Chapter 3.1Mixture, Substances and Compounds

The Law of conservation of Mass

When we burn a sample of metallic magnesium in oxygen, the magnesium combines with the oxygen to form magnesium oxide, a white powder. This reaction is accompanied by a release of a large amount of light and heat.

The final product is heavier than the original piece of magnesium. This increase in the the mass of the solid is due to the combination of oxygen to form magnesium oxide.

Mg2(s) + O2(g) →2 MgO(s)

This process respects the Law of Conservation of Mass which asserts that there is no observable change in the quantity of matter during a chemical reaction or a physical change.

The Law of conservation of Energy

A chemical reaction almost always involves a change in energy, conveniently measured in terms of heat.In the reaction of magnesium with oxygen the chemical energy (potential) of the reactants has been converted into light energy and heat energy (kinetic).

This is an example of exothermic reaction, chemical reaction that releases energy. Other types of reactions are called endothermic, in which the system absorbs energy. Whatever the reaction, all the energy involved appears in some forms before and after the change

The Law of Conservation of Energy asserts that energy can be neither created nor destroyed, but can change form, for instance chemical energy can be converted to kinetic energy in the explosion of dynamite.

Mg2(s) + O2(g) →2 MgO(s)

The Law of conservation of Mass and Energy

From Einstein we know that the energy can be converted into mass. In nuclear reactions matter is transformed into energy fallowing the equation

This equation brings to the Law of Conservation

of Matter and Energy which tells us that the

combined amount of matter and energy in the

universe is fixed.

E = mC2

Chemical and physical changes

The reaction of magnesium as it burns in the oxygen is an example of chemical change.

Chemical changes:• Burning

• Digestion

• Changes in smell, color or temperature

Mg2(s) + O2(g) →2 MgO(s)

In any chemical change:• one or more substances are used (at least partially)• one or more new substances are formed• energy is absorbed or released

Chemical changes are typically irreversible

Chemical and physical changesA physical change occurs when the appearance change but not the substances. Examples of physical changes are:

• Melting

• Boiling

• Breaking

• Cutting

• Dissolving

Chemical or physical? Some examples

State changes are physical. Phase changes are when you melt, freeze, boil, condense, sublimate, or deposit a substance. They do not change the nature of the substance unless a chemical change occurs along with the physical change.

Cutting, tearing, shattering, and grinding are physical.

These may be irreversible, but the result is still composed of the same molecules. When you cut your hair, that is a physical change, even though you can’t put the hair back on your head again.

Mixing together substances is physical. For example, you could mix salt and pepper, dissolve salt in water, or mix molten metals together to produce an alloy.

Gas bubbles forming is chemical. Not to be confused with bubbles from boiling, which would be physical (a phase change). Gas bubbles indicate that a chemical reaction has occurred.

Precipitates forming is chemical. When dissolved substances are mixed, and a cloudy precipitate appears, there has been a chemical change.

Rotting, burning, cooking, and rusting (for example) are chemical.

The resulting substances are entirely new chemical compounds. For instance, wood becomes ash and heat; iron becomes rust; sugar ferments into alcohol.

Changes of color or release of odors (i.e. release of a gas) might be chemical.

As an example, the element chromium shows different colors when it is in different compounds, but a single chromium compound will not change color on its own without some sort of reaction.

Release/absorption of energy (heat, light, sound) is generally not easily categorized.

Hot/cold packs involve dissolving a salt in water to change its temperature; popping popcorn is mostly (but not completely) physical change; a glowstick has a chemical change in it.

Chemical reactionsA chemical reaction is a process that leads to the transformation of one set of chemical substances to another.

H2 + O2 → 2H2Oreactants products

Subscript: number of atoms in the molecule

Coefficient: number of molecules

yields

The substance (or substances) initially involved in a chemical reaction are called reactants. Chemical reactions are usually characterized by a chemical change, and they yield one or more products, which usually have properties different from the reactants.

Reactants and products are separated by an arrow which indicates the direction of the reaction; the arrow is read as "yields". A double arrow (⇄) pointing in opposite directions is used for equilibrium reactions.

Equations should be balanced according to the stoichiometry, the number of atoms of each species should be the same on both sides of the equation. This is achieved by scaling the subscripts.

• Solids have a set volume and shape

• Liquids have a set volume, but change shape

• Gases have neither definite volume nor shape

Classification of Matter by its state (Physical)

Classification of Matter by its composition (Chemical)

• A COMPOUND consists of two or more chemical elements that are chemically bonded together. Water (H2O) is an example of chemical compounds. The ratio of the elements in a compound is always the same. For example in water, the number of H atoms is always twice the number of O atoms.

• A MIXTURE consists of two or more substances (element or compound) mixed together without any chemical bond. A mixture can be separated into its individual components by mechanical means.

Classification of Matter by its composition (Chemical)

Types of MixturesThere are many kinds of mixtures. They are classified by the behavior of the substances (called phases) that have been mixed.

Types of Mixtures: Homogeneous MixturesA homogeneous mixture is uniform, which means that any given sample of the mixture will have the same composition and it is not possible to recognize, at macroscopic level, the different substances. A homogeneous mixture is also referred to as solution.

Classification of Matter

4.1.1 Homogeneous Mixtures

Figure 10: Soda water is a homogeneous mixture. (The straw looks broken because ofREFRACTION5.)

A homogeneous mixture is uniform, which means that any given sample of the mixture will havethe same composition. Air, sea water, and carbonation dissolved in soda are all examples of ho-mogeneous mixtures, or solutions. No matter what sample you take from the mixture, it willalways be composed of the same combination of phases. Chocolate chip ice cream is not ho-

5 HTTP://EN.WIKTIONARY.ORG/WIKI/REFRACTION

22

Air and water are examples of solutions. The substance that gets dissolved is the solute (e.g. NaCl), the substance that does the dissolving is the solvent (e.g. Water). The salty water is now a solution, or homogeneous mixture, of salt and water.

When different gases are mixed, they always form a solution. The gas molecules quickly spread out into a uniform composition, such as the air of the atmosphere.

Types of Mixtures: Heterogeneous Mixtures

A heterogeneous mixture is not uniform. Different samples may have different compositions, like a cup of stracciatella ice cream. One spoonful taken might have two chocolate chips while another spoonful might have several chips.

Concrete, soil, toothpaste and salad are all examples of heterogeneous mixtures.