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"We are all made of stardust."

You may have heard this quote (most famously paraphrased in Carl Sagan's Cosmos) before and wondered how it could be spoken with such certainty.

As initially discovered by Mikhail Lomonosov and Antoine Lavoisier, no reaction can change the mass or elemental identity of the reactants.

Therefore, there is a constant amount of mass in the universe that cannot be changed through any form of reaction. This is called the Law of Conservation of Mass. That means all of the atoms inside of you that carry the mass that you think of as your own have existed since the dawn of the universe, and became the elements they are now during the nuclear reactions inside of stars. You truly are made of star stuff.

Just as mass cannot be changed through any process, energy is converted from one substance to another. It is never created or used up by chemical reactions. When a reaction releases energy in the form of heat, that release comes from the energy within atoms that is no longer needed when they form a lower energy bonded state. This is called the Law of Conservation of Energy.

Law One of Thermodynamics

(Video) The Law of Conservation of Energy | Forms of Energy

by 2 Minute Classroom

Did you know that energy around us is constantly shifting between different forms? It might seem hard to believe at first, but it is never created nor destroyed. It simply transfers between different systems.

In this video, Virgil from 2 Minute Classroom describes this idea, known as the first law of thermodynamics. He also lists examples of five types of energy: kinetic energy, potential energy, chemical energy, electrical energy, and thermal energy.

In 1773, Antoine Lavoisier concluded from his experiments on the mass of reactants and products that matter is never created nor destroyed. By carefully measuring the mass of reactants, and collecting all of the products, he was able to show that the total mass in a chemical reaction is always the same. That is to say, mass is always conserved. This became known as the Law of Conservation of Mass. Lavoisier also noted that mass changed form in invariant amounts, eventually becoming support for the idea that atoms do not change identity in a reaction.

Meanwhile, many scientists were working with a similar idea: the Law of Conservation of Energy. This term was first coined in 1850 by William Rankine. It states that the total energies of the universe are constant. This law is also sometimes referred to as the First Law of Thermodynamics.

These two laws taken together have important implications for chemistry. The Law of Conservation of Mass implies that only the configuration of atoms change within a reaction. The First Law of Thermodynamics implies that the energy released or absorbed within a reaction is due to the change of configuration.

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$\color{red}{\text{Law of Conservation of Mass}}$: Matter is never created nor destroyed. The identity of atoms does not change in a chemical reaction, only the configurations of atoms. $\\$ $\color{red}{\text{First Law of Thermodynamics}}$: Energy is never created nor destroyed, merely changed form.