Chemistry

Physical and chemical changes A physical change does not produce a new substance; changes of state are physical changes. A chemical change produces one or more new substances. Reversibility, colour change, gas formation, temperature change, or a precipitate may provide evidence, but none alone is a complete definition. Rates of reaction The rate of reaction is how quickly reactants are used or products are formed. It can be measured by mass loss, gas volume, colour change, or precipitate formation over time. ...

Exothermic and endothermic reactions An exothermic reaction transfers heat to the surroundings. The surroundings warm and the enthalpy change is negative, so $\Delta H < 0$. An endothermic reaction takes in heat from the surroundings. The surroundings cool and the enthalpy change is positive, so $\Delta H > 0$. Examples include combustion and neutralisation for exothermic reactions, and thermal decomposition and photosynthesis for endothermic reactions. Reaction pathway diagrams A reaction pathway diagram plots energy vertically and reaction progress horizontally. ...

Electrolysis Electrolysis is the decomposition of an ionic compound, molten or aqueous, by an electric current. The electrolyte contains mobile ions. Cathode: negative electrode in an electrolytic cell; reduction occurs. Anode: positive electrode in an electrolytic cell; oxidation occurs. Cations move to the cathode and gain electrons. Anions move to the anode and lose electrons. In general, a metal or hydrogen forms at the cathode; a non-metal other than hydrogen forms at the anode. Electrons move through the external circuit; ions move through the electrolyte. ...

Formulae and equations A molecular formula gives the actual number of atoms in one molecule. A formula for an ionic compound gives the simplest whole-number ratio of ions. Formulae can be deduced from models, diagrams, or ion charges. To deduce an ionic formula, balance total positive and negative charge. For example, $\mathrm{Mg^{2+}}$ needs two $\mathrm{Cl^-}$ ions, giving $\mathrm{MgCl_2}$. Two $\mathrm{Al^{3+}}$ ions and three $\mathrm{O^{2-}}$ ions give $\mathrm{Al_2O_3}$. Common elemental formulae include $\mathrm{H_2}$, $\mathrm{N_2}$, $\mathrm{O_2}$, $\mathrm{F_2}$, $\mathrm{Cl_2}$, $\mathrm{Br_2}$, and $\mathrm{I_2}$. The noble gases are monatomic. ...

Elements, compounds and mixtures An element contains only one type of atom. A compound contains atoms of two or more elements chemically bonded in fixed proportions. A mixture contains two or more substances that are not chemically bonded. Its components can be separated by physical methods. Atomic structure Particle Relative charge Relative mass Location Proton $+1$ $1$ Nucleus Neutron $0$ $1$ Nucleus Electron $-1$ $\frac{1}{1840}$ Shells around nucleus The proton number, or atomic number, is the number of protons. The mass number, or nucleon number, is the total number of protons and neutrons. In a neutral atom, number of electrons equals number of protons. ...