La brasa está muy caliente.

Because Spanish uses estar for temporary states and conditions, and temperature is treated as a condition.

So:

• La brasa está muy caliente = the ember/coals are very hot
• estar is the normal verb for saying something is hot, cold, tired, open, closed, etc.

Using es here would sound wrong in standard Spanish.

A useful shortcut:

• ser = what something is
• estar = how something is

The accent mark shows the correct stress: es-TÁ.

It also helps distinguish it from esta, which can mean this:

• esta brasa = this ember
• está caliente = is hot

So the accent is important both for pronunciation and meaning.

Brasa usually means a glowing coal, ember, or hot charcoal.

Depending on context, it can refer to:

• a single ember
• glowing coals in a fire or barbecue
• heat from charcoal

In many situations, English might translate it more naturally as coal, ember, or even the coals rather than just ember.

Because brasa is a feminine noun in Spanish.

So you say:

• la brasa
• una brasa

And adjectives or other words connected to it may need to agree with it. In this sentence, caliente happens to look the same in masculine and feminine singular, so you do not see a change.

Normally, no. In Spanish, singular count nouns usually need an article or another determiner.

So Spanish wants:

• La brasa está muy caliente
• Una brasa está muy caliente
• Esta brasa está muy caliente

But not usually:

• Brasa está muy caliente

The article makes the noun phrase complete.

• La brasa = the ember / the coal
  You are talking about a specific one, or one already known in the conversation.
• Una brasa = an ember / a coal
  You mean one unspecified ember.

So:

• La brasa está muy caliente = that specific ember is very hot
• Una brasa está muy caliente = an ember is very hot / one ember is very hot

Because muy is used before adjectives and adverbs to mean very.

Here, caliente is an adjective, so you need:

• muy caliente = very hot

Mucho is used differently, often with nouns or verbs:

• mucho calor = a lot of heat
• come mucho = he/she eats a lot

So muy caliente is correct, not mucho caliente.

Yes, it agrees, but caliente has the same form for masculine and feminine singular.

Compare:

• el plato está caliente
• la brasa está caliente

Both use caliente.

But in the plural, it changes:

• las brasas están calientes
• los platos están calientes

So it does agree in number, even if the singular masculine and feminine look the same.

You would say:

Las brasas están muy calientes.

Changes:

• la → las
• brasa → brasas
• está → están
• caliente → calientes

This means The coals/embers are very hot.

No. In this sentence, caliente clearly means hot in temperature.

For spicy, Spanish usually uses picante:

• La sopa está caliente = The soup is hot
• La sopa está picante = The soup is spicy

This is a very common thing for English speakers to mix up.

A simple pronunciation guide is:

lah BRAH-sah es-TAH mooy kah-LYEN-teh

A few notes:

• br in brasa is a rolled or tapped r sound
• está is stressed on the last syllable
• muy sounds roughly like mooy
• caliente is stressed on -lien-

In standard Spain Spanish, the pronunciation is approximately: [la ˈbɾasa esˈta muj kaˈljente]

Normally, no. For temperature, Spanish strongly prefers estar.

A native speaker would say:

• La brasa está caliente
• El café está frío
• La sopa está muy caliente

Using ser for temperature would sound unnatural or simply incorrect in standard usage.

Yes, it is natural and correct.

That said, depending on context, people might also say:

• La brasa quema. = The ember burns / it burns you.
• Las brasas están muy calientes. = The coals are very hot.
• Está ardiendo. = It’s burning / blazing hot.

But La brasa está muy caliente is perfectly normal Spanish.