Science Test 2

What is an element?

A pure substance containing one type of atom.

What is a compound?

Two or more different elements chemically bonded in a fixed ratio.

How do elements form compounds?

Chemical reactions rearrange atoms and form new chemical bonds.

Physical change vs. chemical change?

Physical: no new substance forms. Chemical: one or more new substances form.

Name five signs of a chemical change.

Colour change, gas production, precipitate formation, heat/light change, or new odour.

What is synthesis? Give its pattern.

Two or more substances combine: A + B → AB.

What is decomposition? Give its pattern.

One compound breaks apart: AB → A + B.

What is single displacement?

One element replaces another in a compound: A + BC → AC + B.

What is double displacement?

Two compounds exchange ions: AB + CD → AD + CB.

What is oxidation?

A chemical reaction involving oxygen as a reactant.

What is combustion?

A reaction with oxygen that releases energy.

How do you identify synthesis, decomposition, single displacement, and double displacement?

Synthesis: substances combine. Decomposition: one compound splits. Single displacement: one element replaces another. Double displacement: two compounds exchange ions.

Which reactions can be both synthesis and oxidation?

Reactions where substances combine and oxygen is a reactant, such as 2Mg + O₂ → 2MgO.

What is an exothermic reaction?

Releases energy; surroundings warm up; products have lower energy.

What is an endothermic reaction?

Absorbs energy; surroundings may cool; products have higher energy.

Where is energy written in an exothermic equation?

On the product side: reactants → products + energy.

Where is energy written in an endothermic equation?

On the reactant side: reactants + energy → products.

How do you identify exothermic and endothermic energy graphs?

Exothermic: products lower than reactants. Endothermic: products higher than reactants.

What does the hump on an energy graph represent?

The activation energy barrier.

Which examples are exothermic and which are endothermic?

Exothermic: combustion, freezing, condensation. Endothermic: melting, vaporization, photosynthesis.

What three things does combustion require?

Fuel, oxygen, and heat.

What is the fire triangle?

Fuel, oxygen, and heat. Remove one to extinguish the fire.

How can you extinguish a fire using the fire triangle?

Remove fuel, oxygen, or heat.

What is ignition temperature?

The minimum temperature needed for a substance to start burning.

What are isotopes?

Atoms of the same element with equal protons but different neutrons.

What is atomic number?

The number of protons in an atom's nucleus.

What is mass number?

The total number of protons and neutrons.

How do you calculate neutrons?

Neutrons = mass number − atomic number.

How many electrons does a neutral atom have?

The same number as protons.

What does isotope notation ²³₁₁Na tell you?

11 protons, 12 neutrons, and 11 electrons if neutral.

For ³⁵₁₇Cl, find protons, neutrons, and electrons in a neutral atom.

17 protons, 18 neutrons, and 17 electrons.

What does relative abundance mean?

The percentage of atoms that are a particular isotope.

How do you calculate relative atomic mass?

Sum of (isotope mass × abundance %) ÷ 100.

Isotopes have masses 10 (20%) and 11 (80%). What is the average atomic mass?

(10 × 20 + 11 × 80) ÷ 100 = 10.8 u.

An element has isotopes of masses 20 (25%) and 22 (75%). Find its relative atomic mass.

(20 × 25 + 22 × 75) ÷ 100 = 21.5 u.

What is radioactive decay?

The spontaneous breakdown of an unstable nucleus, releasing radiation or energy.

What is alpha radiation?

2 protons + 2 neutrons; mass number decreases by 4 and atomic number by 2.

What is beta-minus radiation?

An electron is emitted; mass number stays the same and atomic number increases by 1.

What is gamma radiation?

Energy is emitted; mass number and atomic number stay unchanged.

How do you recognize alpha decay in an equation?

Mass number decreases by 4 and atomic number decreases by 2.

How do you recognize beta-minus decay in an equation?

Mass number stays the same and atomic number increases by 1.

How do you recognize gamma decay in an equation?

Neither mass number nor atomic number changes.

What must balance in a nuclear equation?

Total mass numbers and total atomic numbers on both sides.

What is half-life?

The time required for half the radioactive nuclei in a sample to decay.

What is the half-life formula?

N = N₀(1/2)ⁿ, where n = elapsed time ÷ half-life.

What percentage remains after one half-life?

50%.

What percentage remains after two half-lives?

25%.

What percentage remains after three half-lives?

12.5%.

An 80 g sample has a 5-year half-life. How much remains after 15 years?

3 half-lives: 80 → 40 → 20 → 10 g remain.

A sample drops from 160 g to 20 g. How many half-lives passed?

3 half-lives: 160 → 80 → 40 → 20 g.

A 64 g sample has a 3-hour half-life. How much remains after 9 hours?

3 half-lives: 64 → 32 → 16 → 8 g.

A sample falls from 100 g to 12.5 g in 18 hours. What is its half-life?

3 half-lives occurred; 18 ÷ 3 = 6 hours.

What is nuclear fission?

A large nucleus splits into smaller nuclei, releasing energy and usually neutrons.

What is nuclear fusion?

Two light nuclei join to form a larger nucleus, releasing energy.

Which nuclear reaction powers the Sun?

Fusion.

Which nuclear reaction is used in nuclear power plants?

Fission.

What is a nuclear chain reaction?

Neutrons from fission trigger further fission reactions, releasing more energy and neutrons.

Why does nuclear fission release energy?

The products have greater nuclear binding energy per nucleon; some mass is converted into energy.

How does a nuclear power plant generate electricity?

Fission heats water; steam spins a turbine; a generator produces electricity.

Name the three main fossil fuels.

Coal, oil (petroleum), and natural gas.

What are the states of the three fossil fuels?

Coal is solid, oil is liquid, and natural gas is gaseous.

Are fossil fuels renewable? Why?

No. They form over millions of years and are used faster than they form.

How do fossil-fuel power plants generate electricity?

Burning fuel produces heat; steam spins a turbine connected to a generator.