balances and careful experimental technique. Analytical balances are precise to 0.0001 g or better, allowing accurate measurement of tiny mass changes during heating. Proper procedure is crucial – samples must be handled minimally to avoid collecting or losing stray particles, containers should be pre-weighed, and balances should be properly calibrated and isolated from vibration or air currents. Mass measurements before and after heating, subtracted to find the change, must be made once the sample reaches constant mass. This ensures any volatilized components have completely left the sample. With proper balances and technique, mass changes less than 0.1% can be quantified during heating experiments.

Real-World Applications

There are several examples of the law of conservation of mass being applied in real-world processes and activities:

Cooking: When food is cooked, the mass remains the same before and after cooking, even though its volume or appearance may change. For example, when an egg is fried, although it changes from liquid to solid, the mass of the egg remains the same.

Chemical engineering: Chemical engineers rely on the law of conservation of mass to balance chemical equations and account for all inputs and outputs in chemical processes. All atoms present before a reaction must be accounted for after the reaction as well.

Physics experiments: The law is fundamental to many physics experiments that measure mass and changes in mass very precisely. Any discrepancies in mass would indicate experimental errors or incomplete accounting of mass.

Common Misconceptions

There are a few common misconceptions related to mass when matter is heated. Two of the main ones involve confusing mass with weight or volume.

Mass refers to the amount of matter in an object, while weight refers to the force exerted on an object by gravity. An object’s mass remains constant regardless of location, but its weight can change based on gravitational forces. For example, an object has the same mass on Earth and the Moon, but it will weigh less on the Moon due to lower gravity. Heating an object does not change its mass, but it could alter the object’s weight slightly if it expands in size.

Another misconception is thinking that when an object expands in size from heating, its mass must increase as well. But mass and volume are two distinct properties. Volume is the amount of space an object takes up, while mass is the amount of matter it contains. Heating something may cause it to expand in volume, but its mass will remain unchanged. For instance, if you heat a steel block, it will take up more space but it does not gain more steel atoms. Its mass stays the same.

Understanding the differences between mass, weight, and volume is important when learning about how matter behaves when heated. Mass remains constant regardless of physical changes like phase transitions, thermal expansion, or compression. It is a fundamental property of matter that is unaffected by heating.

Educational Resources

Here are some educational activities and demonstrations that can help students understand and apply the law of conservation of mass in a classroom setting:

Conservation of Mass Lab: Students can design an experiment to measure mass before and after a physical or chemical change, such as dissolving salt in water or burning a candle. They record precise measurements using a digital scale and analyze results to see that mass is conserved.

Phase Change Demos: Teachers can perform melting, freezing, vaporization, and condensation demonstrations while tracking mass. This visually shows students that mass stays constant when matter changes phase.

Combustion Analysis: Weigh a piece of wood before and after burning it. Have students account for any mass loss by calculating the oxygen consumed and carbon dioxide and water vapor produced.

Decomposition Experiment: Measure the mass of a compound like mercury oxide before and after heating it to decompose it into mercury and oxygen gas. Show students how mass is conserved despite the change in matter.

Baking Soda and Vinegar Reaction: Students can predict, measure, and analyze mass before and after mixing baking soda and vinegar. The gas produced makes this a dynamic demonstration.

Mass Transfer Simulations: Online simulations allow students to visualize the law of conservation of mass. They can track the mass of reactants and products in a virtual reaction.


The law of conservation of mass is a fundamental concept in chemistry and physics that states the total mass of a closed system remains constant over time, regardless of the processes acting inside the system. This means that in a chemical reaction, the total mass of the reactants equals the total mass of the products.

Matter can change forms through phase changes or chemical reactions, but the total mass remains the same. The law holds true for all ordinary chemical reactions that take place under standard conditions. However, under extreme conditions, such as nuclear reactions or reactions approaching the speed of light, mass may not be conserved.

When matter is heated, several things can happen depending on the material and conditions. Solids and liquids will go through phase changes to gas, resulting in a separation of molecules but no change in total mass. Chemical decomposition can also occur, but the decomposition products will retain the original mass. The law dictates that the overall mass stays constant.

In summary, the fundamental principle that mass is conserved applies to most everyday situations. Understanding where mass goes when matter changes form helps illuminate the deeper mechanisms governing our physical world.

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