Elements, Compounds and Mixtures Chapter 7

Elements Elements are the simplest substances. They are pure. They cannot be broken down into anything else. There is only one type of particle found in an element

Pure Substances Is a substance in which there is only one type of particle. THEREFORE: each element has only one type of particle. The particles are known as atoms.

Properties of Elements Each element is identified by its unique set of properties. These are known as characteristic properties. Characteristic properties are Boiling point Melting point Density Reactivity – for example whether or not the element reacts with an acid.

Identifying elements by their properties. Cobalt Iron Nickel Melting Point: 1495 o C Density: 8.9 g/cm 3 Conducts electric current and heat energy. Unreactive with oxygen in the air. Melting Point: 1535 o C Density: 7.9 g/cm 3 Conducts electric current and heat energy. Combines slowly with oxygen in the air to form rust. Melting Point: 1455 o C Density: 8.9 g/cm 3 Conducts electric current and heat energy. Unreactive with oxygen in the air.

Classifying Elements by Their Properties Elements are grouped into categories by their properties. There are three major categories of elements. Metals Nonmetals Metalloids

Metals Shiny Good conductors of heat and electric current. Malleable Ductile

Nonmetals Dull (not shiny) Poor conductors of heat and electric current. Solid nonmetals tend to be brittle and unmaleable. Few familiar objects are made of only nonmetals.

Metalloids Called semiconductors. Have properties of metals and nonmetals Some are shiny, some are dull. Somewhat maleable and ductile Some conduct heat and electric current. Some do not.

Compounds A pure substance composed of two or more elements, that are chemically combined. Elements combine by reacting, or undergoing a chemical change, with one another. A particle of a compound is a molecule. Most of the substances that you see are compounds.

Familiar Compounds CompoundElements Combined Table saltsodium and chlorine Waterhydrogen and oxygen Vinegarhydrogen, carbon, and oxygen Carbon dioxidecarbon and oxygen Baking Sodasodium, hydrogen, carbon, and oxygen

The Ratio of Elements in a Compound Elements do not randomly join to form compounds. They join in a specific ratio according to their masses to form a compound. For example: The ratio of the mass of hydrogen to the mass of oxygen in water is 1:8. Every water sample has a 1:8 mass ratio of hydrogen:oxygen. If a sample of a compound has a different ratio, it cannot be water.

Properties of Compounds Each compound has its own physical properties. Melting point Density Color Physical properties help to identify unknown substances. Chemical properties can also be used.

Compounds vs. Elements Sodium Chloride (NaCl) is table salt. Table salt is made of two very dangerous elements. Sodium reacts violently with water. Chlorine is a poisonous gas. When combined, these elements for a harmless compound with unique properties. NaCl is save to eat. It dissolves – without exploding – in water.

Water--H 2 O--Hydrogen Hydroxide The most important compound on the planet…. No living thing can survive without water. We will look at the chemistry of water… but then we’ll spend TONS of the year studying it.

Breaking Down Compounds Some are broken down into their elements by chemical changes. Some break down to form simpler compounds… the simpler compounds can be broken down into elements. Energy is required to break down elements. Breaking chemical bonds yields two kinds of reactions. Endothermic – where heat is absorbed Exothermic – where heat is given off

Mixtures A mixture is a combination of two or more substances that are not chemically combined.

Separating Mixtures If you don’t like mushrooms on your pizza…. You simply pick them off. It doesn’t change that they are mushrooms or that they were on a pizza.

Solutions Solutions appear to be a single substance. Solutions have the same properties throughout the mixture.

Not all solutions are liquids!! Gas in gas Dry air Gas in liquid Soft drinks Liquid in liquid Antifreeze – alcohol in water Solid in liquid Salt water Solid in solid Brass – zinc in copper

Particles in solution SOLUTE The substance that is being dissolved SOLVENT The substance that does the dissolving.

Concentrated or Dilute The more solute in the solvent, the more concentrated the substance is said to be.

Suspensions A suspension is a mixture in which particles of a material are dispersed throughout a liquid or gas but are large enough that they settle out.

Colloids A colloid is a mixture in which the particles are dispersed throughout but are not heavy enough to settle out. Milk, mayo, jell-o etc.

REVIEW Solutions Suspensions Colloids All are MIXTURES!