Science Study Guide: The Structure of Matter
Phases of Matter
Matter exists in three phases or states: solid, liquid and gas. The phase depends on the motion of the atoms or molecules in the substance.
Solid: the particles in a solid are tightly bound together in a rigid structure and don't move very much. The atoms vibrate but can't change position.
Liquid: the particles of a liquid aren't bound together as tightly as those of a solid, and can move around and slide past each other.
Gas: the particles of a gas aren't bound together at all. The can move independently and there is space between them. If they are not in a container, they will fly apart.
Raising or lowering the temperature can cause matter to undergo a phase change. Lowering the temperature removes energy, and the particles move more slowly. Raising the temperature adds energy and the particles move faster. If liquid water is cooled to 0 degrees C (32 degrees F), it becomes a solid, ice. If water is heated to 100 degrees C (212 degrees F) it boils and becomes a gas, water vapor.
Physical and Chemical Changes
Physical changes alter the form or state of a substance, but do not produce a new or different substance. The original substance is only changed in shape or form. Examples of physical changes are: tearing paper, breaking glass, melting ice, or freezing water. Dissolving sugar or salt in water is a physical change, because the sugar or salt is not changed; if the water is evaporated, the sugar or salt can be recovered.
Chemical changes involve chemical reactions that produce a new substance that is different from the starting material. Examples of chemical changes are burning paper, rusting iron, or mixing baking soda and vinegar to produce carbon dioxide gas. The original material cannot be recovered if it has undergone a chemical change.
All matter is made up of just over 100 elements. An element is a pure substance that consists of only one type of atom. Elements cannot be broken down into simpler substances. Each element has its own characteristic physical and chemical properties, such as color, melting point, boiling point and density. Examples: gold, oxygen, carbon, aluminum.
A compound is formed when two or more elements undergo a chemical reaction and become a new substance that has different properties than the elements that combined to form it. Example: sodium is a metal and chlorine is a toxic gas, but these two elements can undergo a chemical reaction to form the crystalline compound sodium chloride, table salt, which has very different properties than either sodium or chlorine. The elements of a compound can't be physically separated, they can only be separated by a chemical reaction.
In a mixture, the individual components retain their chemical and physical properties after they are combined. The individual components of a mixture can be physically separated and retain their original form. Example: a salad made of lettuce, tomatoes, cucumbers and carrots. Each component retains its properties, and can be physically separated.
A solution is made of two parts: the solute (what is dissolved) and the solvent (what the solute is dissolved in). Example: when sugar is dissolved in water, the sugar is the solute and the water is the solvent. The solute is spread evenly throughout the solution, so all parts of the solution have the same appearance and properties.
How to Calculate the Concentration of a Solution
Concentration means how much solute is dissolved per a given volume of solvent. Concentration is often expressed in grams per milliliter, or g/mL. We find the concentration by dividing the grams of solute by the volume of solvent.
If 40 grams of salt were dissolved in 100 milliliters of water, what is the concentration of salt? 40 divided by 100 = 0.4 g/mL.
In a suspension, a solid is combined with a fluid, but the solid doesn't dissolve. Instead, the solid remains in tiny particles which will eventually settle out if the suspension is allowed to sit undisturbed. Examples: muddy water, a snow globe.
A colloid is a mixture in which one substance is dispersed in another, but the particles are so small that they remain suspended and do not settle out. Colloids can be combinations of solid, liquid or gas. Examples: whipped cream is gas (air) in liquid; fog is liquid (water) in gas (air); smoke is solid (smoke particles) in gas (air).