Junior Secondary Science Revision Notes - Living Things and Air - PDF

7 Living things and air 7.1 Air (Book 2A, p. 2–13) 1 Air is a mixture of gases. It contains nitrogen (78%), oxygen (21%), carbon dioxide (0.04%), noble gases (about 0.9%), water vapour and other gases. 2 We can carry out the following tests to test for oxygen, carbon di...

7 Living things and air 7.1 Air (Book 2A, p. 2–13) 1 Air is a mixture of gases. It contains nitrogen (78%), oxygen (21%), carbon dioxide (0.04%), noble gases (about 0.9%), water vapour and other gases. 2 We can carry out the following tests to test for oxygen, carbon dioxide and water: Test for… Result of test oxygen It relights a glowing splint. It turns hydrogencarbonate indicator from red to yellow. carbon dioxide It turns lime water from colourless to milky. water It turns dry cobalt chloride paper from blue to pink. There is no simple test for nitrogen. 3 Properties and some uses of oxygen, carbon dioxide and nitrogen: Gas Properties Examples of uses Oxygen Supports burning Mixed with a fuel gas to produce a very hot flame of for cutting and welding metals Used to help patients with breathing difficulties Carbon dioxide Does not support burning Used in some fire extinguishers Denser than air Used on the stage to produce the effect of Changes from solid to dense fog gas directly at −78 °C Used to keep frozen foods cold Nitrogen Unreactive Used in food packaging Very low boiling point Used in the storage of living cells or tissues (−196 °C) Used to freeze foods rapidly Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 1  7 Living things and air 7.2 Photosynthesis (Book 2A, p.14–35) 1 Plants make their own food by photosynthesis. It mainly occurs in the leaves of the plants. 2 Green leaf cells contain chloroplasts. Chlorophyll inside chloroplasts absorbs light energy for photosynthesis. 3 During photosynthesis, light energy is converted to chemical energy. The light energy is used to convert carbon dioxide and water into food (glucose). Oxygen is produced as a by-product. light energy (from the sun) food (glucose) used to produce carbon dioxide — the product useful substances — from air e.g. proteins, chlorophyll chlorophyll water — from soil oxygen (by-product) — released into air 4 Word equation of photosynthesis: carbon dioxide water food (glucose) oxygen reactant reactant product by-product the substances undergoing the reaction the substances produced in the reaction E 5 Chemical equation of photosynthesis: 6 CO2 6 H2O C6H12O6 6 O2 Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 2  7 Living things and air 6 Glucose produced in plants during photosynthesis can be used immediately or energy or stored as starch for later use. Hence, starch is also considered a product of photosynthesis. 7 Iodine solution can be used to test for starch. It turns from brown to blue-black in the presence of starch. 8 Steps of testing for starch in a green leaf and their purposes: Step Purpose 1 Boil the leaf in water. Destroy the cell membranes of the leaf cells. This allows alcohol and iodine solution to get into the cells. 2 Put the leaf into hot alcohol. Dissolve chlorophyll from the leaf so that the colour change can be observed. 3 Wash the leaf in hot water. Remove the remaining alcohol and make the leaf soft. 4 Add iodine solution to the leaf. Test for starch. 9 Light, chlorophyll, carbon dioxide and water are the necessary factors for photosynthesis 10 Before carrying out experiments to investigate the factors necessary for photosynthesis, we have to destarch the plant by putting it in the dark for at least 24 hours. 11 A control set-up is identical to the experimental set-up except that the factor under investigation is absent. It is exposed part important for ensuring that any change (experimental set-up) observed in the experimental set-up is due wrapped part to the factor under investigation but not (control set-up) other factors. 12 Soda lime can be used to absorb carbon dioxide from the air. 13 During photosynthesis, light energy from the sun is converted to chemical energy stored in food. The energy is transferred from plants to animals through feeding. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 3  7 Living things and air E 14 The feeding relationships among different living things can be represented by a food chain. For example, in grassland, when rabbits feed on grass, the chemical energy in grass in transferred to rabbits. The energy is transferred to foxes when the rabbits are eaten by foxes. grass rabbit fox ( means eaten by) E 15 In most food chains, plants are the producers because they can make their own food. Animals that feed on plants or other animals are the consumers. 16 Photosynthesis is important for the survival of other living things on Earth: providing a source of energy by converting light energy to chemical energy supplying oxygen. 7.3 Respiration (Book 2A, p. 36–39) 1 Living things break down food to release energy by respiration. 2 During respiration, some of the chemical energy stored in food is converted to other useful forms of energy to support body activities. 3 Oxygen is usually needed in respiration, and carbon dioxide and water are usually produced. 4 Word equation of respiration: food (glucose) oxygen carbon dioxide water E Chemical equation of respiration: C6H12O6 6 O2 6 CO2 6 H2O Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 4  7 Living things and air 7.4 Gas exchange in plants (Book 2A, p. 40–46) 1 Hydrogencarbonate indicator shows different colours in different carbon dioxide levels: lower than normal level in higher than Carbon dioxide level normal level air (about 0.04%) normal level Colour of purple red yellow hydrogencarbonate indicator 2 The net gas exchange in plants depends on the relative rates of photosynthesis and respiration. Under light conditions Under dark conditions Rate of photosynthesis is higher than Only respiration occurs rate of respiration Net uptake of carbon dioxide Net uptake of oxygen Net release of oxygen Net release of carbon dioxide E 3 Gas exchange in plants is carried out through the stomata on leaves. oxygen carbon dioxide stoma Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 5  7 Living things and air 7.5 Gas exchange in animals (Book 2A, p. 47–66) 1 Compared with inhaled air, exhaled air: contains less oxygen; contains more carbon dioxide; contains more water vapour; has a higher temperature. 2 Main parts of the human breathing system: nasal cavity nostril bronchiole trachea bronchus rib air sac lung intercostal muscle diaphragm 3 Path of air flow when we breathe in and breathe out: Path of air flow when we breathe in Bronchioles air sacs nostrils nasal cavity trachea bronchi (in lung) (in lung) Path of air flow when we breathe out Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 6  7 Living things and air 4 Gas exchange takes places at air sacs. Their surfaces are covered with capillaries. 5 The lungs are protected by the rib cage, which consists of ribs, the backbone and the sternum. 6 The actions of intercostal muscles and diaphragm bring about breathing. lungs trachea sternum rib intercostal muscle diaphragm E 7 The process of gas exchange in air sac: oxygen carbon dioxide 3 Carbon dioxide is removed 1 Inhaled air enters the air sac. from the body when we blood breathe out. 3 The blood then blood carries oxygen to 1 The blood carries the body cells carbon dioxide from for respiration. the body cells to the air sac. 2 Oxygen in the inhaled 2 Carbon dioxide passes air enters the blood in from the blood into the capillaries. the air sac. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 7  7 Living things and air E 8 Tar and some other chemicals in cigarette smoke reduce the efficiency of gas exchange: Tar is deposited in the lungs. It covers some of the surface of the air sacs. Carbon monoxide reduces the oxygen-carrying ability of blood. Less oxygen in the air sacs can enter the blood. Certain chemical cause the walls of air sacs to break down. The surface area for gas exchange becomes smaller. 9 Nicotine in cigarette smoke is addictive. 10 Smoking is harmful to health. It causes many diseases such as stroke, lung cancer and heart diseases. 7.6 Balance of oxygen and carbon dioxide in Nature (Book 2A, p. 67–75) 1 Respiration of living things and photosynthesis of plants keep a balance of oxygen and carbon dioxide in Nature: carbon dioxide respiration photosynthesis of animals of plants and plants oxygen 2 Some human activities (e.g. burning fossil fuel and cleaning forests) are disrupting the balance of carbon dioxide in Nature. 3 Some gases (e.g. carbon dioxide) trap thermal energy in the atmosphere. This causes the Earth’s surface to warm up. This warming effect is called the greenhouse effect and theses gases are called greenhouse gases. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 8  7 Living things and air 4 The greenhouse effect keeps the Earth’s surface at a temperature suitable for life. The increasing amount of carbon dioxide traps more thermal energy and enhances the greenhouse effect. This causes global warming. 5 The rise in global temperature brings about climate change. This may lead to the following possible consequences: Living things in the Polar regions lose their habitats and die. Sea levels rises, causing the flooding of low-lying areas. Unusual weather patterns may result (e.g. more typhoons, heat waves and droughts). Living things that cannot adapt to the warmer climate may die. 7.7 Air quality (Book 2A, p. 76–81) 1 Sulphur dioxide, nitrogen oxides, ozone, carbon monoxide and suspended particulates are some of the major air pollutants. 2 Air pollutants are mainly produced during the burning fossil fuels in power stations, factories and motor cars. 3 The effects of some air pollutants on different parts of our body: Body part Harmful effects of air pollutants Eyes Sulphur dioxide, nitrogen oxides and ozone irritate eyes. Carbon monoxide reduces the oxygen-carrying ability of blood, Brain causing dizziness and headaches. Sulphur dioxide, nitrogen oxides, ozone and suspended particles Breathing system irritate throat, airways and lungs; worsen bronchitis and asthma. 4 The Air Quality Health Index (AQHI) tells the public the possible health risks caused by air pollutants and help them take precautionary measures. 5 The amounts of air pollutants are measured at air quality monitoring stations in different districts. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 9  8 Making use of electricity 8.1 Simple circuits (Book 2A, p.90–103) 1 An electric circuit that forms a closed loop is called a closed circuit, one that does not is called an open circuit.  Closed circuit  Open circuit 2 Conditions for an electrical appliance to work: there must be a source of electrical energy (e.g. a cell or battery). the source of electrical energy and the appliance must form a closed circuit. 3 Materials that conduct electricity are electrical conductors. Examples: metal, pencil lead (graphite), vinegar 4 Materials that do not conduct electricity are electrical insulators. Examples: plastics, wood, distilled water 5 A switch is used to open or close an electric circuit. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 1  8 Making use of electricity 6 Circuit symbols of some circuit components of a circuit: Circuit component Circuit symbol Circuit component Circuit symbol Connecting wire / Cell connector Two or more (connected) Battery or connecting wires / connectors (not connected) (open) Switch Bulb (closed) 7 Drawing circuit diagrams: Steps a Draw the circuit symbol for each component in the same position as it appears in the circuit b Use straight lines to represent connecting wires. c Put a solid dot at each wire junction. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 2  8 Making use of electricity 8.2 Current (Book 2A, p.104–116) 1 Electric current is a flow of electric charges. It flows from the positive terminal of a cell to the negative terminal through the circuit. 2 We use an ammeter to measure the size of a current. The unit of current is the ampere (A). 3 The circuit symbol for an ammeter:  An ammeter 4 Connecting an ammeter: The red knob of the ammeter should lead to the positive terminal of the cell and the black knob to the negative terminal. switch switch black ammeter knob ammeter black red knob knob red knob Never connect the ammeter to the cell directly. Otherwise, a large current may flow through the ammeter and damage it. ammeter cell Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 3  8 Making use of electricity 5 The water flow model can be used as an analogy for an electric circuit: water flow connecting wire current water channel bulb water cell pipe turbine water pump  Water flow model  Electric circuit water pipe and Part in water flow model water pump water flow turbine water channel Part in electric circuit cell connecting wire current bulb provides a path for transfers consumes Function supplies energy water flow / current energy energy 6 When a current flows through a conductor, the temperature of the conductor increases. Some electrical energy is converted to thermal energy. This is the heating effect of current. The effect becomes greater when the current increases. 7 Many appliances work using the heating effect of current. Examples: electric kettle, iron, hairdryer 8 A current produces a magnetic effect. When a current flows through a wire near a compass, the needle of the compass changes direction. When a current flows through a coil of wire, the coil becomes an electromagnet. 9 Electromagnet is useful in many areas. Examples: electromagnetic door lock, electric motor, electromagnetic crane Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 4  8 Making use of electricity 8.3 Voltage (Book 2A, p.117–126) 1 The voltage of a cell indicates how much energy is supplied by the cell to the electric charges in a circuit. 2 We use a voltmeter to measure the size of voltage. The unit of voltage is the volt (V). 3 The circuit symbol for a voltmeter:  A voltmeter 4 Connecting a voltmeter: A voltmeter is connected across a cell to measure its voltage. The red knob of the ammeter should be connected to the positive terminal of the cell and the black knob to the negative terminal. 5 When more than one cell is connected in the same direction, the voltage across the cells is added up. The overall voltage is the sum of the individual voltages. 6 When one of the cells is connected in the opposite direction, the voltage across the cells is lower than the sum of the individual voltages. 7 The higher the voltage, the larger the current flowing through the circuit. 8.4 Resistance (Book 2A, p.127–140) 1 Resistance shows the opposition of a material to the flow of current. Its unit is the ohm (Ω). 2 A resistor is a circuit component that has a certain value of resistance. The circuit symbol for a resistor: Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 5  8 Making use of electricity 3 If a resistor with a higher resistance is connected to a cell, it will result in a smaller current in the circuit. We can connect a resistor to a circuit regardless of its polarity. 4 Resistors are used to control the size of current in a circuit. E 5 Factors affecting resistance: Factor How it affects the resistance of a wire Material of wire A copper wire has a lower resistance than a nichrome wire of the same length and thickness. Thickness of wire A thin wire has a higher resistance than a thick wire of the same material and length. Length of wire A long wire has a higher resistance than a short wire of the same material and thickness. E 6 A variable resister or rheostat is a resistor with an adjustable resistance. It is used to adjust the current in a circuit. E 7 The circuit symbol for a variable resistor or a rheostat: E 8 Volume controls and dimmer switches are examples of applications of rheostats. 8.5 Electrical circuits (Book 2A, p.141–150) 1 In a series circuit, current flows along one path only; the current is the same at all points; the sum of the voltages across each appliance is equal to the voltage of the energy source. 2 If a series circuit is broken at any point, current will not flow in the circuit. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 6  8 Making use of electricity 3 In a parallel circuit, current can flow in more than one path; the current in the main loop is equal to the sum of the currents in the branches; the voltage across each branch is the same; the current flowing in the branch with a lower resistance is larger. 4 If a parallel circuit is broken at any branch, current will flow in other branches. 8.6 Household electricity (Book 2A, p.151–170) 1 The appliances at home are supplied with mains electricity. 2 The mains voltage in Hong Kong is 220 V. 3 Electrical appliances are energy converters. They convert electrical energy to other forms of energy. 4 Many household appliances make use of the heating and magnetic effects of current. The heating element of some appliances becomes hot when a current flows through it. Most of the electrical energy is converted to thermal energy. The electric motors of some appliances make the moving part rotate using the magnetic effect of current. They convert electrical energy into kinetic energy. 5 Wall socket in Hong Kong: earth hole switch neutral hole live hole Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 7  8 Making use of electricity 6 A three-pin plug: earth pin live pin neutral pin 7 The wires of a three-pin plug and their colours: earth wire fuse (green and yellow) neutral wire (blue) live wire (brown) flex grip 8 Electricity is supplied to our homes through the live wire and the neutral wire. 9 All sockets are connected in parallel in a domestic circuit. Switching on or off each appliance does not affect the operation of other appliances. Each socket offers the same voltage for any appliances plugged in. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 8  8 Making use of electricity 10 When several electrical appliances are connected to a single mains socket via a universal adaptor or an extension unit: If too many electrical the current flowing appliances are connected through the socket may to a single mains socket, become very large. The strong heating effect on the mains circuit may Overloading may occur. result in a fire. 11 To avoid overloading, we should connect appliances that draw a large current to different main sockets. 12 In the case of a short circuit: The wire connected in parallel to the bulb provides a branch with very low resistance for current to flow. The bulb is said to be short-circuited. Since resistance of the wire is very This may overheat the circuit or low, the current flowing through it cause a fire. becomes large. 13 Fuse rating shows the maximum current the fuse can stand. 14 The circuit symbol for a fuse: In a fuse, there is a metal wire with a low melting point. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 9  8 Making use of electricity 15 If the current passing through a fuse is larger than the fuse rating, the fuse will blow and cut off the current. This prevents the circuit from overheating. 16 A suitable fuse should have a rating slightly higher than the normal operating current of the electrical appliance. 17 A circuit breaker turns to the ‘OFF’ mode and cuts off the current when the current exceeds its rating. 18 A circuit breaker can be reset by switching it back to the ‘ON’ mode.  Circuit breakers in consumer unit 19 The earth wire connects the metal case of an electrical appliance to the ground. It protects us from an electric shock. 20 Function of the earth wire: If the live wire becomes loose large current flows to the ground through the and touches the metal case of an earth wire and blow the fuse which electrical appliance, disconnect the appliance from mains supply. The current from the mains is cut off. This protects us from an electric shock. 21 Safety precautions in using electricity: We should… We should NOT…  use three-pin plugs with an earth wire. ✗ overload sockets.  only switch on an appliance after firmly ✗ cover appliances while they are plugging it into a socket outlet. working.  use fuses with suitable fuse ratings. ✗ let water seep into any appliances.  ask a registered electrical worker to ✗ touch sockets or switches with wet immediately replace damaged cables, hands. sockets and plugs. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 10  8 Making use of electricity E 8.7 Power and efficiency (Book 2A, p.171–181) 1 The electrical power of an appliance is the amount of electrical energy transferred to the appliance in one second. energy 2 Power = time 3 The unit of power is the watt (W). 1 W = 1 J/s A larger unit of power is the kilowatt (kW). 1 kW = 1000 W 4 The power rating is the power of the appliance when it is working at the indicated voltage rating. 5 The electrical energy consumed by the appliance can be calculated by: Energy = power × time 6 The efficiency of an electrical appliance tells us what percentage of the power input is converted to useful power output. useful power output Efficiency = × 100% power input 7 An appliance with high efficiency uses less power to do the same job than one with low efficiency. 8 We can find an energy label on many appliances. Grade 1 products are the most efficient while grade 5 products are the least efficient. 9 A joulemeter can be used to measure the electrical energy consumed by appliances in joules (J). 10 A kilowatt-hour meter measures the electrical energy in a large unit called kilowatt-hour (kW h). 1 kW h = (1000 W) × (60 × 60 s) = 3 600 000 J  A kilowatt-hour meter Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 11  8 Making use of electricity 11 The amount of electrical energy consumed by an electrical appliance can be read from the kilowatt-hour (218.7 kW h in the photo below). 12 The kilowatt-hour (kW h) is a unit of electrical energy for calculating the cost of electricity. 13 Ways to save electricity at home: Choose energy efficient appliances, i.e. appliances with Grade 1 energy label. Do not leave the appliances in standby mode. Always switch off or plug off appliances from power source after use. Switch lighting on only when you need it. Use daylight whenever possible. Running air conditioner and fan together to spread the cooled air more effectively. Remove battery charger from mains socket after charging is completed. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 12