Everything around us is made of matter. Matter is anything that has mass and volume.
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The physical form in which matter exists is called a state of matter. The term phase can also be used to describe which state matter is in. Most matter on Earth exists in three states: solid, liquid, and gas.
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Examples of solids, liquids, and gases can be found all around us. Often we can determine the state of matter by observing the substance with the naked eye. For instance, the land we walk on is solid, oceans and rain are liquid, and the air we breathe is a mixture of gases. The classification of the state of matter is based on observable characteristic properties.
Atoms and Elements
Matter is made of particles called atoms and molecules.
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Atoms are the basic building blocks of most matter around us. An element is a pure substance made up only of a single type of atom. Atoms are too small to be seen without special magnification.
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For example, a gold ring can be broken into smaller and smaller pieces until the pieces are no longer visible. Scientists have discovered 118 elements. Of these elements, 92 are naturally found in nature such as carbon, oxygen, gold, silver, and iron.
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The remaining elements are synthetic and not found naturally; instead they are made in the laboratory.
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All matter is made of different combinations of about 100 pure substances called elements.
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An atom is the smallest particle of an element that has all the properties of that element. Each element is made up of atoms that differ from other elements. The main differences between atoms are the structure of the atom that makes it up. Atoms are made up of different numbers of subatomic particles (neutrons, protons, and electrons).
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Elements are represented by their chemical symbol on the Periodic Table of Elements. This letter symbol is typically made up of one or two letters. For example, carbon is represented by C, oxygen by O, and He for helium.
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Properties of Elements
Each element also has different properties or characteristics. The structure of the atoms determine an elements’ properties. The Periodic Table of Elements is used to organize elements based on properties such as their reactivity, state of matter, conductivity or density.
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All matter has a variety of properties, some of which are characteristic of the substance. Characteristic properties do not depend on the amount of the substance. Properties such as magnetic attraction, conductivity, density, pH, boiling point and solubility are characteristic properties that can be used to identify substances.
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Some elements, such as iron (Fe) and aluminum (Al) are classified as metals.
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Iron and aluminum are two shinny metals, but they differ slightly in their properties. Iron is silvery grey and heavy, while aluminum is silvery white and lightweight in comparison.
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Individual metallic elements have distinct characteristic properties such as malleability, ductility, and relatively high density. For example, sodium (Na) is a light, soft metal that is nonmagnetic, while iron is a magnetic metal that is denser than sodium and aluminum.
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Other elements, such as carbon (C), hydrogen (H), oxygen (O) and chlorine (Cl), are classified as nonmetal.
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Carbon is a nonmetal that is a solid at room temperature, but exists in several different forms (graphite, diamond, and coal), each of which has distinct properties. One form, graphite, is a soft grey substance that makes up pencil lead.
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Another form that looks very different from graphite is diamond. Diamond is transparent and the hardest material found in nature.
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Hydrogen and oxygen are nonmetals that are similar in that they are both colorless gases; however, each gas has distinct characteristic properties.
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Elements also have different reactivity, which describes there ability to form chemical bonds with other substances. Oxygen is highly reactive and it readily combines with other substances during burning.
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Hydrogen is another element that is highly reactive and it causes explosions when it comes in contact with certain elements. Iron is a metal that reacts easily with water forming rust, while aluminum does not.
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Mixtures and Compounds
Most substances or matter are not made of just one element. Elements can combine in different ways to form other substances called compounds and mixtures.
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Mixtures
Solids, liquids or gases can be combined to form mixtures. A mixture is a combination of two or more substances that are not chemically combined.
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Since substances that form mixtures are not bonded together mixtures are not pure substances. In a mixture, each substance keeps its individual properties and identity. In some mixtures, each of the components can be seen. For example, rocks, twigs, insects and leaves are visible components of soil.
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Salad and a bowl of cereal are other examples of a mixture with distinct parts.
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In other mixtures, the individual substances appear to be a single substance because they are so well blended together. Solutions are mixtures that appear to be single substances because particles have dissolved and spread evenly throughout the mixture.
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Air is a mixture of gases, including oxygen, nitrogen and carbon dioxide.
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Mixtures can be separated using different methods, depending on the physical properties of the component substances. Particle size, density, solubility, boiling point, and magnetic attraction are methods for separating mixtures based on their physical properties.
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However, not all separation methods are effective for separating the components of solutions.
Compounds
Atoms can combine chemically to make a molecule of a new substance with new properties called a compound. Like elements compounds are pure substances that have the same chemical composition throughout.
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A compound forms when two or more elements join chemically creating a molecule. A molecule is the smallest part of a compound and is made of atoms of different elements in specific amounts
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A molecule is a group of two or more atoms held together by very strong chemical bonds.
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These bonds form between atoms that share or transfer their electrons.
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A molecule can be made up of more than one atom of the same element. Oxygen gas (O
2
) is an example of this since it is made of two oxygen atoms joined together. Molecules can also be made of two or more different elements.
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Water (H
2
O) is an example of this since it is made of two hydrogen atoms and one oxygen atom.
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Since they are chemically combined, compounds cannot be separated using the physical properties of the elements that make them up. As a result, compounds often have different properties than the individual elements that make them up. For example, table salt (NaCl) is a compound with very different characteristic properties from the elements sodium and chlorine, which make it up. Sodium (Na) is a soft metal that explodes when combined with water.
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Chlorine (Cl), on the other hand is a poisonous gas.
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When chemically combined sodium and chlorine for sodium chloride (NaCl), an edible white solid we enjoy on French fries, better known as table salt. Water (H
2
O) is another compound with different characteristic properties from the elements that form it.
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Water is a liquid at room temperature; however, hydrogen and oxygen are gases at room temperature. Hydrogen is combustible, but water does not burn. Different amounts of the same elements can also produce compounds with different properties. For example, water (H
2
O) and hydrogen peroxide (H
2
O
2
) are both made up of hydrogen and oxygen atoms, but have very different properties.
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Chemical Equations
Molecules are identified by chemical formulas. A chemical formula is a group of chemical symbols and numbers that show the kids and number of atoms in a molecule. For example, the formula for water is H
2
O. H is the chemical symbol for hydrogen. O is the chemical symbol for oxygen. The small number 2 is called a subscript.
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It shows that the molecule contains two atoms of hydrogen. The O has no subscript, which means that the molecule contains only one atom of oxygen.
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(Note: It is not necessary to write 1 as a subscript in chemical formulas or equation. No subscript recognizes there is only one atom.)
In a chemical reaction, atoms can rearrange to form different molecules of new compounds. For example, during photosynthesis, carbon dioxide (CO
2
) is taken in by green plants and combined with water (H
2
O). The carbon, hydrogen and oxygen atoms rearrange using energy from sunlight to make two new compounds: glucose (C
6
H
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O
6
) (made of atoms of carbon, oxygen, and hydrogen) and oxygen gas (O
2
).
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Balancing Chemical Equations
In a chemical equation, certain conditions must be met. A chemical reaction is simply the rearrangement of atoms to form something with a new chemical formula and different properties.
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In a chemical reaction, the same amount of matter (mass) is present at the start and the end, since the atoms are not created or destroyed but simply rearranged.
Unbalanced chemical equation for photosynthesis:
CO
2
+ H
2
O + Sunlight -> C
6
H
12
O
6
+ O
2
Balanced chemical equation for photosynthesis:
6 CO
2
+ 6 H
2
O + Sunlight -> C
6
H
12
O
6
+ 6 O
2
Balanced chemical equations involve an equal number of atoms on both the product and the reactant side of the equation. In the photosynthesis equation there is 1 C, 3 O, and 2 H on the reactant side and 6 C, 12 H, and 8 O on the product side. The following numbers need to be added in order to make both sides of the equation have 6 C, 18 O, and 12 H.
