Particles, Atoms & Reactions

In a solid, how are the particles arranged and how do they move?

• AFixed in place and motionless, without vibrating
• BSpread out randomly, moving quickly in random directions
• CTightly packed in a fixed, regular pattern, vibrating in place✓
• DLoosely packed, sliding freely past one another

Answer: In a solid, strong forces of attraction hold particles close together in a regular, repeating arrangement. The particles cannot move from place to place, but they do vibrate around a fixed position, which is why solids keep a definite shape and volume. A liquid or gas arrangement would let particles slide past each other or spread out, which a solid does not do.

Why does a gas exert pressure on the walls of its container?

• AIts particles are constantly colliding with the walls✓
• BIts particles cannot move at all
• CIts particles are chemically bonded to the container walls
• DIts particles are far heavier than the walls of the container

Answer: Gas particles move quickly in random directions and are far apart, so they are constantly colliding with each other and with the walls of their container. Each collision pushes on the wall, and the combined effect of countless tiny collisions is felt as gas pressure. This is different from a solid, where particles vibrate in fixed positions and do not travel far enough to strike a container wall.

What happens to the particles when a solid melts and becomes a liquid?

• AThey gain enough energy to move out of their fixed positions✓
• BThey stop vibrating and lock into a fixed lattice
• CThey gain energy and spread far apart to fill the available space
• DThey lose energy and pack into a rigid, regular pattern

Answer: Heating a solid gives its particles more energy, so they vibrate more and eventually gain enough energy to overcome the forces holding them in fixed positions. This lets the particles move around each other while staying close together, which is the particle picture of a liquid. Spreading far apart to fill available space describes a gas forming, not a liquid, and losing energy describes cooling, not melting.

What is happening to the particles of a gas during condensation?

• AThey break free of their fixed positions
• BThey stay the same distance apart, unchanged
• CThey gain energy and spread further apart
• DThey lose energy and move closer together✓

Answer: Condensation happens when a gas cools, so its particles lose kinetic energy and slow down. As they slow, the forces of attraction between them can pull them closer together, forming a liquid. This is the reverse of evaporation, in which particles gain energy and spread apart into a gas.

A sealed, rigid container of gas is heated. What happens to the pressure inside it?

• AIt increases, because the faster-moving particles collide with the walls more often and with more force✓
• BIt stays the same, because the total number of particles inside has not changed
• CIt decreases, because the particles spread out far apart to fill the whole of the larger space available
• DIt decreases, because heating slows down the movement of the particles inside

Answer: Heating a gas transfers energy to its particles, making them move faster on average. In a sealed, rigid container the volume cannot change, so the faster particles collide with the container walls more frequently and with greater force, which increases the pressure. Pressure staying the same would only make sense if the particles were not speeding up, which contradicts what heating actually does.

In particle terms, why is a gas so much less dense than the same substance as a solid?

• AGas particles are much heavier per particle than solid particles of the same substance
• BGas particles are arranged in a neat, regular pattern
• CGas particles are chemically different from solid particles
• DGas particles are spread much further apart than solid particles✓

Answer: Density depends on how much mass is packed into a given volume. In a gas, particles have much more space between them than in a solid, so a given volume of gas contains far fewer particles, and therefore far less mass, than the same volume of solid. The particles themselves are identical in a gas and a solid of the same substance; only their spacing and arrangement change.

Why can gases be compressed much more easily than liquids or solids?

• AThere is a lot of empty space between gas particles✓
• BGas particles are held together by very strong forces of attraction
• CGas particles are arranged in a tightly packed, regular pattern
• DGas particles cannot move or change position at all

Answer: Gas particles are spread far apart with large gaps between them, so squeezing a gas mainly pushes those particles closer together into the empty space. Liquids and solids already have particles touching or nearly touching, so there is very little space left to compress. Strong forces of attraction and a tightly packed, regular pattern describe a solid, not a gas.

What did John Dalton's simple atomic model propose?

• AAtoms are mostly empty space around a tiny, dense nucleus
• BAtoms contain electrons embedded in positive charge
• CAtoms of different elements are identical to each other
• DAtoms are tiny, solid spheres that cannot be divided✓

Answer: Dalton's model, developed in the early 1800s, described atoms as tiny, solid spheres that could not be broken into smaller pieces, with atoms of different elements having different sizes and masses. Later scientists discovered that atoms actually contain smaller particles and mostly empty space, but Dalton's simple model is still a useful starting point. Atoms of different elements are not identical to each other, which is part of what makes each element distinct.

What is the key difference between an element and a compound?

• AAn element can always be broken apart by simple filtration, while a compound cannot be broken apart this way
• BAn element contains only one type of atom, while a compound contains two or more types of atom chemically joined together✓
• CAn element always has a fixed, unchanging chemical formula, while a compound never has a formula at all
• DAn element is always found as a gas at room temperature and pressure, while a compound is always found as a solid at room temperature and pressure

Answer: An element is a pure substance made of only one type of atom, such as oxygen or iron. A compound forms when two or more different types of atom join together chemically in fixed proportions, such as water or carbon dioxide, and has different properties from the elements it contains. Physical state and whether a substance has a formula do not distinguish elements from compounds, since both elements and compounds can exist as solids, liquids or gases and can be represented with a formula.

What does the chemical symbol for an element, such as 'O' for oxygen, represent?

• AA description of the element's colour and physical state
• BA count of how many atoms are present in a sample of the element
• CA shorthand code that identifies which element it is✓
• DA measurement of the element's mass in grams

Answer: Every element has its own one- or two-letter chemical symbol, agreed internationally, which acts as a shorthand way of identifying it without writing its full name. The symbol on its own says nothing about how many atoms are present, what the element looks like, or its mass. Those details come from a chemical formula or from separate data, not from the symbol itself.

A water molecule has the formula H2O. What does this formula tell you?

• AThe substance contains equal masses of hydrogen and oxygen
• BEach molecule contains two hydrogen atoms and one oxygen atom✓
• CThe substance is a mixture of hydrogen gas and oxygen gas, not chemically joined
• DEach molecule contains two oxygen atoms and one hydrogen atom, joined loosely

Answer: A chemical formula shows which elements are present in a compound and how many atoms of each are chemically joined in one unit. The formula H2O shows that each water molecule is made of two hydrogen atoms bonded to one oxygen atom. Water is a compound with its own distinct properties, not simply a mixture of hydrogen and oxygen gases that have not reacted together.

A candle appears to lose mass as it burns. What does the idea of conservation of mass explain about this?

• ASome of the candle's mass turns into light and disappears
• BIts atoms combine with oxygen and escape as gases✓
• CThe candle's atoms are destroyed by the heat of the flame
• DMass is not conserved during a chemical reaction like this

Answer: The law of conservation of mass says that atoms are never created or destroyed in a chemical reaction, only rearranged into new substances. When a candle burns, the wax reacts with oxygen from the air and the atoms end up in gases such as carbon dioxide and water vapour, which escape and are not weighed. Atoms are never destroyed by heat, and mass is always conserved even though it may look like some has vanished.

What is an atom?

• AA group of two or more different elements that have joined together through a chemical reaction
• BA tiny region of empty space inside a molecule that contains no matter at all
• CA mixture of several different substances that have been combined together but not reacted chemically
• DThe smallest part of an element that still has the chemical properties of that element✓

Answer: An atom is the smallest particle of an element that can exist and still show the chemical properties of that element. Joining two or more different elements chemically describes a compound, not a single atom, and an unreacted mixture of substances is not the same as one type of atom either. Atoms are the basic building blocks that make up elements, compounds and everything else in chemistry.

Iron and sulfur are heated together and react to form iron sulfide. What does this show about the product?

• AIt is simply a mixture of iron and sulfur, unchanged
• BIt is exactly the same as iron and sulfur simply mixed together and then gently heated
• CIt is a new compound with different properties from both iron and sulfur✓
• DIt is the element iron with sulfur atoms stuck loosely onto its outer surface

Answer: When iron and sulfur are heated, their atoms react and rearrange to form a new compound, iron sulfide, which has completely different properties from either starting element, such as its colour and magnetism. This is different from simply mixing iron filings and sulfur powder together, where each substance keeps its own separate properties and no chemical change occurs. Because a genuine chemical reaction has taken place, the iron and sulfur atoms are now chemically joined, not just placed side by side.

What does it mean for a substance to be chemically pure?

• AIt has been through a filtration process that removes all solids from it
• BIt consists of a single element only, never a compound
• CIt has an unusually high melting point compared with most other similar substances
• DIt consists of only one substance, with no other substances mixed in✓

Answer: A pure substance is made up of only one element or one compound, with nothing else mixed in, so it has a fixed melting and boiling point. A pure substance can be either an element or a compound; it is not restricted to elements only. Being pure has nothing directly to do with having gone through filtration or having a particularly high melting point.

What is a mixture, in chemical terms?

• AA compound that contains exactly two different types of atom
• BTwo or more elements that have reacted to form a new, single substance
• CA single element that has been broken down into smaller, simpler substances
• DTwo or more substances mixed together but not chemically joined✓

Answer: A mixture contains two or more different substances that are physically combined but not chemically bonded together, so each substance keeps its own individual properties. This is different from a compound, where atoms of different elements are chemically joined to form a genuinely new substance. Because the substances in a mixture are not chemically joined, they can usually be separated again using physical methods.

Sand is mixed with water. Which technique separates the insoluble solid from the liquid?

• AChromatography, which separates coloured substances by how far they travel
• BFiltration, in which the mixture is passed through a filter paper that traps the solid particles✓
• CEvaporation, which boils away the liquid completely until only solid remains
• DDistillation, which relies on differences in boiling point

Answer: Filtration works because a filter paper has tiny holes that let liquid particles pass through but are too small for solid particles, such as sand, to pass through, so the solid is trapped on the paper while the liquid collects below. Evaporation is used to obtain a dissolved solid from a solution, not an insoluble solid mixed with a liquid, and chromatography and distillation solve different separation problems again. Because the sand is insoluble and simply mixed in, rather than dissolved, filtration is the appropriate technique here.

Salt has been dissolved in water. Which technique recovers the dissolved solid from the solution?

• AChromatography, spotting drops of the solution onto absorbent paper
• BEvaporation, heating the solution until all the water has boiled away, leaving the solid behind✓
• CDistillation, collecting the condensed vapour in a separate container
• DFiltration, passing the solution through a sheet of filter paper

Answer: Evaporation works by heating a solution so that the solvent, usually water, turns into vapour and escapes, leaving the dissolved solid behind as a residue in the container. Filtration cannot separate a dissolved solid from its solvent because the dissolved particles are far too small to be trapped by filter paper. Distillation is used when the liquid itself needs to be collected and kept pure, rather than the dissolved solid.

A mixture of two liquids with different boiling points needs to be separated. Which technique is used?

• AEvaporation, boiling away all of the liquid mixture until only a solid residue remains
• BDistillation, heating the mixture so the liquid with the lower boiling point evaporates first and is then condensed and collected✓
• CFiltration, passing the entire mixture very slowly through a sheet of filter paper
• DChromatography, letting the mixture spread slowly up a strip of absorbent paper

Answer: Distillation separates a mixture of liquids by heating it so that the liquid with the lower boiling point evaporates first, then cooling and condensing that vapour in a separate container so it can be collected. Filtration only separates an insoluble solid from a liquid, and chromatography separates substances based on how they move through paper, not by boiling point. Evaporation removes an entire liquid solvent to leave a dissolved solid, which is not the goal when both substances in the mixture are liquids.

How does paper chromatography separate the different dyes in a mixture such as ink?

• AThe dyes react chemically with the paper's fibres
• BThe dyes are filtered out by tiny holes in the paper
• CThe dyes move up the paper at different rates✓
• DThe dyes evaporate off the paper at different rates

Answer: In paper chromatography, a solvent carries the dissolved dyes up a strip of paper, and each dye moves at a different rate depending on how soluble it is and how strongly it sticks to the paper. Because the dyes travel different distances in the same time, they end up separated into distinct spots or bands, which can be compared to identify substances. The dyes do not evaporate off the paper, react chemically with its fibres, or get trapped by tiny holes; the separation happens simply because they move at different speeds.

Someone opens a bottle of perfume in one corner of a room, and soon the smell can be noticed right across the room, even with no draught. Which particle-model process explains this?

• AFiltration, perfume particles passing through the air as though through a filter
• BDiffusion, the natural spreading out of particles from a region of high concentration to a region of low concentration✓
• CEvaporation, the liquid perfume slowly turning entirely into a gas inside the bottle
• DCondensation, perfume gas particles turning back into a liquid on the skin

Answer: Diffusion is the movement of particles from a region where they are more concentrated to a region where they are less concentrated, caused by their constant random motion, and it happens fastest in gases because gas particles move quickly and are far apart. As perfume particles diffuse through the air, they spread out until they can be smelled throughout the room, even without any draught to carry them. Evaporation only describes the liquid perfume turning into vapour in the bottle, not the further spreading of that vapour through the room.

In a chemical reaction, what happens to the atoms of the reactants?

• AThey are created from nothing to form the products
• BThey are destroyed and replaced with entirely new atoms
• CThey are rearranged into new substances, but no atoms are created or destroyed✓
• DThey stay in exactly the same substances, unchanged

Answer: During a chemical reaction, the bonds between atoms in the reactants break, and the same atoms then join together in new arrangements to form different substances, the products. No atoms are ever created or destroyed in the process, which is why the total mass stays the same before and after the reaction. Atoms simply swapping partners, rather than disappearing or appearing from nowhere, is what makes a chemical reaction different from the reactants staying unchanged.

What is combustion?

• AA fuel reacting with oxygen, releasing heat and light energy✓
• BA single substance breaking down into simpler substances, caused by heating
• CA solid changing directly into a gas without becoming a liquid first
• DA more reactive element taking the place of a less reactive one in a compound

Answer: Combustion is the chemical reaction between a fuel and oxygen, usually from the air, which releases energy as heat and often light in the form of a flame. This is different from thermal decomposition, where a single substance is broken apart by heating without needing oxygen to react with, and from displacement, where one element swaps places with another in a compound. Combustion needs a fuel and oxygen to react together; a solid turning straight into a gas is a change of state, not a chemical reaction at all.

Copper carbonate is heated strongly and breaks down into copper oxide and carbon dioxide. What type of reaction is this?

• AThermal decomposition✓
• BA neutralisation reaction
• CA displacement reaction
• DA simple combustion reaction

Answer: Thermal decomposition is a reaction in which a single compound is broken down by heating into two or more simpler substances, exactly as copper carbonate splits into copper oxide and carbon dioxide. No fuel is reacting with oxygen here, so it is not combustion, and no element is swapping places with another in a compound, so it is not a displacement reaction. There is also no acid and alkali reacting, so neutralisation does not describe this change either.

An iron nail left outdoors slowly rusts. In terms of oxidation, what has happened to the iron?

• AThe iron has lost oxygen to the surrounding air around it
• BThe iron has gained oxygen from the air and water around it, forming iron oxide✓
• CThe iron has dissolved into the surrounding rainwater
• DThe iron has melted, then re-solidified into a new shape

Answer: Oxidation is a reaction in which a substance gains oxygen, and rusting is an everyday example: iron reacts slowly with oxygen and water in the air to form iron oxide, the substance known as rust. Losing oxygen is the opposite process, called reduction, so it cannot describe rusting. The iron has not melted or dissolved; it has undergone a chemical change into a new compound, not simply a change of state.

A strip of magnesium is added to a solution of copper sulfate, and the magnesium becomes coated in copper. What type of reaction has taken place?

• AA neutralisation, in which an acid reacts with an alkali
• BA thermal decomposition, in which copper sulfate breaks apart on its own
• CA displacement reaction, in which magnesium takes the place of copper in the compound✓
• DA change of state, in which copper simply melts onto the magnesium

Answer: In a displacement reaction, a more reactive element takes the place of a less reactive element within a compound; here, magnesium is more reactive than copper, so it displaces copper from copper sulfate, forming magnesium sulfate and leaving copper metal behind. No heating is decomposing a single substance on its own, so this is not thermal decomposition, and there is no acid and alkali involved, so it is not neutralisation. Copper appearing on the magnesium is due to a chemical reaction, not copper simply melting.

Hydrochloric acid is added to magnesium metal, and bubbles of gas form as the metal reacts and disappears. What are the products of this reaction?

• AA salt and hydrogen gas✓
• BA metal oxide and some water
• CThe original acid, unchanged
• DOnly a salt, with no gas at all

Answer: When an acid reacts with a reactive metal such as magnesium, the products are a salt and hydrogen gas, which is why bubbles of gas are seen as the metal is gradually used up. A salt and water are the products when an acid reacts with an alkali, not with a metal, so that combination does not fit here. The acid clearly reacts and changes, since a new gas is produced and the metal is used up, so the acid cannot remain unchanged.

Sodium hydroxide solution, an alkali, is added to hydrochloric acid until the mixture is exactly neutral. What are the products of this neutralisation reaction?

• AWater, but no salt
• BA salt and water✓
• CThe acid and alkali
• DA salt, but no water

Answer: Neutralisation is the reaction between an acid and an alkali, which produces a salt and water; the particular salt formed here would be sodium chloride, from the sodium and chloride ions present. A salt and hydrogen gas are produced when an acid reacts with a reactive metal, not with an alkali, so that is a different reaction entirely. Because the mixture has become neutral, the acid and alkali have reacted and been used up, so they cannot remain as they started.