4. Rewrite the sentence using the modal verbs.
a I’m sure you know the difference between a solid and a gas! MUST
You must know the difference between a solid and a gas!
________________________________________________________________________
b If a substance does not contain hydrogen, then we are sure that it isn’t an acid. CAN’T
__________________________________________________________________________________
c If we put some litmus paper in a solution and it turns red, then we are sure that the solution
is an acid. MUST
___________________________________________________________________________
d If we add a substance to a red-cabbage solution and it stays red, then we are sure that the
substance isn’t an alkali. CAN’T
___________________________________________________________________________
e If we add a substance to a red-cabbage solution and it turns green, then the substance is sure
to be an alkali. MUST
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__
f If we put some litmus paper in a solution and it turns red, then we are sure that the solution
isn’t an alkali. CAN’T
___________________________________________________________________________
g If it’s not an acid, or an alkali, then it’s sure to be neutral. MUST
___________________________________________________________________________
h If we use a universal indicator to check a substance, and it turns purple, then we are sure
that the substance isn’t an acid. CAN’T
___________________________________________________________________________
35
5. Look at the picture and comment on it from the point of view of chemistry.
6. Read the text. Which pH indicator is the most acidic? Which one is the most basic?
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