Metals, non-metals and combustion
Class 8 Science · Chemistry · 33 questions with answers
Class 8 Chemistry is where a change starts being described by an equation rather than by a sentence, and where the exceptions matter as much as the rules. Sodium is a metal you can cut with a knife; graphite is a non-metal that conducts. These questions cover both the pattern and the places it breaks.
- Lists the physical properties of metals and non-metals with an exception for each
- Predicts a displacement reaction from the order of reactivity
- Writes a word equation for a simple reaction
- Names the three conditions for combustion and applies them to putting out a fire
- Identifies the zones of a flame and which is hottest
Exercise 1 of 3 · 12 questions
Metal or non-metal? Give the property that shows it, and name an exception to that property.
Write the answer · Warm-up
Here’s one done for you
Copper is a metal - it conducts heat and electricity well and can be beaten into sheets. The exceptions are what the paper likes: mercury is a liquid metal, sodium and potassium are soft enough to cut with a knife, graphite is a conducting non-metal, and iodine is a lustrous one. Four exceptions, and knowing them is usually worth more than knowing a longer list of properties.
- 1)Copper
- 2)Sulphur
- 3)Iron
- 4)Iodine
- 5)Mercury
- 6)Graphite
- 7)Aluminium
- 8)Phosphorus
- 9)Silver
- 10)Oxygen
- 11)Sodium
- 12)Carbon (diamond)
Answers
- 1) Metal - it conducts heat and electricity well and is malleable. The exception among metals is mercury, which is liquid at room temperature.
- 2) Non-metal - it is dull and brittle, and breaks into pieces when hammered rather than flattening.
- 3) Metal - it is sonorous, malleable and a good conductor. It is also the metal most obviously affected by rusting.
- 4) Non-metal, and the exception to the rule that non-metals are dull - iodine is lustrous.
- 5) Metal, and the exception to the rule that metals are solid at room temperature.
- 6) Non-metal, and the exception to the rule that non-metals do not conduct electricity - graphite does, which is why it is used in electrodes.
- 7) Metal. It is malleable and conducts heat and electricity well.
- 8) Non-metal. It is brittle, dull and a poor conductor.
- 9) Metal. It is lustrous, ductile and the best conductor of electricity.
- 10) Non-metal. It is a gas at room temperature and does not conduct.
- 11) Metal. It is lustrous when freshly cut and conducts electricity, though it is soft enough to cut with a knife.
- 12) Non-metal. It is a very hard non-metal and does not conduct electricity, unlike graphite.
Exercise 2 of 3 · 10 questions
Will a reaction happen? If yes, write the word equation and say what you would see.
Write the answer · Guided
Here’s one done for you
An iron nail in copper sulphate: iron is more reactive than copper, so it displaces it - iron sulphate and copper. The observation is half the answer and is what proves the reaction happened: the blue solution turns pale green and a reddish-brown layer forms on the nail. An answer with the equation and no observation is missing exactly the part that shows the child understood what a displacement reaction is.
- 1)An iron nail placed in copper sulphate solution
- 2)A copper wire placed in iron sulphate solution
- 3)Zinc granules placed in copper sulphate solution
- 4)Zinc added to dilute sulphuric acid
- 5)Copper turnings placed in silver nitrate solution
- 6)Silver wire placed in copper sulphate solution
- 7)Magnesium ribbon placed in zinc sulphate solution
- 8)Iron filings added to dilute hydrochloric acid
- 9)Copper placed in dilute hydrochloric acid
- 10)Zinc placed in magnesium sulphate solution
Answers
- 1) Yes. Iron sulphate + copper. Iron is more reactive than copper, so it displaces it. The blue solution turns pale green and a reddish-brown deposit forms on the nail.
- 2) No reaction. Copper is less reactive than iron, so it cannot displace iron from its salt. The solution stays as it was.
- 3) Yes. Zinc sulphate + copper. Zinc is more reactive than copper, so the blue colour fades and copper is deposited.
- 4) Yes. Zinc sulphate + hydrogen. Bubbles of hydrogen gas are given off, and the gas burns with a pop when a lighted splint is brought near.
- 5) Yes. Copper is more reactive than silver: copper + silver nitrate → copper nitrate + silver.
- 6) No. Silver is less reactive than copper, so it cannot displace it.
- 7) Yes. Magnesium is more reactive than zinc: magnesium + zinc sulphate → magnesium sulphate + zinc.
- 8) Yes. Iron + hydrochloric acid → iron chloride + hydrogen.
- 9) No. Copper is below hydrogen in the reactivity series, so it does not displace hydrogen from an acid.
- 10) No. Zinc is less reactive than magnesium, so no reaction takes place.
Exercise 3 of 3 · 11 questions
Answer in two or three sentences.
Write the answer · Practice
Here’s one done for you
Water puts out a fire by doing two of the three things at once - it cools the fuel below its ignition temperature and it cuts off the air. That is why the three conditions are worth learning as a set rather than as a list: every question about putting out a fire is really asking which of the three you are removing, and every question about starting one is asking which is missing.
- 1)What three conditions are needed for combustion, and how does water put out a fire?
- 2)Why is water not used on an oil fire or an electrical fire?
- 3)Which zone of a candle flame is the hottest, and why?
- 4)Why does a matchstick catch fire when struck but not when simply held in air?
- 5)What is calorific value, and why does it matter when choosing a fuel?
- 6)Why does a candle go out when covered with a glass?
- 7)What is the ignition temperature of a substance?
- 8)Why is a fire extinguisher that gives out carbon dioxide effective?
- 9)Why does coal burn without a flame while a candle burns with one?
- 10)Why should a burning object never be carried through a crowded room?
- 11)Why is LPG considered a good fuel?
Answers
- 1) A fuel, a supply of air to provide oxygen, and heat to reach the ignition temperature. Water works by bringing the temperature below the ignition temperature and by cutting off the air.
- 2) Oil floats on water, so the burning oil spreads instead of going out. Water conducts electricity, so using it on an electrical fire risks a shock. Carbon dioxide is used instead, because it is heavier than air and smothers the fire without conducting.
- 3) The outermost blue zone, because combustion there is complete - the fuel meets enough oxygen to burn fully. The inner yellow zone glows brightly but burns only partly, and the dark innermost zone is unburnt vapour.
- 4) Because it has not reached its ignition temperature. Striking it produces heat by friction, and that heat takes the chemical on the head above its ignition temperature.
- 5) The amount of heat produced by burning one kilogram of a fuel completely, measured in kilojoules per kilogram. A fuel with a higher calorific value gives more heat for the same mass, which is one of the things that makes a fuel worth using.
- 6) Because the supply of air is cut off. Combustion needs oxygen, and once the oxygen inside the glass is used up the flame dies.
- 7) The lowest temperature at which a substance catches fire. Below it, the substance will not burn however long it is heated by a spark.
- 8) Because carbon dioxide is heavier than air and does not support combustion. It forms a blanket over the fire, cutting off the oxygen supply.
- 9) Because a flame is produced only by substances that vaporise on heating. Wax vaporises and the vapour burns as a flame, while coal burns as a glowing solid.
- 10) Because it can set light to clothing or other objects it passes, and a fire in a crowded space traps people. It is a hazard even if it looks small.
- 11) Because it has a high calorific value, burns cleanly without much smoke or ash, and is easy to store and transport as a liquid under pressure.
While your child works
- The exceptions are what the paper asks about. Mercury, sodium, graphite, iodine - four of them, and they are worth more than a longer list of properties.
- In displacement questions, insist on the observation as well as the equation. That is where the mark for understanding sits.
- For any fire question, ask 'which of the three conditions is being removed?' Every version of the question reduces to that.