What does a physical change alter in a substance?
Its state or shape without changing chemical composition.
What results from a chemical change in substances?
New substances with different chemical properties.
What is a chemical reaction in terms of atomic bonds?
It is breaking bonds in reactants and forming new bonds in products.
What are the main signs indicating a chemical reaction?
Precipitate formation, gas evolution, color change, consistency change, odour, light and heat emission.
What gas is produced when hydrochloric acid reacts with magnesium?
Hydrogen gas is produced.
How can hydrogen gas from magnesium and hydrochloric acid be tested?
It burns with a pop sound near a burning match.
What happens to the blue color when zinc is placed in copper sulphate solution?
The blue color disappears.
What forms on the zinc plate in copper sulphate solution?
Reddish-brown copper precipitates form.
What forms in solution when zinc reacts with copper sulphate?
Colorless zinc sulphate forms.
What can reactants do during a chemical reaction?
They may combine, decompose, or replace one another.
What causes bonds to break during a chemical reaction?
The chemical reaction process causes bonds to break.
What are the components of every chemical reaction?
Reactants and products.
How is a chemical reaction represented?
By a chemical equation.
What happens when a magnesium strip burns in air?
It glows brightly with heat.
What is the product formed when magnesium burns in air?
White magnesium oxide powder.
Is magnesium oxide formed by burning magnesium soluble in water?
Yes, it is soluble in water.
How are products formed in a chemical reaction?
By bond breaking in reactants, atom rearrangement, and new bond formation.
How is soap formed from oil and sodium hydroxide?
By adding oil dropwise to NaOH, heating to 40–50°C, and stirring well in one direction.
What does the reaction of oil with concentrated sodium hydroxide produce?
It produces a solid substance called soap and changes the mixture's consistency.
Name one example of a physical change involving water.
Melting ice.
What happens to water during freezing as a physical change?
It expands.
Give one example of a chemical change involving combustion.
Wood combustion.
What chemical change occurs when milk turns into yogurt?
Yogurt formation from milk.
What chemical change causes iron to rust?
Rusting of iron.
Which chemical change involves limewater turning milky?
Reaction with carbon dioxide.
What precipitate forms when silver nitrate reacts with sodium chloride?
A white silver chloride precipitate forms.
What happens to silver chloride precipitate when exposed to sunlight?
It changes to violet.
What change occurs when carbon dioxide gas passes into limewater?
The limewater becomes milky or turbid.
What gas is released when baking soda reacts with vinegar?
Carbon dioxide gas is released.
What changes occur when sugar burns?
It changes from white solid to brown molten caramel-smelling substance, then to black solid with strong odour and smoke.
Answer 13 questions on Chemical Reactions and Changes.
1. A sample of ice melts into liquid water without producing a different substance. How should this change be classified?
2. Why is the formation of rust on iron classified as a chemical change?
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