GCSE Science₈₄₆₅

Recall and explain the main features of the particle model in terms of the states of matter and change of state, distinguishing between physical and chemical changes.

(HT only) Explain the limitations of the particle model in relation to changes of state when particles are represented by inelastic spheres.

The three states of matter are solid, liquid and gas. Melting and freezing take place at the melting point, boiling and condensing take place at the boiling point.

The three states of matter can be represented by a simple model. In this model, particles are represented by small solid spheres. Particle theory can help to explain melting, boiling, freezing and condensing.

(HT only) Limitations of the simple model include that there are no forces between the spheres, and that atoms, molecules and ions are not solid spheres.

WS 1.2

Recognise/draw simple diagrams to model the difference between substances in the solid, liquid and gas states.

WS 3.5

Predict the states of substances at different temperatures given appropriate data.

MS 1d

Relate the size and scale of atoms to objects in the physical world.

Define density and explain the differences in density between the different states of matter in terms of the arrangements of the atoms or molecules.

The density of a material is defined by the equation:

$\mathit{d}\mathit{e}\mathit{n}\mathit{s}\mathit{i}\mathit{t}\mathit{y}\mathsf{=}\mathit{ }\frac{\mathit{m}\mathit{a}\mathit{s}\mathit{s}}{\mathit{v}\mathit{o}\mathit{l}\mathit{u}\mathit{m}\mathit{e}}$

$\mathsf{[}\mathit{\rho }\mathsf{=}\frac{\mathit{m}}{\mathit{V}}\mathsf{}\mathsf{]}$

density, ρ , in kilograms per metre cubed, kg/m³

mass, m , in kilograms, kg

volume, V , in metres cubed, m³

MS 1a, 1b, 1c, 3c

Recall and apply this equation to changes where mass is conserved.

WS 3.3

Carry out and represent mathematical and statistical analysis.

WS 4.3, 4.5

Use and interconvert SI units in calculations.

Explain how the motion of the molecules in a gas is related both to its temperature and its pressure: hence explain the relation between the temperature of a gas and its pressure at constant volume (qualitative only).

The molecules of a gas are in constant random motion. The temperature of the gas is related to the average kinetic energy of the molecules. The higher the temperature, the greater the average kinetic energy and so the faster the average speed of the molecules.

When the molecules collide with the wall of their container they exert a force on the wall. The total force exerted by all of the molecules inside the container on a unit area of the walls is the gas pressure.

Changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas.

Describe how heating a system will change the energy stored within the system and raise its temperature or produce changes of state.

Describe how, when substances melt, freeze, evaporate, condense or sublimate, mass is conserved but that these physical changes differ from chemical changes because the material recovers its original properties if the change is reversed.

Energy is stored inside a system by the particles (atoms and molecules) that make up the system. This is called internal energy.

The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance. The nature of the particles involved depends on the type of bonding and the structure of the substance. The stronger the forces between the particles the higher the melting point and boiling point of the substance.

This topic links with Structure and bonding.

WS 3.3

Carry out and represent mathematical and statistical analysis.

Define the term specific heat capacity and distinguish between it and the term specific latent heat.

The increase in temperature of a system depends on the mass of the substance heated, the type of material and the energy input.

The following equation, given on the Physics equations sheet, applies:

$\mathit{c}\mathit{h}\mathit{a}\mathit{n}\mathit{g}\mathit{e}\mathit{ }\mathit{i}\mathit{n}\mathit{ }\mathit{t}\mathit{h}\mathit{e}\mathit{r}\mathit{m}\mathit{a}\mathit{l}\mathit{ }\mathit{e}\mathit{n}\mathit{e}\mathit{r}\mathit{g}\mathit{y}\mathit{ }\mathsf{=}\mathit{m}\mathit{a}\mathit{s}\mathit{s}\mathit{ }\mathsf{\times }\mathit{s}\mathit{p}\mathit{e}\mathit{c}\mathit{i}\mathit{f}\mathit{i}\mathit{c}\mathit{ }\mathit{h}\mathit{e}\mathit{a}\mathit{t}\mathit{ }\mathit{c}\mathit{a}\mathit{p}\mathit{a}\mathit{c}\mathit{i}\mathit{t}\mathit{y}\mathit{ }\mathsf{\times }\mathit{t}\mathit{e}\mathit{m}\mathit{p}\mathit{e}\mathit{r}\mathit{a}\mathit{t}\mathit{u}\mathit{r}\mathit{e}\mathit{ }\mathit{c}\mathit{h}\mathit{a}\mathit{n}\mathit{g}\mathit{e}$

$\mathsf{[}\mathsf{∆}\mathit{E}\mathit{ }\mathsf{=}\mathit{m}\mathit{ }\mathit{c}\mathit{ }\mathsf{∆}\mathit{\theta }\mathsf{}\mathsf{]}$

change in thermal energy, ∆ E , in joules, J

mass, m , in kilograms, kg

specific heat capacity, c , in joules per kilogram per degree Celsius, J/kg °C

temperature change, ∆ θ , in degrees Celsius, °C

The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.

The energy needed for a substance to change state is called latent heat. When a change of state occurs, the energy supplied changes the energy stored (internal energy) but not the temperature.

WS 3.5, MS 4a

Interpret heating and cooling graphs that include changes of state.

WS 4.3, 4.5, MS 1a, 3c, 3d

Apply this equation, which is given on the Physics equations sheet, to calculate energy changes when a material is heated or cooled.

WS 3.3

Carry out and represent mathematical and statistical analysis.

Describe and calculate the changes in energy involved when a system is changed by heating (in terms of temperature change and specific heat capacity).

The specific latent heat of a substance is the amount of energy required to change the state of one kilogram of the substance with no change in temperature. The following equation, given on the Physics equations sheet , applies:

$\mathit{e}\mathit{n}\mathit{e}\mathit{r}\mathit{g}\mathit{y}\mathit{ }\mathit{f}\mathit{o}\mathit{r}\mathit{ }\mathit{a}\mathit{ }\mathit{c}\mathit{h}\mathit{a}\mathit{n}\mathit{g}\mathit{e}\mathit{ }\mathit{o}\mathit{f}\mathit{ }\mathit{s}\mathit{t}\mathit{a}\mathit{t}\mathit{e}\mathit{ }\mathsf{=}\mathit{m}\mathit{a}\mathit{s}\mathit{s}\mathit{ }\mathsf{\times }\mathit{s}\mathit{p}\mathit{e}\mathit{c}\mathit{i}\mathit{f}\mathit{i}\mathit{c}\mathit{ }\mathit{l}\mathit{a}\mathit{t}\mathit{e}\mathit{n}\mathit{t}\mathit{ }\mathit{h}\mathit{e}\mathit{a}\mathit{t}$

$$ $\mathsf{[}\mathit{E}\mathit{ }\mathsf{=}\mathit{m}\mathit{ }\mathit{L}\mathsf{}\mathsf{]}$

energy, E , in joules , J

mass, m , in kilograms, kg

specific latent heat, L , in joules per kilogram, J/kg

Specific latent heat of fusion – change of state from solid to liquid.

Specific latent heat of vaporisation – change of state from liquid to vapour.

WS 4.3, 4.5, MS 1a, 3c, 3d

Apply this equation, which is given on the Physics equations sheet, to calculate energy changes during changes of state.

Explain what is meant by the purity of a substance, distinguishing between the scientific and everyday use of the term ‘pure’.

In chemistry, a pure substance is a single element or compound, not mixed with any other substance.

Pure elements and compounds melt and boil at specific temperatures. Melting point and boiling point data can be used to distinguish pure substances from mixtures.

In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state.

WS 3.5

Use melting point data to distinguish pure from impure substances.

Describe how and why the atomic model has changed over time.

Stages in the development of atomic models:

• Dalton atoms (1804) – spherical atoms that cannot be split up to explain the properties of gases and the formulae of compounds
• Plum pudding model (1897) – it was found that the mass of electrons, which had recently been discovered, was very much less than the mass of atoms so they must be sub-atomic particles
• the nuclear atom (1911) – an experiment which showed that most of the alpha particles directed at thin gold foil passed through but a few bounced back, suggesting the positive charge was concentrated at the centre of each gold atom
• discovery of neutrons in the nucleus (1932) – explained why the mass of atoms was greater than could be accounted for by the mass of the protons.

Students are not required to recall dates or the names of scientists.

WS 1.1

Explain, with examples, why new data from experiments or observations led to changes in atomic models.

Decide whether or not given data supports a particular theory.

Recall the typical size (order of magnitude) of atoms and small molecules.

Atoms are very small, having a radius of about 0.1 nm (1 x 10^–10 m).

The radius of a small molecule such as methane, CH₄ , is about 0.5 nm (5 x 10^–10 m).

MS 1b

Interpret expressions in standard form.

WS 4.4

Use SI units and the prefix nano.

MS 1d

Estimate the size of atoms based on scale diagrams.

Describe the atom as a positively charged nucleus surrounded by negatively charged electrons, with the nuclear radius much smaller than that of the atom and with almost all of the mass in the nucleus.

Recall that atomic nuclei are composed of both protons and neutrons, that the nucleus of each element has a characteristic positive charge, but that elements can differ in nuclear mass by having different numbers of neutrons.

Recall relative charges and approximate relative masses of protons, neutrons and electrons.

The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10^–14 m).

The relative masses and charges of protons, neutrons and electrons are:

The number of protons in an atom of an element is its atomic number. All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons.

In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.

WS 1.2

Interpret and draw diagrams to represent atoms.

Relate differences between isotopes to differences in conventional representations of their identities, charges and masses.

The sum of the protons and neutrons in an atom is its mass number.

Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.

Atoms can be represented as shown in this example:

WS 1.2

Work out numbers of protons, neutrons and electrons in atoms and ions, given atomic number and mass number of isotopes.

Recall that in each atom its electrons are arranged at different distances from the nucleus.

The electrons in an atom occupy the lowest available energy levels (innermost available shells closest to the nucleus). The electronic structure of an atom can be represented by numbers or by a diagram. For example, the electronic structure of sodium is 2,8,1 or

showing two electrons in the lowest energy level, eight in the second energy level and one in the third energy level.

This topic links with Atomic number and the periodic table .

Explain how electron microscopy has increased our understanding of sub-cellular structures.

An electron microscope has a much higher resolving power than a light microscope. This means that it can be used to study cells in much finer detail. An electron microscope can magnify up to a million times (× 1000 000) or more, which is much more than a light microscope which has a useful magnification of only about a thousand times (× 1000).

$\mathsf{m}\mathsf{a}\mathsf{g}\mathsf{n}\mathsf{i}\mathsf{f}\mathsf{i}\mathsf{c}\mathsf{a}\mathsf{t}\mathsf{i}\mathsf{o}\mathsf{n}\mathsf{ }\mathsf{=}\frac{\mathsf{s}\mathsf{i}\mathsf{z}\mathsf{e}\mathsf{ }\mathsf{o}\mathsf{f}\mathsf{ }\mathsf{i}\mathsf{m}\mathsf{a}\mathsf{g}\mathsf{e}}{\mathsf{s}\mathsf{i}\mathsf{z}\mathsf{e}\mathsf{ }\mathsf{o}\mathsf{f}\mathsf{ }\mathsf{r}\mathsf{e}\mathsf{a}\mathsf{l}\mathsf{ }\mathsf{o}\mathsf{b}\mathsf{j}\mathsf{e}\mathsf{c}\mathsf{t}}$

MS 2a, 2h

Demonstrate understanding of number, size and scale and the quantitative relationship between units.

WS 4.5

Interconvert units.

MS 1a,1b, 1c, 2h

Carry out calculations involving magnification, real size and image size including numbers written in standard form.

WS 3.3

Carry out and represent mathematical and statistical analysis.

WS 4.6

Use an appropriate number of significant figures.

WS4.4

Use prefixes centi, milli, micro and nano.

MS 1d, 2h

Make order of magnitude calculations.

MS 1d

Use estimations and explain when they should be used.

Explain how the main sub-cellular structures of eukaryotic cells (plants and animals) and prokaryotic cells are related to their functions, including the nucleus/genetic material, plasmids, mitochondria, chloroplasts and cell membranes.

Plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm and a nucleus containing the genetic material.

Bacterial cells (prokaryotic cells) are much smaller in comparison. They have cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and may have one or more small rings of DNA called plasmids.

Most animal cells have the following parts:

• a nucleus
• cytoplasm
• a cell membrane
• mitochondria
• ribosomes.

Most human cells are like most other animal cells.

In addition to the parts found in animal cells, plant cells often have:

• chloroplasts
• a permanent vacuole filled with cell sap.

Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

Explain how substances are transported into and out of cells through diffusion, osmosis and active transport.

Some substances move across cell membranes via diffusion. Diffusion is a spreading out and mixing process. Particles move from a region where they are in higher concentration to a region where their concentration is lower.

Factors that affect the rate of diffusion across a membrane are:

• the difference in concentration
• the temperature
• the surface area of the membrane.

Water may move across cell membranes by osmosis. Cell membranes are partially permeable: they allow small molecules such as water through but not larger molecules. During osmosis water diffuses from where it is more concentrated (because the solute concentration is lower), through a partially permeable membrane to where water is less concentrated (because the solute concentration is higher).

Some substances move across cell membranes via active transport. Active transport involves the movement of a dissolved substance from a region where it is less concentrated to a region where it is more concentrated. This requires energy from respiration.

Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood with a higher sugar concentration.

MS 4a, 4b, 4c, 4d

Plot, draw and interpret appropriate graphs.

WS 3.4

Represent the distribution of results and make estimations of uncertainty.

MS 1c

Calculate percentage gain and loss of mass.

WS 3.3

Carry out and represent mathematical and statistical analysis.

Describe the process of mitosis in growth, including the cell cycle.

The nucleus of body cells contains chromosomes. In body cells the chromosomes are normally found in pairs. There are 46 chromosomes in human body cells. DNA is in the chromosomes and each chromosome carries a large number of genes.

Cells divide so that organisms can grow during the development of multicellular organisms, and repair damaged tissues.

Dividing cells go through a series of stages called the cell cycle. During the cell cycle the genetic material doubles and then divides to give two new cells that are genetically identical to each other and to the original cell. Knowledge of the stages of the cell cycle and mitosis is not required.

Before a cell can divide it must grow, and make copies of all the organelles such as mitochondria and ribosomes. It must also replicate the chromosomes in the nucleus. Then it can divide by mitosis. During mitosis, the two complete sets of chromosomes are pulled to opposite sides of the cell. Two new nuclei form. Then the cell splits into two.

Explain the role of meiotic cell division in halving the chromosome number to form gametes.

Cells in reproductive organs divide by meiosis to form gametes (egg and sperm cells). Knowledge of the stages of meiosis is not required.

When a cell divides to form gametes:

• copies of the genetic information are made
• the cell divides twice to form four gametes, each with a single set of chromosomes
• all gametes are genetically different from each other.

Gametes join at fertilisation to make a new cell with the normal number of chromosomes.

The new cell divides by mitosis to grow.

Describe the function of stem cells in embryonic and adult animals.

Explain the importance of cell differentiation.

At first the cells in an embryo can grow and divide to form any type of cell. They are stem cells.

As an embryo develops most of the cells differentiate and become specialised. Specialised cells carry out a particular function. Differentiation is essential to produce a variety of cells with different functions in multicellular organisms (animals and plants).

Cells that have become specialised cannot later change into different kinds of cells. However, there are some stem cells in most adult tissues that are ready to start dividing to replace old cells or to repair damage in the tissues where they are found.

This section links with Stem cells .

Describe the difference between transverse and longitudinal waves.

Describe how ripples on water surfaces are examples of transverse waves whilst sound waves in air are longitudinal waves, and how the speed of each may be measured.

Describe evidence that in both cases it is the wave and not the water or air itself that travels.

In a transverse wave the oscillations are perpendicular to the direction of energy transfer. The ripples on a water surface are an example of a transverse wave.

In a longitudinal wave the oscillations are parallel to the direction of energy transfer. Longitudinal waves show areas of compression and rarefaction. Sound waves travelling through air are longitudinal.

WS 2.3

Describe one method to measure the speed of sound waves in air.

WS 2.2, 2.3

Describe one method to measure the speed of ripples on a water surface.

WS 3.5

Interpret given data from experiments to measure the speed of sound or water waves.

Describe wave motion in terms of amplitude, wavelength, frequency, and period; define wavelength and frequency and describe and apply the relationship between these and the wave velocity.

Waves are described by their amplitude, wavelength, frequency and period.

The amplitude of a wave is the maximum displacement of a point on a wave away from its undisturbed position.

The wavelength of a wave is the distance from a point on one wave to the equivalent point on the adjacent wave.

The frequency of a wave is the number of waves passing a point each second.

The wave speed is the speed at which the energy is transferred (or the wave moves) through the medium.

All waves obey the wave equation:

$\mathit{w}\mathit{a}\mathit{v}\mathit{e}\mathit{ }\mathit{s}\mathit{p}\mathit{e}\mathit{e}\mathit{d}\mathit{ }\mathsf{=}\mathit{f}\mathit{r}\mathit{e}\mathit{q}\mathit{u}\mathit{e}\mathit{n}\mathit{c}\mathit{y}\mathit{ }\mathsf{\times }\mathit{w}\mathit{a}\mathit{v}\mathit{e}\mathit{l}\mathit{e}\mathit{n}\mathit{g}\mathit{t}\mathit{h}$

$\mathsf{[}\mathit{v}\mathit{ }\mathsf{=}\mathit{f}\mathit{ }\mathit{\lambda }\mathsf{}\mathsf{]}$

wave speed, v , in metres per second, m/s

frequency, f , in hertz, Hz

wavelength, λ , in metres, m

Students should be able to apply the relationship:

$\mathit{p}\mathit{e}\mathit{r}\mathit{i}\mathit{o}\mathit{d}\mathsf{=}\mathit{ }\frac{\mathsf{1}}{\mathit{f}\mathit{r}\mathit{e}\mathit{q}\mathit{u}\mathit{e}\mathit{n}\mathit{c}\mathit{y}}$

$\mathsf{[}\mathit{T}\mathsf{=}\mathit{ }\frac{\mathsf{1}}{\mathit{f}}\mathsf{}\mathsf{]}$

period, T , in seconds, s

frequency, f , in hertz, Hz

WS 4.6, MS 1b, 2a

Calculate with numbers written in standard form and give answers to an appropriate number of significant figures.

MS 1c, 3b, 3c

Recall and apply the wave equation.

MS1a, 1c, 3b, 3c

Apply the equation for relationship between period and frequency, which is given on the Physics equations sheet.

WS 3.3

Carry out and represent mathematical and statistical analysis.

Recall that electromagnetic waves are transverse, are transmitted through space where all have the same velocity, and explain, with examples, that they transfer energy from source to absorber.

Recall that light is an electromagnetic wave.

Describe the main groupings of the spectrum – radio, microwave, infrared, visible (red to violet), ultraviolet, X-rays and gamma rays, that these range from long to short wavelengths and from low to high frequencies, and that our eyes can only detect a limited range.

Give examples of some practical uses of electromagnetic waves in the radio, microwave, infrared, visible, ultraviolet, X-ray and gamma ray regions.

Electromagnetic waves form a continuous spectrum.

Examples of uses of electromagnetic waves include:

• radio waves – television, radio and radio telescopes
• microwaves – satellite communications, cooking food
• infrared – electrical heaters, cooking food, infrared cameras
• visible light – fibre optic communications
• ultraviolet – fluorescent lamps, sun tanning
• X-rays – medical imaging and treatments
• gamma rays – sterilising surgical instruments, treatment of cancer.

WS 1.2

Show that the uses of electromagnetic waves illustrate the transfer of energy from source to absorber.

MS 1a, 1c, 3c

Recall and apply the relationship between frequency and wavelength across the electromagnetic spectrum.

Recall that radio waves can be produced by, or can themselves induce, oscillations in electrical circuits.

When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit.

Recall that different substances may refract, or reflect these waves; explain how some effects are related to differences in the velocity of the waves in different substances.

Shiny surfaces act as mirrors when they reflect waves. Rough surfaces scatter waves in all directions.

Electromagnetic waves change speed when they travel between different substances such as from air to glass or water. As a result they change direction. This is refraction.

WS 1.2

Construct ray diagrams to illustrate the refraction of a wave at the boundary between two different media.

Use wavefront diagrams to explain refraction in terms of the change of wave speed.