The nature of substances and chemical reactions

Q: What is an element?

A: A substance made of only one type of atom.

Q: Can elements be broken down into simpler substances?

A: No, elements cannot be broken down into a simpler substance.

Q: What happens when iron is heated?

A: It produces molten iron.

Q: What are elements the building blocks of?

A: All other substances, such as compounds.

Q: How are atoms arranged in solid elements?

A: They are tightly packed together.

Atoms in gases often go round in pairs, what is a molecule with two atoms in it called?

A diatomic molecule

Q: What is a compound?

A: A substance made of two or more different types of atom that are chemically bonded.

Q: How is carbon dioxide formed?

A: One carbon atom reacts with two oxygen atoms in a chemical reaction.

Q: Is it easy to separate elements in a compound?

A: No, it is very difficult to separate them.

Q: How are the properties of a compound compared to the original elements?

A: They are often totally different.

Q: Does iron sulfide behave like iron or sulfur?

A: No, it’s not magnetic like iron or yellow like sulfur.

Q: How are mixtures different from compounds?

A: There is no chemical bond between the parts of a mixture.

Q: How can mixtures be separated?

A: By physical methods like distillation.

Q: What are the properties of a mixture like?

A: A combination of the properties of its separate parts.

Q: What is evaporation used to separate?

A: A soluble solid from a solution.

Q: What happens to the solvent during evaporation?

A: The solvent evaporates, leaving the soluble solid behind as crystals.

Q: Why must the solid not decompose during evaporation?

A: Because if the solid breaks down when heated, evaporation won't successfully separate it.

How do we use evaporation to separate soluble solids from solutions.

Q: What is filtration used to separate?

A: An insoluble solid from a liquid.

Q: What is the role of filter paper in filtration?

A: It lets the liquid pass through while trapping the insoluble solid.

Q: How can filtration be useful in purification?

A: It separates solid impurities from a mixture.

Q: What is simple distillation used for?

A: Separating a liquid from a solution or separating liquids with very different boiling points.

Q: What happens during simple distillation?

A: The component with the lowest boiling point evaporates, then condenses back to liquid and is collected.

Q: What is left behind after simple distillation?

A: The rest of the solution or mixture that didn’t evaporate.

Q: What is the main limitation of simple distillation?

A: It only works well when the boiling points of the substances are very different.

How do we use simple distillation to separate out solutions?

Q: What is the baseline in paper chromatography?

A: A pencil line drawn near the bottom of the filter paper where ink or dye samples are placed. It's drawn in pencil because pencil is insoluble in the solvent.

Q: Why must the solvent be placed below the baseline?

A: To prevent the ink spots from dissolving directly into the solvent at the start.

Q: What carries the dyes up the paper in chromatography?

A: The solvent, which seeps up the paper, carrying the inks with it.

Q: Why do different dyes move different distances in chromatography?

A: some dyes will be more attracted to the paper so spend more time stuck to it and not travel as far, whilst others will be more soluble in the solvent so will travel further up the paper

Q: What is a chromatogram?

A: The final pattern of separated spots formed on the chromatography paper.

Q: What does the lid on the container do during chromatography?

A: It prevents the solvent from evaporating.

Q: What does an Rf value tell you in chromatography?

A: An Rf value is the ratio between the distance travelled by the substance and the distance travelled by the solvent

Q: How do you calculate the Rf value?

A: Rf = Distance moved by substance ÷ Distance moved by solvent front

Q: How do you measure the distance a dye traveled?

A: From the baseline to the center of the dye spot.

Q: How can chromatography help identify a substance in a mixture?

A: Compare the Rf value of a known pure sample to the Rf of a spot in the mixture—if they match, it's likely the same substance.

Q: What are ionic compounds made of?

A: A positively charged part and a negatively charged part.

Q: What is the overall charge of any ionic compound?

A: Zero — all the positive and negative charges are balanced

Q: How do you work out the formula for an ionic compound?

A: You can use the charges on the individual ions present to work out the formula for the ionic compound

Q: What do chemical symbols represent?

A: Atoms of each element can be represented by a one or two letter symbol.

Q: What does a molecular formula show?

A: The number of atoms and each type of atom in a molecule.

Q: What does CH₄ tell you about methane?

A: It contains 1 carbon atom and 4 hydrogen atoms.

Q: What is a structural formula?

A: A structural formula shows the atoms and covalent bonds in a molecule as a picture.

Q: What do brackets in a formula indicate, like in CH₃(CH₂)₂CH₃?

A: They show repeated groups; (CH₂)₂ means there are 2 CH₂ units.

Q: How do you find the molecular formula from a structural formula?

A: Count up how many atoms of each element are in the structure and write them down.

Q: What is the molecular formula of CH₃(CH₂)₂CH₃?

A: C₄H₁₀ (4 carbon atoms, 10 hydrogen atoms).

Q: What are the reactants and products in the reaction of magnesium with oxygen?

A: Reactants: magnesium and oxygen; Product: magnesium oxide

Q: What is the word equation for the reaction between magnesium and oxygen?

A: magnesium + oxygen → magnesium oxide

Q: What does (g) represent in a chemical equation?

A: (g) means the substance is in the gas state.

Q: What does (aq) represent in a chemical equation?

A: (aq) means the substance is dissolved in water (aqueous).

Q: What does (s) represent in a chemical equation?

A: (s) means the substance is a solid.

Q: What does (l) represent in a chemical equation?

A: (l) means the substance is a liquid.

Q: Can you change chemical formulas when balancing an equation?

A: No, you can only place numbers in front of formulas, not change them.

How do we balance an equation

Q: What is the key rule to follow when balancing chemical equations?

A: The number of atoms of each element must be the same on both sides of the equation.

Q: What happens to atoms during chemical reactions?

A: Bonds between atoms break and the atoms rearrange to form new substances (products).

Q: Do the number of atoms change during a chemical reaction?

A: No, the number of atoms in the reactants equals the number in the products.

Q: Why are chemical changes often hard to reverse?

A: Because new substances are formed with different properties.

Q: What observations show a chemical reaction is happening?

A: Change in colour, temperature change, or effervescence (bubbles/fizzing).

Q: What can a colour change in a chemical reaction indicate?

A: It can indicate a pH change, a displacement reaction, or a redox reaction.

Q: What causes a temperature change in a chemical reaction?

A: Energy being transferred either to or from the surroundings.

Q: What is an exothermic reaction?

A: A reaction where energy is transferred from the reactants to the surroundings.

Q: What is an endothermic reaction?

A: A reaction where energy is absorbed from the surroundings into the reactants.

Q: What is effervescence?

A: The release of gas during a reaction, seen as bubbling or fizzing.

Q: What are common gases released during chemical reactions?

A: Hydrogen, oxygen, and carbon dioxide.

Q: What happens when an acid reacts with a carbonate?

A: Carbon dioxide is released and the solution effervesces (bubbles).

Q: What does 'Mr' stand for in chemistry?

A: Mr stands for relative formula mass, the total of all relative atomic masses (Ar) in a compound.

Q: What does 'Ar' stand for?

A: Ar stands for relative atomic mass, the average mass of an element's atoms taking into account its isotopes.

Q: How is the relative formula mass (Mr) of a compound calculated?

A: By adding together the relative atomic masses (Ar) of all the atoms in the compound's formula.

Q: What's the difference between Ar and mass number?

A: Ar is the average atomic mass considering isotopes; mass number is the total number of protons and neutrons in an atom.

Q: What’s the difference between relative formula mass and relative molecular mass?

A: Relative molecular mass refers to compounds with molecules, but it is calculated the same way as relative formula mass.

Q: What is the formula for calculating the percentage mass of an element in a compound?

A: Percentage mass = (Ar × number of atoms of the element ÷ Mr of the compound) × 100

Q: What is the Ar of an element based on?

A: The average mass of all its isotopes relative to 1/12th the mass of a carbon-12 atom.

Q: If MgCl₂ has Ar(Mg) = 24 and Ar(Cl) = 35.5, what is its Mr?

A: Mr of MgCl₂ = 24 + (2 × 35.5) = 95

Q: What does the mass number of an atom represent?

A: The total number of protons and neutrons in its nucleus.

Q: What is a mole in chemistry?

A: A mole is the name given to an amount of a substance containing 6 × 10²³ particles, known as the Avogadro number.

Q: What types of particles can one mole contain?

A: A mole can contain atoms, molecules, ions, or electrons.

Q: Why is the number used in a mole (6 × 10²³) so large?

A: Because it represents the number of atoms or molecules in a sample that has a mass in grams equal to its relative atomic mass (Aᵣ) or relative formula mass (Mᵣ).

Q: What does the mass of one mole of atoms or molecules equal?

A: The relative atomic mass (Aᵣ) or relative formula mass (Mᵣ) in grams.

Q: What is the relative atomic mass (Aᵣ) of carbon?

A: 12

Q: What is the relative formula mass (Mᵣ) of nitrogen gas (N₂)?

A: 28 (14 × 2)

Q: What is the relative formula mass (Mᵣ) of carbon dioxide (CO₂)?

A: 44 (12 + 2 × 16)

Q: How many particles are in 12 g of carbon, 28 g of N₂, or 44 g of CO₂?

A: Each contains one mole, or 6 × 10²³ particles.

Q: What is the formula for finding the number of moles in a given mass?

A: Number of moles = Mass (g) ÷ Mᵣ (relative formula mass)

Q: Example: How many moles are there in 66 g of CO₂?

A:

1. Calculate Mᵣ of CO₂ = 44

2. Number of moles = 66 ÷ 44 = 1.5 moles

Q: How can the mole formula be rearranged using the formula triangle?

A:

• Mass = No. of moles × Mᵣ

• Mᵣ = Mass ÷ No. of moles

• No. of moles = Mass ÷ Mᵣ

Q: Example: What mass of carbon is there in 4 moles of carbon dioxide?

A:There are 4 moles of carbon in 4 moles of CO₂.

Mass = 4 × 12 = 48 g

Q: How do you calculate the number of moles from a given mass?

A:Number of moles = Mass ÷ Mᵣ

Q: How many moles are in 60 g of magnesium? (Ar of Mg =24)

A:Moles = 60 ÷ 24 = 2.5 moles

Q: What is the mass of 2.5 moles of MgO? (Mr of MgO = 40)

A:Mass = Moles × Mᵣ = 2.5 × 40 = 100 g

Q: What does the term ‘yield’ mean in a chemical reaction?

A: Yield is the amount of product obtained from a chemical reaction.

Q: What is theoretical yield?

A: The maximum amount of product that could be formed from a given amount of reactants, calculated from the balanced equation.

Q: What is the formula for percentage yield?

A:Percentage yield = (actual yield ÷ theoretical yield) × 100

Q: What does a 100% yield mean?

A: It means all the reactants were converted into product — you got all the product you expected.

Q: What does a 0% yield mean?

A: No reactants were converted into product; no product was made.

Q: Why is percentage yield usually less than 100%?

A: In practice, you never get 100% yield due to losses or incomplete reactions.