Year 8 Science Revision Notes Christmas Test 2023 PDF

Year 8 Science revision notes Christmas test 2023 Biology ● Photosynthesis is a process that occurs in the leaves of a plant and needs chlorophyll and light energy. ● During photosynthesis, the chlorophyll in leaves helps convert carbon dioxide and water into the products oxygen and glucose. ● The...

Year 8 Science revision notes Christmas test 2023 Biology ● Photosynthesis is a process that occurs in the leaves of a plant and needs chlorophyll and light energy. ● During photosynthesis, the chlorophyll in leaves helps convert carbon dioxide and water into the products oxygen and glucose. ● The product glucose acts as a vital source of food for the plant. ● Carbon dioxide, water and light are all needed for photosynthesis to take place. ● Photosynthesis takes place inside plant cells in chloroplasts. Chloroplasts contain a green substance called chlorophyll. This absorbs the light energy needed to make photosynthesis happen. Plants and algae can only carry out photosynthesis in the light. ● Plants get carbon dioxide from the air through their leaves, and water from the ground through their roots. Light energy comes from the Sun. ● The oxygen produced is released into the air from the leaves. The glucose produced can be turned into other substances, such as starch and plant oils, which are used as an energy store. This energy can be released by respiration. ● The word equation for photosynthesis in the presence of light and chlorophyll is: Carbon dioxide + water → glucose + oxygen ● Plants are adapted to carry out the process of respiration and photosynthesis. Gases such as oxygen and carbon dioxide can travel through tiny holes underneath the leaf called stomata. Carbon dioxide diffuses into the cells found within the leaf to help carry out the process of photosynthesis, whilst oxygen is released from these cells as a product of photosynthesis. Plants have adaptations to help them to photosynthesise and ultimately to survive. ● Photosynthesis can be investigated to show the production of starch and the importance of chlorophyll. Starch test Starch testing → Iodine solution is used to test leaves for the presence of starch. You need to: 1. Heat a leaf in boiling water for 30 seconds (this stops its chemical reactions) 2 Put the leaf I in boiling ethanol for a few minutes (this removes the chlorophyll) 3 Wash with water and spread onto a white tile 4 Add iodine solution from a dropping pipette After a few minutes, the parts of the leaf that contain starch turn blue-black. ● Variegated leaves have green parts (where the cells contain chlorophyll) and white parts (where there is no chlorophyll). Only the parts that were green become blue black with iodine solution, showing the importance of chlorophyll in photosynthesis. ● A plant can be ‘de-starched’ by leaving it in the dark for a few hours. Parts of its leaves are covered with dark paper, and the plant is left in the light for a few hours. Only the uncovered parts become blue-black with iodine solution, showing the importance of light in photosynthesis. ● Transpiration is the evaporation of water at the surfaces of the spongy mesophyll cells in leaves, followed by loss of water vapour through the stomata. Water moves through the xylem vessels in a continuous transpiration stream: root → stem → leaf ● The root hairs are where most water absorption happens. They are long and thin so they can penetrate between soil particles, and they have a large surface area for absorption of water. Xylem moves water from roots to the leaves Phloem tubes carry sucrose from the leaf to the shoots and roots. , Chemistry States of matter ● Almost everything is made of particles. ● Particles can be atoms, molecules, compounds. ● Particles behave differently in solids, liquids and gases. ● The particle model explains the differences between solids, liquids and gases. The particles in solids are very close together, therefore they cannot usually be compressed or squashed. Forces of attraction between the particles hold them together and keep them in place. The particles in solids are arranged in a regular way. The particles in solids move only by vibrating about a fixed position. This gives solids a fixed shape and means that they cannot flow like liquids. ● The particles in liquids are arranged in a random way, and are close together, touching many of their neighbours. There are some gaps, but liquids cannot usually be compressed or squashed. The particles of a liquid have enough energy to break free of some of the forces of attraction between the particles. So particles in liquids can move around and can move over each other, allowing liquids to flow and be poured. ● The particles in gases are widely spaced and randomly arranged, meaning they can be easily compressed or squashed. ● The particles in a gas have enough energy to overcome the forces of attraction between the particles, so are free to move in any direction. They move quickly in straight lines, colliding with each other and the walls of their container. Heating up ● Melting - When a solid is heated, it absorbs energy and it melts, turning into a liquid. ● Boiling - If the liquid is heated, it absorbs more energy and it boils, turning into a gas. ● Evaporating is when a liquid turns into a gas slowly, at temperatures that are below its boiling point. Puddles dry up because they evaporate – they don’t boil. Cooling down ● Condensing - If a gas is cooled, it transfers energy to the surroundings, and turns into a liquid. ● Freezing - If the liquid is cooled, it transfers energy to the surroundings, and turns into a solid. ● Pressure in gases is caused by particles colliding with the walls of the container. ● Gas pressure is increased when the temperature increases or the volume of the container decreases. ● A car tyre contains gas under pressure. This means that there are more air particles pushing on the inside of the tyre than on the outside of the tyre. Elements compounds and mixtures ● Atoms are the building blocks of everything. Atoms are the smallest particles of an element, which are far too small to see. ● Atoms can form strong bonds with each other, making molecules. ● A pure substance made from only one type of atom is called an element. Elements are listed on the periodic table. Examples of elements include oxygen, hydrogen and carbon. ● Molecules are made when two or more atoms chemically bond together. Atoms from different elements can combine. When the atoms are from different elements, the molecule can also be called a compound. ● Water is made of molecules. Each water molecule is made from two hydrogen atoms chemically bonded to one oxygen atom. This means that the chemical formula of water is H2O. ● Oxygen gas is a molecule made of two oxygen (O) atoms represented as O2. ● Sulfuric acid is a combination of hydrogen (H), sulfur (S) and oxygen (O) represented as H2SO4. ● The periodic table can be divided into metals and non-metals. Metals are found on the left and in the middle, whereas non-metals are on the right. There is a zig-zag diagonal line dividing metals and non metals in the periodic table. Hydrogen doesn’t fit into this grouping and is placed over the table, this is because of Hydrogen's atomic structure. ● Chemical reactions make new chemicals. ● Atoms are rearranged during a chemical reaction, but the number of atoms does not change. ● Evidence of chemical reactions includes a large temperature change, bubbles, or a colour change. ● Chemical reactions can be represented using equations. The substances that react together are called the reactants. The substances that are formed in the reaction are called the products ● A mixture is formed when two or more elements or compounds are present without being chemically bonded together. In a mixture, the two ingredients can be separated using physical processes, without chemical reactions. This is because they are not chemically bonded together. ● A mixture of sand and water can be separated using filtration. ● A solution of salt and water can be separated using crystallisation or distillation. ● A mixture of iron filings and sulfur powder can be separated using a magnet. Separation of mixtures ● Filtration - This technique is used to separate an insoluble solid from a liquid. It can be used to obtain a product that is free from unreacted chemicals, by-products or solvent. ● Evaporation - to separate a soluble solid from its solution is to make crystals. This involves evaporating the solution to a much smaller volume and then leaving it to cool. As the solution cools, crystals form, and these can be obtained by filtration. ● Distillation- separates a liquid from a solution. For example, water can be separated from salty water by simple distillation. This method works because the water evaporates from the solution, but is then cooled and condensed in a condenser and collected in a separate container. The salt does not evaporate and so it stays behind. ● Chromatography- can be used to separate mixtures of coloured compounds. Mixtures that are suitable for separation by chromatography include inks, dyes and colouring agents in food. Simple chromatography is carried out on paper. A spot of the mixture is placed on a pencil line near the bottom of a piece of chromatography paper – the line must be in pencil because pencil is insoluble in water and so will not move as the chromatography progresses. The paper is then placed upright in a suitable solvent, such as water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out. Metals and non-metals ● Most elements in the periodic table are metals, while non-metals account for around 20% of known elements. ● Metals are found on the left and in the middle, whereas non-metals are all on the right. It is possible to use information about an element’s physical properties to classify an element. ● Most metals share similar properties with each other. For example: - They have high melting and boiling points meaning they are solid at room temperature - They are good conductors of heat and electricity - They are shiny in their appearance - They are malleable ● Other common properties of metals are: - They are hard and strong - Have a high density - They are sonorous ● Non-metals have properties in common with each other. For example, they are often: Poor conductors of heat and electricity - Dull in their appearance - Weak and brittle ● Some other common properties of non-metals are: - Generally low melting and boiling points, meaning they are gases and liquids at room temperature - Not sonorous ● Some non-metals do not have all of these common properties. For example, carbon has two main forms - graphite found in pencils, and diamond. Both graphite and diamond have very high melting points and are shiny. Graphite conducts electricity, which is not typical of non metals. However, graphite is also brittle which is a typical property of non-metals. ● Metals and non-metals can react with oxygen to make compounds called oxides. Metal oxides are bases. This means they can neutralise an acid. ● For example, magnesium oxide has the chemical formula MgO, and contains only magnesium (Mg) and oxygen (O) ● Non-metal oxides are mostly liquids or gases. Non-metal oxides dissolve in water to make acidic solutions. This means the solutions have pH values which are lower than 7. ● In an oxidation reaction, a substance gains oxygen atom. Oxidation reactions occur when a substance reacts with the element oxygen to produce an oxide. ● The word and symbol equations are: Magnesium + oxygen → magnesium oxide ● Combustion (burning) is an example of an oxidation reaction. This is because a fuel reacts with oxygen to release energy. ● Rusting is another example of an oxidation reaction. ● Iron reacts with oxygen in the air and produces iron oxide. Iron oxide is the scientific name for rust! ● The word equation for rusting is: Iron + oxygen → iron oxide9Hydrated) ● Corrosion happens when a metal continues to oxidise. The metal becomes weaker over time, and eventually all of it may become metal oxide. The nail only rusts in the left-hand test tube does not rust: In the middle test tube, where there was water but no oxygen the nail does not rust In the right-hand test tube, where there was oxygen but no water the nail cannot rust. Iron can only rust in the presence of oxygen AND water. Rust prevention ● Rusting can be prevented by creating a physical barrier to oxygen and water. Ways to do this include: painting, oiling and greasing, coating with plastic ● Iron can be protected from rusting if it is in contact with a more reactive metal, such as zinc. The more reactive metal oxidises more readily than iron, so it ‘sacrifices’ itself while the iron does not rust. ● When iron is coated in zinc, the process is called galvanising. The zinc layer stops oxygen and water reaching the iron. Zinc is more reactive than iron, so it also acts as a sacrificial metal. This protection works, even if the zinc layer is scratched.