AQA C2 Acids, Bases & Salts Help

Hiya,
I'm just doing the C2 2015 paper and came across the following question:

5 (b) Metal oxides react with acids to make salts.
What type of compound is a metal oxide? [1 mark]

I put 'alkali' but the mark scheme states:
5 (b) base | ignore alkali

I'm just wondering why this is - I've never been able to get my head around this topic but I thought alkali/base was pretty much interchangable; is there some way to know whether a product is soluble or not?

Thank you

An alkali is a type of base that can dissolve in water - not all metal oxides can dissolve, hence they're bases, not alkalis.

And I'm pretty sure that you can tell whether something is soluble or not by looking at the chemical formula of the compound. I think I saw a table on it some while ago.

Edit: Here's the list.

Soluble
Any sodium or potassium salts
Any nitrate
Most chlorides, bromides and iodides (exceptions are in Insoluble list)
Most sulphates (exceptions are in Insoluble list)

Insoluble
Silver chloride, bromide and iodide
Lead chloride, bromide and iodide
All carbonates (except for sodium and potassium carbonate)
Barium, calcium, silver and lead sulphate

Hiya,
I'm just doing the C2 2015 paper and came across the following question:

5 (b) Metal oxides react with acids to make salts.
What type of compound is a metal oxide? [1 mark]

I put 'alkali' but the mark scheme states:
5 (b) base | ignore alkali

I'm just wondering why this is - I've never been able to get my head around this topic but I thought alkali/base was pretty much interchangable; is there some way to know whether a product is soluble or not?

Thank you

bases are metal oxides and metal hydroxides.
alkalis are type of base: a soluble base (i.e. a metal hydroxide)
thus, a metal oxide is a base, but not an alkali
and a metal hydroxide is both a base and and alkali.

Soluble
Any sodium or potassium salts
Any nitrate
Most chlorides, bromides and iodides (exceptions are in Insoluble list)
Most sulphates (exceptions are in Insoluble list)

Insoluble
Silver chloride, bromide and iodide
Lead chloride, bromide and iodide
All carbonates (except for sodium and potassium carbonate)
Barium, calcium, silver and lead sulphate

Do we need to know these ?


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I can't see it on the specification

Soluble
Any sodium or potassium salts
Any nitrate
Most chlorides, bromides and iodides (exceptions are in Insoluble list)
Most sulphates (exceptions are in Insoluble list)

Insoluble
Silver chloride, bromide and iodide
Lead chloride, bromide and iodide
All carbonates (except for sodium and potassium carbonate)
Barium, calcium, silver and lead sulphate