Exam Grade 8 2025
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Questions and Answers

What is the maximum number of electrons that can occupy the third electron shell?

  • 18 (correct)
  • 2
  • 32
  • 8

    • Which statement accurately describes a compound?

  • It is listed in the Periodic Table.
  • It contains two or more different types of atoms bonded together. (correct)
  • It consists of only one type of atom.
  • It can be separated by physical means.

    • What is the atomic mass of an element based on?

  • The total number of protons and neutrons. (correct)
  • The total number of neutrons only.
  • The number of protons only.
  • The total number of protons and electrons.

    • In a mixture, what can be said about its components?

    <p>They can be present in varying quantities.</p> Signup and view all the answers

    What best describes an element?

    <p>A pure substance consisting of the same type of atom.</p> Signup and view all the answers

    What is the primary role of producers in an ecosystem?

    <p>To produce their own food through photosynthesis</p> Signup and view all the answers

    Which statement accurately describes the flow of energy in food chains?

    <p>Energy decreases progressively as it moves through levels.</p> Signup and view all the answers

    What is the correct description of decomposers?

    <p>Organisms that recycle matter back into the soil</p> Signup and view all the answers

    What type of consumer only eats plants?

    <p>Herbivores</p> Signup and view all the answers

    Which of the following best illustrates the concept of energy flow in a food chain?

    <p>Energy flows from producers to multiple consumers.</p> Signup and view all the answers

    Why are food chains typically short with only a few steps?

    <p>Energy is lost at each trophic level.</p> Signup and view all the answers

    Which process describes how plants convert carbon dioxide and water into sugars?

    <p>Photosynthesis</p> Signup and view all the answers

    Which of the following best defines omnivores?

    <p>Organisms that can feed on both plants and animals.</p> Signup and view all the answers

    Which of the following is the correct way to represent the balancing of HgO → Hg + O2?

    <p>2HgO → 2Hg + O2</p> Signup and view all the answers

    What is the coefficient for HCl in the balanced equation BaCl2 + H2SO4 → BaSO4 + HCl?

    <p>2</p> Signup and view all the answers

    In the chemical formula C2H6, how many hydrogen atoms are present?

    <p>6</p> Signup and view all the answers

    Which formula represents the correct ratio of Na to Br in the balanced equation Cl2 + 2NaBr → 2NaCl + Br2?

    <p>2:1</p> Signup and view all the answers

    How many total hydrogen atoms are present in the formula 3 Al(OH)3?

    <p>9</p> Signup and view all the answers

    What is the balanced coefficient for magnesium in the equation Mg + O2 → MgO?

    <p>2</p> Signup and view all the answers

    Why should you balance hydrogen and oxygen atoms last when balancing chemical equations?

    <p>It simplifies balancing the other elements.</p> Signup and view all the answers

    What does a subscript in a chemical formula indicate?

    <p>The number of atoms of that element in a molecule.</p> Signup and view all the answers

    How do you know that the equation Fe + O2 → Fe2O3 is not balanced?

    <p>There is an unequal number of oxygen atoms on each side.</p> Signup and view all the answers

    What is the main objective when you balance a chemical equation?

    <p>To ensure mass is conserved during the reaction.</p> Signup and view all the answers

    Which of the following describes coefficients in a chemical equation?

    <p>They indicate the number of molecules present.</p> Signup and view all the answers

    What does the conservation of mass principle state in a chemical reaction?

    <p>The total mass of reactants equals the total mass of products.</p> Signup and view all the answers

    Given the unbalanced equation Fe + O2 → Fe2O3, what is the correct coefficient for Fe to start balancing?

    <p>4</p> Signup and view all the answers

    After balancing iron in the equation 4 Fe + O2 → 2 Fe2O3, how many oxygen atoms are present on the product side?

    <p>6</p> Signup and view all the answers

    What is the final balanced equation for the reaction involving iron and oxygen?

    <p>4 Fe + 3 O2 → 2 Fe2O3</p> Signup and view all the answers

    What is a characteristic of the final balanced equation concerning coefficients?

    <p>They must be whole numbers only</p> Signup and view all the answers

    Which of the following is true about balancing equations for charge and mass?

    <p>Mass and charge balancing is often required together</p> Signup and view all the answers

    What are the three main components of an ecosystem?

    <p>Physical surroundings, living organisms, and environmental factors</p> Signup and view all the answers

    Which of the following best describes intraspecific competition?

    <p>Competition within the same species for mates</p> Signup and view all the answers

    What term describes the relationship where one organism benefits while the other is unaffected?

    <p>Commensalism</p> Signup and view all the answers

    Which factor is classified as abiotic?

    <p>Temperature</p> Signup and view all the answers

    What is mutualism?

    <p>Both organisms benefit from the interaction</p> Signup and view all the answers

    What is an example of interspecific competition?

    <p>Chickens and ducks competing for food</p> Signup and view all the answers

    Which of the following describes parasitism?

    <p>One organism benefits while the other is harmed</p> Signup and view all the answers

    What is true about the sustainability of natural ecosystems?

    <p>Most natural ecosystems are sustainable.</p> Signup and view all the answers

    What is one primary factor that affects the stability of an ecosystem?

    <p>Climate change</p> Signup and view all the answers

    Which type of ecosystem is characterized by high levels of biodiversity and significant resource cycling?

    <p>Coral reef</p> Signup and view all the answers

    How do natural disturbances impact ecosystems?

    <p>They can reshape environments and provide new opportunities for species.</p> Signup and view all the answers

    Which statement best reflects the relationship between species adaptations and ecosystem characteristics?

    <p>Species adaptations have a direct correlation with ecosystem qualities.</p> Signup and view all the answers

    Which human activity is known to significantly disturb ecosystems?

    <p>Deforestation</p> Signup and view all the answers

    Which of the following best describes the role of consumers in an ecosystem?

    <p>They obtain energy by consuming other organisms.</p> Signup and view all the answers

    What is a key difference between abiotic and biotic factors in ecosystems?

    <p>Abiotic factors are non-living components.</p> Signup and view all the answers

    How does energy flow through an ecosystem?

    <p>In a one-way direction from producers to consumers.</p> Signup and view all the answers

    Which nutrient cycle is critical for returning essential elements to the ecosystem?

    <p>Carbon cycle</p> Signup and view all the answers

    What can result from significant changes in abiotic factors within an ecosystem?

    <p>It can drastically alter the ecosystem.</p> Signup and view all the answers

    Which of the following is NOT considered an ecosystem service?

    <p>Energy creation</p> Signup and view all the answers

    Which statement correctly describes the importance of biodiversity in ecosystems?

    <p>High biodiversity can enhance ecosystem stability and resilience.</p> Signup and view all the answers

    Which of the following accurately describes producers in an ecosystem?

    <p>They rely on photosynthesis to create their own food.</p> Signup and view all the answers

    What is the balanced coefficient for hydrogen in the reaction $N_2 + H_2 \rightarrow NH_3$?

    <p>3</p> Signup and view all the answers

    In the equation $Pb(OH)_2 + HCl \rightarrow H_2O + PbCl_2$, how many moles of water are produced?

    <p>2</p> Signup and view all the answers

    What is the balanced form of the combustion reaction $C_3H_8 + O_2 \rightarrow CO_2 + H_2O$?

    <p>$C_3H_8 + 2 O_2 \rightarrow 3 CO_2 + 4 H_2O$</p> Signup and view all the answers

    What is the correct coefficient for $Na$ when balancing the equation $Na + H_2O \rightarrow NaOH + H_2$?

    <p>1</p> Signup and view all the answers

    In the equation C₃H₈ + O₂ → CO₂ + H₂O, what is the correct coefficient for O₂ after balancing?

    <p>5</p> Signup and view all the answers

    After balancing the equation NH₄NO₃ → N₂O + H₂O, how many H atoms are present on the product side?

    <p>4</p> Signup and view all the answers

    Which of the following can disrupt mutualistic relationships?

    <p>Habitat destruction.</p> Signup and view all the answers

    What is the primary characteristic of parasitism?

    <p>One organism benefits at the expense of the other.</p> Signup and view all the answers

    Which of the following options describes a mutualistic relationship?

    <p>Bacteria helping digest food in the stomach of animals.</p> Signup and view all the answers

    What is the primary reason energy transfer between trophic levels is inefficient?

    <p>A large portion of energy is lost as heat during cellular processes.</p> Signup and view all the answers

    What is one consequence of the inefficiency of energy transfer in food webs?

    <p>A limitation on the number of trophic levels supported.</p> Signup and view all the answers

    Which of the following describes the role of decomposers within a food web?

    <p>They metabolize waste and dead matter, converting it to heat and nutrients.</p> Signup and view all the answers

    What distinguishes producers from consumers in an ecosystem?

    <p>Producers can make their own organic molecules.</p> Signup and view all the answers

    Which of the following describes the relationship between the number of trophic levels and the efficiency of energy transfer?

    <p>Increased trophic levels decrease energy availability.</p> Signup and view all the answers

    Which statement accurately describes the role of primary consumers in a food chain?

    <p>They primarily feed on primary producers.</p> Signup and view all the answers

    Which of the following best defines apex consumers in a food chain?

    <p>Consumers that have no natural predators.</p> Signup and view all the answers

    In which trophic level are primary consumers found?

    <p>One level above primary producers</p> Signup and view all the answers

    Which of the following accurately describes a tertiary consumer?

    <p>A carnivore that eats other carnivores.</p> Signup and view all the answers

    What is the function of decomposers in an ecosystem?

    <p>To convert dead organic matter into nutrients for producers.</p> Signup and view all the answers

    How are trophic levels defined within an ecological community?

    <p>By counting how many transfers of energy occur.</p> Signup and view all the answers

    What best describes energy transfer within a food chain?

    <p>It occurs inefficiently, losing energy at each step.</p> Signup and view all the answers

    Which of the following statements is true regarding apex consumers?

    <p>They are at the top of the food chain and have no predators.</p> Signup and view all the answers

    In a food web, what might be a consequence of removing a primary producer?

    <p>Potential collapse of the entire food web.</p> Signup and view all the answers

    Study Notes

    Chemistry Basics

    • Atoms are the smallest particles of matter.

    • Atoms consist of a nucleus and electron shells.

    • The nucleus contains protons (positive charge) and neutrons (neutral charge).

    • Electrons (negative charge) orbit the nucleus in shells.

    • Electron shells have maximum capacities:

      • First shell = 2 electrons
      • Second shell = 8 electrons
      • Third shell = 18 electrons (stable at 8)
      • Fourth shell = 32 electrons

    • Elements are made of the same type of atom.

    • Elements are listed on the Periodic Table ordered by atomic number (number of protons). Elements include hydrogen (H), carbon (C), nitrogen (N), silver (Ag), lead (Pb), mercury (Hg).

    • Each element has a symbol (e.g., Hydrogen - H, Carbon - C).

    • Molecules are formed when two or more atoms bond together.

    • Molecules of the same type of atom = molecule of element (e.g., O₂).

    • Molecules of different types of atoms = molecule of compound (e.g., CO₂).

    • Compounds are formed when two or more different types of atoms bond together.

    • Examples include water (H₂O), carbon dioxide (CO₂), methane (CH₄), glucose (C₆H₁₂O₆), Iron (Fe), Oxygen (O₂), Mercury (Hg), sodium (Na), chlorine (Cl₂), sodium bromide (NaBr), copper(II) chloride (CuCl₂)., hydrogen sulfide (H₂S), copper sulfide (CuS), hydrogen chloride (HCl), barium sulfate (BaSO₄), water (H₂O₂), nitrogen (N₂), ammonia (NH₃)., aluminum hydroxide(Al(OH)₃)., potassium chloride, potassium chlorate, sodium fluoride, potassium chlorate, aluminum bromide, potassium sulfate, potassium chlorate, potassium iodide.

    • Mixtures are combinations of atoms, elements, and compounds.

    • Mixtures may have varying amounts of each component.

    • Mixtures do not have a set formula.

    Periodic Table

    • A list of all known elements.
    • Elements are listed in order of atomic number.
    • Rows are called periods and columns are called groups.
    • Metals are on the left side of the table, non-metals are on the right. Metalloids are the elements in between.
    • The table provides information about each element, including:
      • Atomic number
      • Chemical symbol
      • Element name
      • Atomic mass

    Balancing Chemical Equations

    • Chemical equations show reactants (starting materials) and products (results of the reaction).
    • Balanced equations show equal numbers of atoms of each element on both sides of the equation.
    • Follow steps to balance equations:
      • Write unbalanced equation.
      • Count atoms of each element on each side.
      • Add coefficients (numbers in front of formulas) as needed to balance each element. Start with the least manipulated element.
      • Indicate the state of matter for each substance (s, l, g, aq).

    Mixtures

    • Mixtures can be separated by physical means.
    • Examples of mixtures include: salt water, air, soft drinks, perfumes.

    Ecosystem Interactions

    • Environment includes all living and non-living factors in an organism's surroundings.

    • Ecosystem: a group of living things interacting with their environment.

    • Ecosystem components:

      • Physical surroundings (soil, water, rocks, light intensity, temperature, nutrients, concentration of gases in water, texture and pH of soil, etc.).
      • Living organisms (community)
      • Living and non-living factors (e.g. competition, predation).

    • Abiotic factors are non-living, while biotic factors are living.

    • Types of Symbiosis

      • Mutualism: both species benefit (e.g., clownfish and anemone, oxpecker bird and antelope).
      • Commensalism: one species benefits, the other is unaffected (e.g., barnacle on a whale, possum in tree hollow).
      • Parasitism: one species benefits, the other is harmed (e.g., tick on a dog, parasitic wasp and caterpillar).

    • Ecosystems show sustainability when maintaining living conditions for the community is possible.

    • Organisms interact with each other and the environment, influencing the ecosystem.

    Food Chains

    • Energy flows from producers (e.g., plants) to consumers (e.g., herbivores, carnivores).
    • Decomposers break down dead materials and cycle nutrients, so the ecosystem maintains sustainability.
    • Food chains show energy flow and trophic levels (producers, primary consumers, secondary consumers, tertiary consumers, etc.).
    • Pyramids of energy show the progressive loss of usable energy as you move up the trophic levels.
    • Only about 10% of energy gets passed from one level to the next. This loss limits the number of organisms at each level.
    • Pyramids of energy can display the total mass of organisms at each stage of a food chain thus showing the productivity of an area.

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    Description

    Test your knowledge on the fundamental concepts of chemistry, including atoms, elements, and molecules. This quiz covers atomic structure, the periodic table, and compound formation. Perfect for beginners or anyone looking to refresh their chemistry knowledge.

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