Integrated Science — MCQ Practice (Answer Key)

Q	Ans	Why	Topic
1	B	A dichotomous key is a series of paired questions; each answer splits the group into two, so the number of possibilities halves at every step until one organism is left.	Dichotomous keys
2	C	Volume = 6.0 × 4.0 × 2.5 = 60 cm³. Density = mass / volume = 480 / 60 = 8.0 g/cm³.	Density
3	B	Volume of the stone = 62 - 40 = 22 cm³ (the water displaced). Density = 55 / 22 = 2.5 g/cm³.	Density by displacement
4	B	A pure substance has a fixed melting and boiling point. Freezing at exactly 0 °C is the fixed point of pure water; boiling over a range, or leaving a residue, means impurities are present.	Purity
5	B	Sand is insoluble, so it is removed by filtration. The salt is dissolved in the filtrate, so evaporating (or crystallising) the filtrate recovers the solid salt. The order cannot be reversed — evaporating first would leave sand mixed with salt.	Separation techniques
6	B	Total magnification = magnification of eyepiece x magnification of objective = 10 × 40 = 400.	Microscope
7	B	Brownian motion is the uneven bombardment of the larger visible particles by the much smaller, fast-moving air particles. It is evidence that air particles are in constant, random motion — the light only makes them visible.	Brownian motion
8	C	Gas particles are very far apart with almost no forces between them, so they move randomly at high speed. Solid particles are closely packed in a fixed pattern; liquid particles are close but able to slide past one another.	Particulate model of matter
9	B	Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration. Raising the temperature raises the average kinetic energy of the particles, so both the water and dye particles move faster and mixing is quicker.	Diffusion
10	B	Iodine sublimes: it changes from a solid directly to a gas, which is a change of state (physical change). Burning and rusting form new substances, and dissolving is a physical change but not a change of state.	Changes of state

Q	Ans	Why	Topic
11	C	Temperature measures the average kinetic energy of the particles, so the hotter block B has particles of greater average kinetic energy. With equal mass and the same material, the hotter block also has the greater total thermal energy.	Temperature and thermal energy
12	A	Methane and carbon dioxide are greenhouse gases: they trap heat radiated by the Earth, contributing to global warming. Sulfur dioxide and nitrogen oxides cause acid rain.	Greenhouse gases
13	C	Sulfur dioxide dissolves in rain water to form an acidic solution, so it is the main cause of acid rain. Carbon dioxide and methane are greenhouse gases.	Air pollution
14	C	Unpolluted air is mainly nitrogen and oxygen, with small amounts of carbon dioxide, water vapour and noble gases. Carbon monoxide, nitrogen dioxide and sulfur dioxide are pollutants and are not normal components of clean air.	Composition of air
15	C	Mg²+ has 12 - 2 = 10 electrons; F- has 9 + 1 = 10; Ne has 10; S²- has 16 + 2 = 18 electrons, which is the largest.	Electrons in ions
16	A	12 protons identify the element as magnesium. It has 12 - 10 = 2 more protons than electrons, so the charge is 2+ and the particle is Mg²+ (a magnesium ion).	Subatomic particles
17	B	Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, so they have different nucleon numbers but the same chemical properties. Both are neutral atoms, so both have 6 electrons.	Isotopes
18	B	The angle of deflection is proportional to charge / mass. ¹H+ has charge 1 and mass 1, giving 12°. ³H+ has charge 1 and mass 3, which is one third of the charge/mass ratio, so it is deflected through 12 / 3 = 4°. ²H+ and ⁴He²+ each give half the ratio (6°), and ⁶Li³+ also gives half (6°).	Behaviour of particles in an electric field
19	B	An ionic bond forms by transfer of electrons: the metal atom loses its outer-shell electrons to form a positive ion, and the non-metal atom gains them to form a negative ion. The oppositely charged ions then attract electrostatically.	Formation of ionic bonds
20	B	A simple molecular compound is held together by weak forces between its molecules, so only a little energy is needed to separate them (low melting point). There are no ions or free electrons, so it does not conduct electricity in any state.	Simple molecular compounds

Q	Ans	Why	Topic
21	C	In the solid state the ions are held in a fixed lattice and cannot move, so a solid ionic compound does NOT conduct electricity — it conducts only when molten or dissolved, when the ions are free to move. The other three statements are correct.	Properties of ionic compounds
22	B	A metal conducts because its electrons are free to move through the lattice. A molten ionic compound conducts because its positive and negative ions are free to move.	Conduction in metals and ionic compounds
23	B	In graphite each carbon atom forms three strong covalent bonds, so one electron per atom is delocalised and free to move along the layer — that is what carries the current. In diamond every atom is bonded to four others, so all the outer electrons are used in bonds and none are free to move.	Diamond and graphite
24	B	Metallic bonding is the attraction between the layers of positive metal ions and the sea of delocalised electrons. When the layers slide, the bonding is maintained, so a metal is malleable rather than brittle like an ionic solid.	Metallic bonding
25	B	Angles are always measured from the normal (the line perpendicular to the mirror). 30° to the surface means 90 - 30 = 60° to the normal, and by the law of reflection the angle of reflection equals the angle of incidence, so it is 60°.	Reflection
26	B	A plane mirror forms a virtual image behind the mirror. The image is the same size as the object, upright, laterally inverted (left and right are swapped) and cannot be caught on a screen.	Images in plane mirrors
27	B	Light travels more slowly in an optically denser medium such as glass, so when it enters the block at an angle it bends towards the normal. Leaving the block it speeds up again and bends away from the normal.	Refraction
28	B	Refractive index = speed of light in air / speed of light in the medium = 3.0 × 10⁸ / 2.0 × 10⁸ = 1.5.	Refractive index
29	C	Chloroplasts contain chlorophyll and are the site of photosynthesis, so they are found in plant cells such as leaf cells but not in animal cells. Cell membrane, nucleus and cytoplasm are present in both.	Plant cells
30	D	No wings takes step 1b to step 3. Six legs takes step 3b, which gives Ant. Dragonfly and Housefly both have wings, and a spider has eight legs.	Using a key