The laboratory reactions of acids could be classified into four categories, but only two of them show the distinct physical changes in the material. But before considering these changes and the type of the acid reactions, it is vital to get familiarize with the acids and bases. Basically, acids are those substances which release H3O+ ions and bases are those substances which release OH- ions while dissolved in the water. However, there are several definitions of acids and bases given by various scientists. But, here you must consider upon the reactions of acids, keeping the fact in the mind that acids decrease the pH of water below 7.0, whereas base increase the pH towards 14.0.

So, let’s consider the reactions of acids, which depict the visual physical changes. Well, in case when acids react with the metal carbonates, the carbon dioxide gas is released in the form of bubbles. And in case when the acids react with most of the  metal, they release hydrogen gas, which shows more bubbles than the carbon-dioxide. This is a type of redox reaction in which the oxidation number of the reactants is changed. However, in rest of the reactions of acids, you can’t predict any physical and distinct changes. 

Moreover, you must keep in mind that acids are electrolytes, and you can identify them easily by their chemical names, which usually begin with H. So, first of all, let’s talk about the first category of reactions of acids, which include their reactions with the bases. In this case, it is worth noting that bases are also electrolytes. Therefore, whenever an acid reacts with a base, in these reactions of acids, two different products are formed but of which one product is always water.
Well, if we talk about the other reactions of acids, they react with the metal oxides to. However, in this case the visual evidence of the reaction is not available. The chemical process of these reactions of acids is same as that of acid-base reactions. In fact, you can consider metal oxides as bases without water. The following example will eliminate your all doubts regarding these reactions:
H2SO4 + MgO → MgSO4 + H2O
Acid + Metal oxide ( Salt + Water

The third type of reactions of acids is their reaction with metal carbonates. Now, you must be wondering about the properties of metal carbonates. Well, metal carbonates could be considered as metal oxide, which has picked up molecules of carbon-dioxide. The following example precisely defines the reactions of acids with metal carbonates:
2 HCl        +    K2CO3 → 2 KCl + H2O + CO2                
                    Acid + metal carbonate → Salt + Water + Carbon-dioxide

Now, let’s consider the last but the most important reactions of acids. Usually, these reactions don’t belong to the group of acid reactions, as there is a transfer of electrons in them. While, in all other reactions of acids, there is a transfer of H+ ion. Well, this exception reflects the fact that these reactions include redox process. The following example is quite enough to explain this type of acid reaction in which H form of the acid is replaced by the metal group.
2 HCl + Mg → MgCl2 + H2
                       Acid + Metal→ Salt + Hydrogen