Edexcel IGCSE Chemistry - Topic 1: Principles of Chemistry

Comprehensive practice flashcards covering Edexcel IGCSE Chemistry Topic 1: Principles of Chemistry, including states of matter, atomic structure, periodic trends, bonding types, stoichiometry, and electrolysis.

How are an element, a compound, and a mixture defined and distinguished from one another?

An element is a substance made from only one type of atom. A compound is made from two or more elements chemically reacted together. A mixture consists of two or more elements or compounds not chemically combined, with the chemical properties of each substance remaining unchanged.

How do pure substances and mixtures differ in their melting and boiling behavior?

A pure substance melts and boils at fixed, specific temperatures, whereas a mixture melts or boils over a range of temperatures.

What is the difference between simple distillation and fractional distillation?

Simple distillation is used to separate a solvent from a solution (such as water from salt solution) when the dissolved solute has a much higher boiling point. Fractional distillation is used to separate a mixture of liquids with different boiling points, such as separating crude oil into fractions using a fractionating column.

When are filtration and crystallisation respectively used to separate mixtures?

Filtration separates an insoluble solid (such as a precipitate) from a solution. Crystallisation separates a soluble salt from a solution by gently warming the solution to evaporate solvent until saturated, cooling it to grow crystals, and then collecting and drying them.

Based on this paper chromatogram, which substance is the most soluble in the solvent and why?

Substance B is the most soluble because the more soluble a substance is in the mobile phase, the further up the paper it travels.

What is the formula used to calculate the $R_f$ value in paper chromatography?

$$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$$

How are elements organized in the Periodic Table in terms of atomic number, groups, and periods?

Elements are arranged in order of atomic (proton) number. Columns are called groups and contain elements with the same number of outer-shell electrons (giving them similar chemical properties). Rows are called periods and indicate the number of electron shells.

What does the stepped red line on the Periodic Table indicate about the classification of elements?

It marks the division between metals (located to the left and bottom, which react to form positive ions) and non-metals (located to the right and top, which do not form positive ions).

How can electrical conductivity and the acid-base character of oxides classify an element as a metal or a non-metal?

An element is a metal if it is electrically conductive and forms a basic oxide. An element is a non-metal if it is non-conductive (excluding graphite) and forms an acidic oxide.

Why are the noble gases (Group 0) unreactive?

They have a stable arrangement of electrons with $8$ electrons in their outer shell (except helium, which has $2$), meaning they do not easily gain, lose, or share electrons to form molecules.

What are the relative masses, relative charges, and positions of the three subatomic particles in an atom?

Proton: relative mass $1$, relative charge $+1$, located in the nucleus. Neutron: relative mass $1$, relative charge $0$, located in the nucleus. Electron: relative mass $\frac{1}{1836}$, relative charge $-1$, located in shells around the nucleus.

What is meant by the terms atomic number, mass number, and isotopes?

Atomic number is the number of protons in the nucleus of an atom. Mass number is the total number of protons and neutrons. Isotopes are different atoms of the same element containing the same number of protons but different numbers of neutrons.

How is the relative atomic mass ($A_r$) of chlorine calculated from a sample of $75\%$ chlorine-35 and $25\%$ chlorine-37?

$$A_r = \frac{(35 \times 75) + (37 \times 25)}{100} = \frac{3550}{100} = 35.5$$

What type of chemical bonding and structure is illustrated in this diagram?

Metallic bonding, which consists of a giant regular lattice of positive metal ions surrounded by a sea of delocalised electrons held together by strong electrostatic attractions.

Why are metals good conductors of electricity and malleable?

Metals conduct electricity because delocalised outer-shell electrons are free to move throughout the whole structure. They are malleable because the regular layers of atoms can slide over each other without breaking the metallic bond.

How do the arrangement, movement, and energy of particles differ among the three states of matter shown?

Solids have particles closely packed in a regular arrangement vibrating in fixed positions with the least energy. Liquids have particles close together in a random arrangement that can move around each other with moderate energy. Gases have particles widely spaced in a random arrangement moving rapidly in all directions with the most energy.

Why does diffusion occur in gases and liquids, but not in solids?

Diffusion requires particles to move from an area of high concentration to an area of low concentration. In solids, particles cannot move from place to place and can only vibrate, preventing diffusion.

What is the difference between a solute, a solvent, a solution, and a saturated solution?

A solute is a substance that dissolves in a liquid. A solvent is the liquid in which the solute dissolves. A solution is the mixture formed when a solute dissolves in a solvent. A saturated solution is one in which no more solute can dissolve at that temperature.

Why does the solubility of a solid solute in water generally increase as temperature rises?

Water particles gain more kinetic energy at higher temperatures, enabling them to overcome the intermolecular forces of attraction holding the solid particles together and causing them to break apart more readily.

What is a covalent bond, and what produces the electrostatic attraction holding it together?

A covalent bond is formed between non-metal atoms by the sharing of a pair of electrons. The bond is held together by the electrostatic attraction between the positive nuclei of the atoms and the shared pair of negative electrons.

Why do simple molecular substances have low melting and boiling points, whereas giant covalent structures have high melting and boiling points?

Simple molecular substances have weak intermolecular forces between molecules that require little energy to overcome when melted or boiled (covalent bonds are not broken). Giant covalent substances are held by many strong covalent bonds throughout the entire structure that require large amounts of energy to break.

What allotrope of carbon is shown in this diagram, and how does its structure explain its physical properties?

Diamond. Each carbon atom is covalently bonded to $4$ other carbon atoms in a giant tetrahedral network, making it very hard with a high melting point. It does not conduct electricity because it contains no free or delocalised electrons.

Why is graphite soft and slippery, and why is it able to conduct electricity?

In graphite, each carbon atom forms $3$ covalent bonds into layers of hexagonal rings with weak intermolecular forces and no covalent bonds between the layers, allowing layers to slide over each other. One electron per carbon atom is delocalised and free to move, enabling graphite to conduct electricity.

Using this calculation triangle, how many moles are present in $42\,g$ of carbon ($A_r = 12$)?

$$\text{Moles} = \frac{\text{Mass}}{M_r} = \frac{42}{12} = 3.5\,\text{moles}$$

What is the formula for percentage yield, and why is the actual yield of a reaction often less than $100\%$?

$\text{Percentage yield} = \frac{\text{Amount of product produced}}{\text{Maximum amount of product possible}} \times 100$. Yield is less than $100\%$ because: the reaction may not go to completion because it is reversible, some product may be lost during separation, or reactants may react in unexpected side reactions.

What is the difference between an empirical formula and a molecular formula?

An empirical formula is the simplest whole number ratio of atoms of each element in a compound. A molecular formula is the actual number of atoms of each element in a compound.

What volume is occupied by $1\,mol$ of any gas at room temperature and pressure (RTP), and what volume do $5\,moles$ of $H_2$ occupy?

$1\,mol$ of any gas occupies $24\,dm^3$ ($24000\,cm^3$) at RTP. Therefore, $5\,moles$ of $H_2$ occupies: $\text{Volume} = 5 \times 24 = 120\,dm^3$.

What are the rules for predicting the products formed at the cathode and anode during the electrolysis of an aqueous solution using inert electrodes?

At the cathode ($-$): hydrogen gas ($H_2$) is produced unless the metal in the compound is less reactive than hydrogen (e.g., copper), in which case the pure metal is formed. At the anode ($+$): oxygen gas ($O_2$) is produced from $OH^-$ unless the solution contains halide ions ($Cl^-$, $Br^-$, $I^-$), in which case the halogen gas ($Cl_2$, $Br_2$, $I_2$) is produced.

What are the products of the electrolysis of molten lead(II) bromide ($PbBr_2$)?

Lead ($Pb\,(s)$) is produced at the cathode, and bromine ($Br_2\,(l)$) is produced at the anode.

Does this ionic half-equation represent oxidation or reduction, and what mnemonic defines this classification?

It represents oxidation because electrons are lost ($2Cl^- \rightarrow Cl_2 + 2e^-$). This is remembered by the mnemonic OILRIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

In the electrolysis of aqueous copper(II) sulfate using copper electrodes, what occurs at the anode and cathode?

The impure copper anode loses mass as copper atoms lose electrons to form $Cu^{2+}$ ions, leaving impurities as sludge below. The pure copper cathode gains mass as $Cu^{2+}$ ions gain electrons and are discharged as a layer of pure copper.

What structure is represented in this diagram, and why does it have a high melting and boiling point?

A giant ionic lattice of sodium chloride ($NaCl$). It has a high melting and boiling point because strong electrostatic forces of attraction between oppositely charged ions ($Na^+$ and $Cl^-$) act in all directions and require large amounts of energy to overcome.

Why do ionic compounds conduct electricity when molten or in aqueous solution, but not when solid?

In solid ionic compounds, the ions are fixed in a rigid lattice and cannot move. When molten or in aqueous solution, the ions become free to move and carry an electric charge.

What are the characteristic charges of ions formed by elements in Groups 1, 2, 3, 5, 6, and 7?

Group 1 forms $+1$ ions; Group 2 forms $+2$ ions; Group 3 forms $+3$ ions; Group 5 forms $-3$ ions; Group 6 forms $-2$ ions; Group 7 forms $-1$ ions.

What are the chemical formulas and charges for the ammonium, hydroxide, carbonate, nitrate, and sulfate ions?

Ammonium: $NH_4^+$; Hydroxide: $OH^-$; Carbonate: $CO_3^{2-}$; Nitrate: $NO_3^-$; Sulfate: $SO_4^{2-}$.