Cambridge Lower Secondary Stage 7 Chemistry Flashcards

What is the strongest general evidence that a chemical reaction has taken place?

Topic: Evidence of reactions
Answer: Reactants are used up and new products with different properties are formed.

A student says, 'A reaction happened because the material changed shape.' Why is this weak evidence?

Topic: Evidence of reactions
Answer: Changing shape can be a physical change. A chemical reaction needs evidence that new substances with different properties formed.

A mixture starts colourless and becomes blue. Why may this suggest a chemical reaction?

Topic: Evidence of reactions
Answer: A new coloured product may have formed, showing the products have different properties from the reactants.

Why is bubbling during mixing often evidence of a chemical reaction?

Topic: Evidence of reactions
Answer: It may show that a gas product is being formed from the reactants.

A liquid is heated and bubbles. Why is this not automatically evidence of a chemical reaction?

Topic: Evidence of reactions
Answer: The liquid may simply be boiling; boiling is a physical change and does not necessarily make a new substance.

What observation shows that a solid product may have formed when two clear solutions are mixed?

Topic: Evidence of reactions
Answer: A cloudy solid or precipitate appears in the mixture.

Two clear solutions are mixed and the mixture stays clear with no temperature, colour or gas change. What can you conclude?

Topic: Evidence of reactions
Answer: There is no clear observational evidence of a chemical reaction, although a reaction cannot be proved impossible from this alone.

Why is the loss of reactants important evidence in a chemical reaction?

Topic: Evidence of reactions
Answer: It shows the starting substances are being changed into products rather than simply remaining unchanged.

A piece of metal becomes smaller while bubbles form in an acid. What does this suggest?

Topic: Evidence of reactions
Answer: The metal reactant is being used up and a gas product is forming, so a chemical reaction is likely.

A white powder is mixed with water and disappears. Why is this not enough to show a chemical reaction?

Topic: Evidence of reactions
Answer: It may have dissolved to form a solution; dissolving often does not make a new substance.

A reaction forms a product that is a different colour from all reactants. What key idea does this demonstrate?

Topic: Evidence of reactions
Answer: Products can have different properties from the reactants.

A student records 'fizzing' but does not identify the gas. What should the student still understand about the fizzing?

Topic: Evidence of reactions
Answer: Fizzing is evidence that a gas may be produced, but further tests are needed to identify the gas.

Why is 'it looks different' an incomplete conclusion in chemistry?

Topic: Evidence of reactions
Answer: It does not explain whether a new substance formed; the change could be physical or chemical.

Give three observations that can show new products are forming in a chemical reaction.

Topic: Evidence of reactions
Answer: Evolution of a gas, formation of a precipitate, and a change of colour.

A colour change happens when food colouring is added to water. Why is this not a good example of a chemical reaction?

Topic: Evidence of reactions
Answer: The colour change is caused by mixing and spreading of dye particles, not necessarily by forming a new substance.

Why should observations be made before and after mixing reactants?

Topic: Evidence of reactions
Answer: Comparing before and after helps show whether reactants changed and products with different properties formed.

A gas is produced but both liquids were colourless before mixing. Why can a reaction still be identified?

Topic: Evidence of reactions
Answer: The gas is a new product with different properties, even if there is no colour change.

A precipitate forms but no gas is produced. Is this enough evidence for a chemical reaction?

Topic: Evidence of reactions
Answer: Yes, if a new insoluble solid product forms from soluble reactants, it is evidence of a chemical reaction.

Why should a scientist avoid relying on only one observation when deciding if a reaction happened?

Topic: Evidence of reactions
Answer: Some observations can be caused by physical changes, so several pieces of evidence make the conclusion stronger.

An orange solid and a clear liquid react to make a green solution. Which change supports the idea of new products?

Topic: Evidence of reactions
Answer: The colour change from orange to green suggests products with different properties have formed.

What is a precipitate?

Topic: Precipitates
Answer: An insoluble solid that forms in a liquid during a chemical reaction.

Why can two clear solutions produce a cloudy mixture when mixed?

Topic: Precipitates
Answer: Soluble reactants can react to form an insoluble solid product, called a precipitate.

In a precipitation reaction, what must be true about at least one product?

Topic: Precipitates
Answer: At least one product must be insoluble in the liquid.

A student says, 'The solid was already hidden in one solution.' Correct this idea.

Topic: Precipitates
Answer: The precipitate forms when particles from soluble reactants react to make a new insoluble product.