Matter exists as solid, liquid or gas, and is classified as a mixture or a pure substance. Pure substances are elements or compounds. Mixtures separate by physical methods; compounds need chemical ones.
Matter is anything that has mass and occupies space. A book, air and water are matter. Chemistry studies what matter is made of, its properties, and how it changes.
Matter exists in three common states: solid, liquid and gas. The state depends on how closely the particles are held and how freely they move.
Heating or cooling can change a substance from one state to another. The substance itself stays the same: ice, water and steam are all H₂O.
Matter is also classified by what it is made of. Every sample is either a mixture or a pure substance, and each of those divides again.
The classification of matter. Matter is a mixture or a pure substance. A mixture is homogeneous or heterogeneous. A pure substance is an element or a compound.
A mixture contains two or more substances that are not chemically combined, in proportions that can vary. Sugar dissolved in water, air and soil are mixtures. Each component keeps its own properties.
Because the components are not chemically bonded, a mixture can be separated by physical methods: filtration, evaporation, distillation, or picking out by hand.
A homogeneous mixture has the same composition throughout, and its components cannot be seen separately. Sugar solution and air are homogeneous. A heterogeneous mixture has a composition that varies from place to place, and its components can often be seen. Sand in water is heterogeneous.
Is salt water a mixture or a pure substance? How could you recover the salt without a chemical reaction?
Salt water is a homogeneous mixture. The salt is spread evenly through the water and cannot be seen.
Heating the solution evaporates the water and leaves solid salt behind. This is a physical method: the salt and the water are separated, but neither is changed into a new substance. Separation by a physical method is a test for a mixture.
A pure substance has a fixed composition and the same properties throughout. Copper, water and glucose are pure substances.
An element is a pure substance made of only one kind of atom. Copper contains only copper atoms. Oxygen gas contains only oxygen atoms, joined in pairs as O₂ molecules. 118 elements are known; about 94 occur naturally on Earth.
A compound is a pure substance made of atoms of two or more elements chemically combined in a fixed ratio. Water always contains two hydrogen atoms for each oxygen atom. A compound can be separated into its elements only by a chemical reaction, not by a physical method.
The properties of a compound differ from those of its elements. Hydrogen is a flammable gas. Oxygen is a gas that supports burning. Water, the compound they form, is a liquid at room temperature that does not burn and is used to put out fires.
Hydrogen burns and oxygen supports burning, but water puts out fires. What does this show about a compound compared with a mixture?
A mixture of hydrogen and oxygen gases would keep the properties of both. It would still burn, explosively if ignited.
Water does not, because its hydrogen and oxygen atoms are chemically bonded into H₂O molecules. The bonded molecule has its own properties. In a mixture, components keep their properties; in a compound, the elements form a new substance with different properties.
A physical property can be observed or measured without changing the substance into a different one. Colour, smell, melting point, boiling point and density are physical properties.
A chemical property describes how a substance changes into other substances. Whether it burns, how it reacts with an acid, and whether it rusts are chemical properties. Measuring a melting point leaves the substance unchanged. Testing whether it burns changes it.
1What defines a pure substance?
2Classify each as a mixture or a pure substance.
3How does an element differ from a compound?
4How can the components of a mixture be separated?
5Water does not burn, although hydrogen does. This shows that a compound…
6Which is a physical property?
Next: measurement in chemistry, and how many digits a measured value can claim.
Noescio