📚 Year 7 SQA Science: Unit Test Mock Paper Explained | Year 7 SQA科学：单元测试模拟卷解析
Welcome to this detailed walkthrough of a Year 7 SQA style unit test mock paper. This exam covers key topics from biology, chemistry and physics, helping you consolidate your understanding and prepare for your assessments. We will go through each question, provide model answers and highlight common mistakes.
欢迎收看这套Year 7 SQA风格单元测试模拟卷的详细解析。本试卷涵盖生物、化学和物理的核心主题，帮助你巩固理解并为评估做准备。我们将逐题讲解，给出标准答案并指出常见错误。
1. Animal and Plant Cells | 动植物细胞
A typical exam question shows a diagram of a plant cell and asks you to label the nucleus, cell wall and chloroplast. In both animal and plant cells, the nucleus controls all cell activities and contains genetic material. It is often the largest organelle visible under a light microscope.
一个典型的考题会展示植物细胞简图，并要求你标记细胞核、细胞壁和叶绿体。在动植物细胞中，细胞核控制所有细胞活动并含有遗传物质。它通常是光学显微镜下可见的最大细胞器。
Plant cells have a rigid cell wall made of cellulose, which provides structural support and gives the cell a regular shape. Animal cells lack this wall.
植物细胞有由纤维素构成的坚固细胞壁，它提供结构支撑并使细胞呈现规则形状。动物细胞没有细胞壁。
Chloroplasts contain chlorophyll, the green pigment that absorbs light energy for photosynthesis. These are found only in plant cells and some algae.
叶绿体含有叶绿素，这种绿色色素吸收光能进行光合作用。它们只存在于植物细胞和某些藻类中。
2. Specialised Cells | 特化细胞
Red blood cells are often examined in transport topic questions. They have a unique biconcave disc shape, which increases the surface area to volume ratio for efficient oxygen absorption and release.
红细胞经常在运输主题的问题中出现。它们具有独特的双凹圆盘形状，增大了表面积与体积之比，可高效地吸收和释放氧气。
They contain haemoglobin, an iron-rich protein that binds to oxygen in the lungs and carries it to body tissues. Mature red blood cells have no nucleus, providing more space for haemoglobin.
它们含有血红蛋白，一种富含铁的蛋白质，在肺部与氧气结合并将其输送至身体组织。成熟的红细胞没有细胞核，为血红蛋白提供了更多空间。
This adaptation allows each cell to transport as much oxygen as possible, which is crucial for aerobic respiration in cells.
这种适应使每个细胞尽可能多地运输氧气，这对细胞内的有氧呼吸至关重要。
3. States of Matter | 物质的三态
Questions on particle arrangement are fundamental. In a solid, particles are packed closely together in a regular pattern. They vibrate about fixed positions and cannot move freely, giving solids a definite shape and volume.
有关粒子排列的问题是基础性的。在固体中，粒子紧密排列成规则图案。它们在固定位置振动，无法自由移动，因此固体具有确定的形状和体积。
In a liquid, particles are still close together but are arranged randomly. They can slide past each other, allowing liquids to flow and take the shape of their container while maintaining a constant volume.
在液体中，粒子仍然彼此靠近但排列无序。它们可以相互滑动，使液体能够流动并呈现容器的形状，同时保持恒定的体积。
In a gas, particles are far apart with no regular arrangement. They move rapidly in all directions and can be compressed. Gases have no fixed shape or volume, expanding to fill any container.
在气体中，粒子相距很远且无规则排列。它们快速向各个方向运动并可以被压缩。气体没有固定的形状或体积，会膨胀充满任何容器。
4. Changes of State | 物态变化
Sublimation is the process where a solid turns directly into a gas without passing through the liquid state. A common example is solid carbon dioxide (dry ice) changing into carbon dioxide gas at room temperature.
升华是固体不经过液态直接转变为气体的过程。常见例子是固态二氧化碳（干冰）在室温下变成二氧化碳气体。
When water boils, the heat energy supplied is used to overcome the intermolecular forces holding the liquid particles together, rather than increasing the kinetic energy. Therefore, the temperature stays at the boiling point (100 °C at standard pressure) until all water has vaporised.
水沸腾时，供应的热能用于克服液体粒子间的分子间力，而不是增加动能。因此，温度保持在沸点（标准压力下为100 °C），直到所有水都蒸发为止。
5. Pure Substances and Mixtures | 纯净物与混合物
A pure substance consists of only one type of atom or molecule, such as distilled water (H₂O). It has a fixed melting point and boiling point. A mixture contains two or more different substances not chemically combined, like salt water or air, and it melts over a range of temperatures.
纯净物仅由一种原子或分子组成，如蒸馏水（H₂O）。它具有固定的熔点和沸点。混合物含有两种或多种未化学结合的不同物质，如盐水或空气，并在一个温度范围内熔化。
Tap water is a mixture because it contains dissolved minerals and gases. When heated, its melting point is lower than 0 °C and occurs over a range, unlike pure ice which melts sharply at 0 °C.
自来水是一种混合物，因为它含有溶解的矿物质和气体。加热时，其熔点低于0 °C且在一个范围内发生，不像纯冰在0 °C时急剧融化。
6. Acids and Alkalis | 酸与碱
A green colour on universal indicator indicates a neutral solution with a pH of 7. This is neither acidic (pH <7) nor alkaline (pH >7).
通用指示剂呈绿色表示中性溶液，pH值为7。这既不是酸性（pH<7）也不是碱性（pH>7）。
reaction produces a salt, water and carbon dioxide: acid + carbonate → salt + water + CO₂.
要中和盐酸（HCl），可以使用小苏打（碳酸氢钠，NaHCO₃）。反应生成盐、水和二氧化碳：酸 + 碳酸盐 → 盐 + 水 + CO₂。
Alternatively, an alkali such as sodium hydroxide could be used, but baking soda is safer for household spills because it is milder and produces visible fizzing to indicate neutralisation.
也可以使用碱液如氢氧化钠，但对于家庭泄漏，小苏打更安全，因为它较温和并且会产生可见的气泡以指示中和过程。
7. Forces and Friction | 力与摩擦力
The key forces acting on a box pushed across a rough floor are: an applied push force to the right, a friction force to the left opposing motion, the weight acting downwards due to gravity, and the normal reaction force from the floor acting upwards.
作用在粗糙地板上被推的箱子上的关键力有：向右的推动力、向左阻碍运动的摩擦力、因重力作用向下的重量，以及地板向上的法向反作用力。
Friction is a force that opposes motion between two surfaces in contact. As the box slides, kinetic friction converts kinetic energy into thermal energy, gradually slowing it down. When the applied force is removed, the unbalanced frictional force decelerates the box until it stops.
摩擦力是阻碍两个接触表面之间运动的力。箱子滑动时，动摩擦力将动能转化为热能，逐渐使其减速。当撤去推力时，未平衡的摩擦力使箱子减速直至停止。
8. Balanced and Unbalanced Forces | 平衡力与不平衡力
When a car travels at constant speed in a straight line, the forces are balanced. The driving force from the engine equals the total resistive forces (air resistance and friction). This results in no net force and therefore no acceleration (Newton's first law).
当汽车沿直线匀速行驶时，力是平衡的。发动机的驱动力等于总阻力（空气阻力和摩擦力）。这使得净力为零，因此没有加速度（牛顿第一定律）。
For the car to accelerate, the driving force must exceed the total resistive forces, producing an unbalanced force.
F = m × a
要使汽车加速，驱动力必须大于总阻力，产生净力。牛顿第二定律 F = m × a 表明加速度与净力成正比，与质量成反比。
9. Energy Transfers | 能量传递
When a ball is held at a height, it possesses gravitational potential energy (GPE). As it is released, GPE is converted into kinetic energy (KE). The total mechanical
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