General Certificate of Secondary Education (GCSE) Combined Science Syllabus

Describe how blood glucose concentration is reduced when it is too high.

The pancreas detects high blood glucose and secretes insulin. Insulin causes body cells, especially liver and muscle, to take up glucose; the liver converts excess glucose into glycogen for storage, lowering blood glucose.

Compare nervous and hormonal (endocrine) coordination.

Nervous responses use electrical impulses along neurones, are very fast, short-lived and act on precise areas. Hormonal responses use chemicals carried in the blood, are slower, longer-lasting and often act on larger areas.

Trace the path of a reflex arc from stimulus to response.

Stimulus $\rightarrow$ receptor $\rightarrow$ sensory neurone $\rightarrow$ relay neurone (in the CNS/spinal cord) $\rightarrow$ motor neurone $\rightarrow$ effector (muscle or gland) $\rightarrow$ response. Synapses connect the neurones.

Define a gene, an allele, and the difference between genotype and phenotype.

A gene is a small section of DNA coding for a protein. An allele is a different version of a gene. Genotype is the combination of alleles an organism has; phenotype is the physical characteristics expressed.

What do the terms dominant, recessive, homozygous and heterozygous mean?

A dominant allele is expressed even if only one copy is present; a recessive allele is only expressed if two copies are present. Homozygous means two identical alleles; heterozygous means two different alleles.

For a monohybrid cross between two heterozygous parents (Bb × Bb), what is the expected phenotype ratio?

$3:1$ dominant to recessive (genotype ratio $1\,BB : 2\,Bb : 1\,bb$).

What are the four bases in DNA and how do they pair?

Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). They pair complementarily: A with T, and C with G.

Distinguish between sexual and asexual reproduction in terms of variation.

Sexual reproduction involves two parents and gametes (fusion via fertilisation), producing genetically varied offspring. Asexual reproduction involves one parent and mitosis, producing genetically identical clones with no variation.

Define biodiversity and explain why high biodiversity is important.

Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. High biodiversity makes ecosystems more stable and resilient, as species are less dependent on one another for resources.

Name the abiotic and biotic factors that affect a community.

Abiotic (non-living): light intensity, temperature, moisture, soil pH and mineral content, wind, CO2 and O2 levels. Biotic (living): availability of food, new predators, new pathogens, competition between species.

What is the structure of an atom in terms of subatomic particles and their charges?

A central nucleus contains protons (charge $+1$) and neutrons (charge $0$), surrounded by electrons (charge $-1$) in shells. Atoms have no overall charge because protons equal electrons.

Define atomic (proton) number and mass number.

Atomic number = the number of protons in an atom. Mass number = the total number of protons plus neutrons. Number of neutrons = mass number − atomic number.

What is an isotope?

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (same atomic number, different mass number).

How is the modern periodic table arranged, and what do groups and periods represent?

Elements are arranged in order of increasing atomic (proton) number. Vertical columns (groups) contain elements with the same number of outer-shell electrons and similar properties; horizontal rows (periods) represent the number of occupied electron shells.

Describe the trend in reactivity down Group 1 (alkali metals) and explain it.

Reactivity increases down Group 1. The outer electron is further from the nucleus and more shielded, so it is lost more easily, making the metal more reactive.

Compare the three types of chemical bonding: ionic, covalent and metallic.

Ionic bonding: transfer of electrons between a metal and non-metal, forming oppositely charged ions held by electrostatic attraction. Covalent bonding: sharing of electron pairs between non-metals. Metallic bonding: positive metal ions in a sea of delocalised electrons.

Why do ionic compounds have high melting points and conduct electricity only when molten or dissolved?

Strong electrostatic forces in the giant ionic lattice need a lot of energy to break, giving high melting points. Ions are fixed in the solid so it cannot conduct; when molten or in solution the ions are free to move and carry charge.

What is the formula relating number of moles, mass and relative formula mass?

$\text{moles} = \dfrac{\text{mass (g)}}{M_r}$, where $M_r$ is the relative formula mass.

State Avogadro's constant and what one mole represents.

Avogadro's constant is $6.02 \times 10^{23}$ per mole. One mole of a substance contains this number of particles (atoms, molecules or ions) and has a mass in grams equal to its relative formula mass.

How do you calculate the percentage yield of a reaction?

$\text{percentage yield} = \dfrac{\text{mass of product actually made}}{\text{maximum theoretical mass}} \times 100$

What is the pH scale, and what do acids, neutral solutions and alkalis correspond to?

The pH scale runs from $0$ to $14$ and measures acidity. Acids are pH $< 7$, neutral is pH $= 7$, and alkalis are pH $> 7$. Lower pH means a higher concentration of $\ce{H+}$ ions.

Give the general word equation for an acid reacting with a metal, and with a metal carbonate.

Acid + metal $\rightarrow$ salt + hydrogen. Acid + metal carbonate $\rightarrow$ salt + water + carbon dioxide.

Define oxidation and reduction in terms of electrons (OIL RIG).

OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons. Example: $\ce{Mg -> Mg^2+ + 2e-}$ is oxidation.

What happens at the cathode and anode during electrolysis of a molten ionic compound?

Positive metal ions are attracted to the negative cathode where they gain electrons (reduction); negative ions are attracted to the positive anode where they lose electrons (oxidation).

Define exothermic and endothermic reactions and give an example of each.

Exothermic reactions transfer energy to the surroundings (temperature rises), e.g. combustion or neutralisation. Endothermic reactions take in energy from the surroundings (temperature falls), e.g. thermal decomposition or the reaction of citric acid with sodium hydrogencarbonate.

State the collision theory and four factors that increase the rate of reaction.

Reactions occur when particles collide with enough energy (the activation energy). Rate increases with higher temperature, higher concentration/pressure, larger surface area of solids, and adding a catalyst.

How does a catalyst speed up a reaction?

A catalyst provides an alternative reaction pathway with a lower activation energy, so more collisions are successful. It is not used up in the reaction.

Name the main gases in today's atmosphere and their approximate percentages.

About $78\%$ nitrogen, about $21\%$ oxygen, and small amounts of other gases including around $0.04\%$ carbon dioxide, plus argon and water vapour.

Name three greenhouse gases and one human activity that increases each.

Carbon dioxide (burning fossil fuels, deforestation), methane (livestock farming, landfill/rice paddies), water vapour. These gases absorb and re-emit infrared radiation, contributing to global warming.

What is the difference between potable water and pure water?

Potable water is safe to drink but may contain dissolved substances (e.g. some salts and microbes within safe limits). Pure water contains only $\ce{H2O}$ molecules with nothing dissolved in it.

State the equation for kinetic energy and define each term.

$$E_k = \frac{1}{2}mv^{2}$$ where $E_k$ is kinetic energy in joules (J), $m$ is mass in kilograms (kg) and $v$ is speed in metres per second (m/s).

State the equation for gravitational potential energy.

$$E_p = mgh$$ where $m$ is mass (kg), $g$ is gravitational field strength ($\approx 9.8\ \text{N/kg}$) and $h$ is height (m).

How do you calculate the efficiency of an energy transfer?

$\text{efficiency} = \dfrac{\text{useful output energy transfer}}{\text{total input energy transfer}}$ (can be $\times 100$ for a percentage). Power may be used in place of energy.

What is the difference between a scalar and a vector quantity? Give two examples of each.

A scalar has magnitude only (e.g. speed, mass, distance, energy). A vector has both magnitude and direction (e.g. velocity, force, acceleration, displacement).

State Newton's three laws of motion.

1st: an object stays at rest or at constant velocity unless acted on by a resultant force. 2nd: $F = ma$ (resultant force = mass × acceleration). 3rd: when two objects interact, they exert equal and opposite forces on each other.

Give the equation linking force, mass and acceleration.

$$F = ma$$ where $F$ is resultant force (N), $m$ is mass (kg) and $a$ is acceleration (m/s$^2$).

Write the equation for weight and state the value of gravitational field strength on Earth.

$$W = mg$$ where $W$ is weight (N), $m$ is mass (kg) and $g$ is gravitational field strength, about $9.8\ \text{N/kg}$ on Earth.

Define speed, velocity and acceleration.

Speed is distance travelled per unit time (scalar). Velocity is speed in a given direction (vector). Acceleration is the change in velocity per unit time, $a = \dfrac{\Delta v}{t}$.

State the equation for acceleration and the uniform-acceleration (suvat) equation linking velocity and distance.

$a = \dfrac{v - u}{t}$, and $$v^{2} = u^{2} + 2as$$ where $u$ is initial velocity, $v$ final velocity, $a$ acceleration and $s$ distance.

What does the gradient and the area under a velocity-time graph represent?

The gradient (slope) of a velocity-time graph represents acceleration; the area under the line represents the distance travelled (displacement).

State Ohm's law equation linking potential difference, current and resistance.

$$V = IR$$ where $V$ is potential difference (volts), $I$ is current (amperes) and $R$ is resistance (ohms, $\Omega$).

How do current, potential difference and resistance behave in a series circuit?

Current is the same at all points. Potential difference is shared between components and adds up to the supply p.d. Total resistance is the sum of individual resistances: $R_{total} = R_1 + R_2 + \dots$

How do current and potential difference behave in a parallel circuit?

Potential difference is the same across each branch. The total current is the sum of the currents through the separate branches. Adding resistors in parallel decreases the total resistance.

Give the two equations for electrical power.

$$P = VI \qquad \text{and} \qquad P = I^{2}R$$ where $P$ is power (W), $V$ is potential difference (V), $I$ is current (A) and $R$ is resistance ($\Omega$).

State the equation for charge in terms of current and time.

$$Q = It$$ where $Q$ is charge (coulombs, C), $I$ is current (A) and $t$ is time (s).

Describe the arrangement, movement and energy of particles in solids, liquids and gases.

Solid: particles closely packed in a regular pattern, vibrate in fixed positions, lowest energy. Liquid: close together but random, can move/flow around each other. Gas: far apart, random, move quickly in all directions, highest energy.

State the equation for density and give the units.

$$\rho = \frac{m}{V}$$ where $\rho$ is density (kg/m$^3$), $m$ is mass (kg) and $V$ is volume (m$^3$).

What is specific latent heat, and what is the equation for energy in a change of state?

Specific latent heat is the energy needed to change the state of $1\ \text{kg}$ of a substance with no temperature change. $$E = mL$$ where $E$ is energy (J), $m$ is mass (kg) and $L$ is the specific latent heat (J/kg).

Describe the rules for magnetic poles and how to identify a magnetic field direction.

Like poles repel, unlike poles attract. Magnetic field lines run from north to south outside the magnet; the field is strongest where the lines are closest together (at the poles).

Distinguish between transverse and longitudinal waves and give an example of each.

In a transverse wave the oscillations are perpendicular to the direction of energy transfer (e.g. light, water waves). In a longitudinal wave the oscillations are parallel to the energy transfer, with compressions and rarefactions (e.g. sound).

State the wave speed equation and define each term.

$$v = f\lambda$$ where $v$ is wave speed (m/s), $f$ is frequency (Hz) and $\lambda$ is wavelength (m).

List the electromagnetic spectrum in order of increasing frequency.

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. (Increasing frequency means decreasing wavelength and increasing energy.)

Name the three types of nuclear radiation and rank them by ionising power and penetration.

Alpha ($\alpha$): most ionising, least penetrating (stopped by paper). Beta ($\beta$): moderate (stopped by aluminium). Gamma ($\gamma$): least ionising, most penetrating (reduced by thick lead/concrete).

Define the half-life of a radioactive isotope.

The half-life is the time taken for the number of undecayed nuclei (or the activity/count rate) of a radioactive sample to halve.